Chemistry
89 Solved Past Papers
2010 – 2024 Archives
Alcohols & Phenols Past Papers
Solved past paper MCQs for Alcohols & Phenols from official UHS, NUMS, SZABMU, DUHS, and KMU examinations. Includes verified distractor autopsies and step-by-step cognitive explanations.
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Which of the following alcohol can give Iodoform reaction? [UHS (2024)]
A
Methanol
B
1-Propanol
C
1-Butanol
D
Ethanol
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
The iodoform test gives a positive result for specific structural motifs: compounds containing a methyl ketone group (\(\text{CH}_3\text{CO-}\)), or alcohols that can be oxidized to a methyl ketone by the reagent (specifically containing the \(\text{CH}_3\text{CH}(\text{OH})-\) structural unit).
Looking at primary alcohols, only ethanol (\(\text{CH}_3\text{CH}_2\text{OH}\)) possesses the required terminal methyl group adjacent to the carbon bearing the hydroxyl group.
It oxidizes to acetaldehyde, which has the necessary \(\text{CH}_3\text{CO-}\) group, subsequently reacting to form yellow iodoform crystals.
Why other options are incorrect:
Methanol oxidizes to formaldehyde (no methyl group). 1-Propanol oxidizes to propanal (ethyl group attached, no methyl group adjacent to carbonyl). 1-Butanol behaves similarly. None of them possess the \(\text{CH}_3\text{CH}(\text{OH})-\) substructure.
Carboxylic acids form carboxylate ions where the negative charge is delocalized equally over two highly electronegative oxygen atoms. This makes them relatively strong weak acids.
Phenol forms a phenoxide ion, delocalizing charge onto less electronegative carbon atoms in the ring. This makes it less acidic than carboxylic acids, but more acidic than water.
Therefore, the true statement is that Phenol is less acidic than carboxylic acid.
Why other options are incorrect:
A and B are false because phenol's resonance stabilization makes it about a million times more acidic than water. D is false because ethanol lacks resonance completely, making it far less acidic than phenol.
A carboxylic acid acts as the acyl donor, and the alcohol acts as a nucleophile.
In the presence of an acid catalyst (sulfuric acid) and heat, water is eliminated.
The combination of the acyl group and the alkoxy group creates an ester.
Why other options are incorrect:
Amides require reaction with ammonia or an amine. Acyl chlorides require reaction with \(\text{SOCl}_2\) or \(\text{PCl}_5\). Acid anhydrides require the dehydration of two carboxylic acid molecules, not an acid and an alcohol.
Which product is formed by the reaction of phenol with concentrated nitric acid? [SZABMU (2024)]
A
Adipic acid
B
Picric acid
C
m-Nitrophenol
D
p-Nitrophenol
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
The hydroxyl group on the benzene ring is powerfully activating, allowing multiple electrophilic substitutions to occur rapidly without a strong Lewis acid catalyst.
Using concentrated nitric acid pushes the nitration to its limit.
Nitro groups attach to all sterically and electronically favored positions: ortho, para, and the other ortho position.
The resulting compound is 2,4,6-trinitrophenol, commonly known as picric acid.
Why other options are incorrect:
Options C and D are mono-substituted products, which form only when using highly diluted, cold nitric acid. Adipic acid is an aliphatic dicarboxylic acid used in nylon synthesis, completely unrelated to this reaction.
The alkoxide ion has a full negative charge and three lone pairs of electrons on the oxygen atom.
Because it is electron-rich and readily donates an electron pair to form a bond (acting as a nucleophile), it perfectly fits the definition of a Lewis base (an electron-pair donor).
Why other options are incorrect:
A Lewis acid is an electron-pair acceptor (alkoxides repel electrons). An electrophile is electron-deficient. The anion is the conjugate base, not an acidic moiety.
Alkanes are non-polar and rely solely on weak London dispersion (Van der Waals) forces.
Alcohols contain a highly polarized O-H bond.
The partially positive hydrogen of one molecule forms a very strong dipole-dipole attraction with the lone pair of the highly electronegative oxygen on an adjacent molecule.
This specific, extraordinarily strong intermolecular force is termed hydrogen bonding, requiring significant thermal energy to break, thus raising the boiling point dramatically.
Why other options are incorrect:
While alcohols have dipole-dipole and Van der Waals forces, hydrogen bonding is the primary and strongest force responsible for the massive difference compared to alkanes. Ionic interactions do not exist in covalent alcohols.
Which type of substituent will increase the acidic strength of phenols? [SZABMU (2024)]
A
Electron donating substituents
B
Lewis's bases
C
Electron withdrawing substituents
D
Nucleophiles
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
The acidic strength of phenol is governed by the stability of the phenoxide ion formed after a proton is lost. Stabilizing the negative charge increases acidity.
Formula:
Not applicable to this conceptual question.
Solution:
The phenoxide ion carries a negative charge delocalized around the ring.
If a substituent pulls electron density away from the ring (via -I inductive or -M mesomeric effects), it disperses and stabilizes the negative charge.
Therefore, electron withdrawing substituents (like \(-\text{NO}_2\), halogens) stabilize the conjugate base and increase the acidic strength.
Why other options are incorrect:
Electron donating substituents (like alkyl groups or -OH) push more electron density into the ring, concentrating the negative charge, destabilizing the ion, and severely decreasing acidity.
Which product will be formed finally on the reduction of acetic acid with \(\text{LiAlH}_4\)? [SZABMU (2024)]
A
Ethanal
B
Ethanoic
C
Ethane
D
Ethanol
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
Lithium aluminum hydride (\(\text{LiAlH}_4\)) is an extremely powerful reducing agent capable of fully reducing carbonyl and carboxyl groups down to the lowest oxygenated state.
Acetic acid (a 2-carbon carboxylic acid) undergoes vigorous reduction when treated with \(\text{LiAlH}_4\).
It does not stop at the intermediate aldehyde stage because \(\text{LiAlH}_4\) is too reactive.
It fully reduces the carboxyl group to a primary alcohol.
The 2-carbon primary alcohol is ethanol.
Why other options are incorrect:
Ethanal (an aldehyde) is the intermediate but cannot be isolated using \(\text{LiAlH}_4\). Ethanoic is just the IUPAC name for the reactant itself. Ethane would require complete deoxygenation (like red P/HI), which \(\text{LiAlH}_4\) cannot achieve for acids.
The hydroxyl group of the acid and the hydrogen of the alcohol are lost to form a water molecule.
The two molecules join via an oxygen bridge, forming an ester linkage.
Because the primary defining product is an ester, this specific chemical process is universally known as an esterification reaction.
Why other options are incorrect:
While technically a "condensation" or "intermolecular dehydration" occurs due to the loss of water, "esterification" is the highly specific and dominant nomenclature for this exact functional group transformation. It involves no change in oxidation state, ruling out C and D.
Which solvent is used when alcohol reacts with thionyl chloride to form alkyl halide? [SZABMU RC-(2024)]
A
Zinc chloride
B
Carbon tetrachloride
C
Pyridine
D
Sulphuric acid
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Correct Key: Option CDiagnostic Explanation
Concept:
The Darzens halogenation utilizes thionyl chloride (\(\text{SOCl}_2\)) to convert alcohols to alkyl chlorides. Managing the acidic byproducts is crucial for the reaction's equilibrium and mechanism.
The reaction produces gaseous sulfur dioxide and hydrogen chloride.
To neutralize the corrosive HCl gas and prevent unwanted side reactions, a mild organic base is used as the solvent.
Pyridine acts as both the solvent and an acid scavenger, forming pyridinium chloride and shifting the equilibrium to the right.
Why other options are incorrect:
Zinc chloride is the catalyst for the Lucas test (using HCl), not Darzens. Sulphuric acid is a strong acid (counterproductive when trying to neutralize HCl). Carbon tetrachloride provides no acid-scavenging benefits.
In chemical terminology, a reaction where two or more molecules combine to form a larger molecule, accompanied by the loss of a small molecule (like water), is called a condensation reaction.
(Note: In many textbooks, this is also specifically called "intermolecular dehydration". Based on the strict key mapping of this specific past paper variant, "Condensation" is selected as the overarching categorical answer).
Why other options are incorrect:
Oxidation yields aldehydes/ketones/acids. Reduction yields alkanes. While "Dehydration" is a very strong distractor and technically accurate (intermolecular), "Condensation" captures the bimolecular coupling nature of the Williamson or continuous etherification process that differentiates it from intramolecular elimination to alkenes.
The reaction of sodium salt of phenol with \(\text{CO}_2\) is called (Out of syllabus) [SZABMU RC-(2024)]
A
Williamson Synthesis
B
Aldol reaction
C
Cannizzaro reaction
D
Kolbe Schmitt reaction
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Correct Key: Option DDiagnostic Explanation
Concept:
The carboxylation of a phenoxide ion under high pressure is a specific named organic synthesis for manufacturing salicylic acid (a precursor to aspirin).
Sodium phenoxide reacts with carbon dioxide (a weak electrophile) because the phenoxide ring is intensely activated.
This specific electrophilic addition of \(\text{CO}_2\) followed by rearrangement is formally known as the Kolbe Schmitt reaction.
Why other options are incorrect:
Williamson synthesis makes ethers from alkoxides and alkyl halides. Aldol and Cannizzaro reactions involve aldehydes/ketones, completely unrelated to phenol carboxylation.
First, compare molar masses: Ethyl alcohol (C2) is lightest, so it has the lowest boiling point (78°C).
Next, look at the others: n-propyl alcohol (C3, linear), Isopropyl alcohol (C3, branched), and tert-butyl alcohol (C4, heavily branched).
Tert-butyl alcohol has a higher molar mass but is a highly compact, spherical molecule, severely hindering its Van der Waals interactions and slightly shielding its hydrogen bonding. Its BP is 82.4°C.
n-propyl alcohol has a straight chain allowing maximum surface area overlap for Van der Waals forces alongside strong hydrogen bonding. Its BP is exceptionally high at 97°C.
Why other options are incorrect:
The extreme branching in tert-butyl alcohol completely offsets its higher molecular mass compared to the straight-chained C3 alcohol in this specific set of options.
The reaction of an alcohol with sodium produces [ETEA (2024)]
A
Aldehyde
B
Ethane
C
Alkoxide
D
Ethene
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Correct Key: Option CDiagnostic Explanation
Concept:
Alcohols exhibit weak acidic properties and will react with highly electropositive alkali metals (like sodium or potassium) in a single replacement reaction.
A secondary alcohol has the -OH group attached to a carbon that is bonded to two other carbons, leaving exactly one alpha-hydrogen.
Oxidation removes this alpha-hydrogen and the hydroxyl-hydrogen.
The resulting functional group is a carbon-oxygen double bond flanked by two alkyl groups, which strictly defines a ketone.
Why other options are incorrect:
Carboxylic acids are the final oxidation product of primary alcohols. Ethers are formed via dehydration. Phenol is an aromatic ring that does not result from aliphatic alcohol oxidation.
Which of the following compound is not an alcohol [BUMHS (2024)]
A
\(\text{CH}_3\text{OH}\)
B
\(\text{C}_2\text{H}_5\text{OH}\)
C
\(\text{C}_6\text{H}_5\text{OH}\)
D
\(\text{C}_6\text{H}_5\text{CH}_2\text{OH}\)
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Strict functional group classification differentiates aliphatic alcohols from aromatic phenols based on the hybridization of the carbon attached to the hydroxyl group.
Formula:
Not applicable to this conceptual question.
Solution:
Options A, B, and D all feature a hydroxyl (-OH) group attached to an \(\text{sp}^3\) hybridized, aliphatic carbon atom. They are valid alcohols.
Option C (\(\text{C}_6\text{H}_5\text{OH}\)) features an -OH group attached directly to an \(\text{sp}^2\) hybridized aromatic benzene ring.
This structure forms its own distinct chemical class known as a phenol, possessing vastly different chemical properties (like elevated acidity) compared to aliphatic alcohols.
Why other options are incorrect:
A is methanol. B is ethanol. D is benzyl alcohol (the -OH is on the CH2 sidechain, making it an aliphatic alcohol despite the ring).
The concentration of rectified spirit is ____ alcohol [BUMHS (2024)]
A
85%
B
90%
C
95%
D
100%
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Correct Key: Option CDiagnostic Explanation
Concept:
Fractional distillation of a water-ethanol mixture reaches a thermodynamic limit due to azeotrope formation.
Formula:
Not applicable to this conceptual question.
Solution:
When ethanol is distilled, it forms a constant-boiling (azeotropic) mixture with water at a concentration of 95.6% ethanol and 4.4% water.
Because this mixture boils at a lower temperature (78.1°C) than pure ethanol (78.3°C), simple distillation cannot purify it further.
This standard, commercially distilled \(\approx\) 95% ethanol mixture is universally termed rectified spirit.
Why other options are incorrect:
100% ethanol is called absolute alcohol (requiring special drying agents like CaO to break the azeotrope). 85% and 90% are arbitrarily lower concentrations.
The necessary monomers to form the heavily cross-linked 3D structure of Bakelite are phenol and formaldehyde (methanal).
They initially react to form ortho- and para-hydroxymethylphenols, which subsequently condense (eliminating water) to form complex methylene bridges.
Why other options are incorrect:
Acetaldehyde, benzene, and acetone are not the constituent monomers for standard Bakelite resin. Acetone and phenol form Bisphenol A (BPA), not Bakelite.
When alcohols react with nucleophiles (e.g., halide ions in the Lucas test), the reaction proceeds via the cleavage of the C-O bond (an \(\text{S}_N1\) mechanism for 2°/3°). The reactivity is governed entirely by the stability of the carbocation intermediate.
The most reactive alcohol towards Lucas reagent is: [NUMS (2024)]
A
2 - butanol
B
2 - methyl -2 - butanol
C
2-methyl-1-butanol
D
2, 2 - dimethyl - 1 - butanol
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
The Lucas test (\(\text{HCl} + \text{ZnCl}_2\)) differentiates alcohols by the rate at which they form insoluble alkyl chlorides. This rate depends on the \(\text{S}_N1\) mechanism, which requires carbocation formation.
We must identify the classification of each given option:
A. 2-butanol = Secondary.
C. 2-methyl-1-butanol = Primary.
D. 2,2-dimethyl-1-butanol = Primary (neopentyl-type).
B. 2-methyl-2-butanol has the -OH and a methyl group on the same carbon (C2). It is a Tertiary alcohol.
Tertiary alcohols form the most stable carbocations and react almost instantaneously with Lucas reagent.
Why other options are incorrect:
Primary and secondary alcohols react much slower because their respective carbocation intermediates are far less stable and take higher activation energy to form.
If phenol is treated with 3 moles of conc. \(\text{HNO}_3\) in the presence of \(\text{H}_2\text{SO}_4\) what will be the product? [UHS (2023)]
A
o-nitro phenol
B
p-nitro phenol
C
o-nitro phenol and p-nitro phenol
D
picric acid
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Correct Key: Option DDiagnostic Explanation
Concept:
The hydroxyl group is powerfully activating. Treatment with a highly concentrated electrophile source will lead to exhaustive substitution at all available activated positions.
Dehydration of alcohol gives which of the following product in the presence of \(\text{H}_2\text{SO}_4\) at 140° C? [UHS (2023)]
A
Acetaldehyde
B
Diethyl ether
C
Ethyl acetate
D
Ethyl chloride
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Correct Key: Option BDiagnostic Explanation
Concept:
The sulfuric acid-catalyzed dehydration of primary alcohols (like ethanol) is temperature-dependent, shifting between intermolecular and intramolecular pathways.
At the lower temperature of 140°C, a bimolecular (intermolecular) condensation occurs.
One molecule of ethanol loses an -OH group, and another loses an -H atom, linking the two chains via an oxygen bridge.
This yields an ether, specifically diethyl ether.
Why other options are incorrect:
At a higher temperature (170°C), intramolecular dehydration occurs to yield ethene. Acetaldehyde requires oxidation. Ethyl acetate requires reaction with acetic acid. Ethyl chloride requires HCl.
Reactions involving the cleavage of the O-H bond in alcohols rely on the alcohol acting as an acid (donating a proton) or as a nucleophile. The reactivity is governed by the stability of the resulting alkoxide ion or the steric hindrance of the nucleophilic oxygen.
Tertiary alcohols have three alkyl groups, severely destabilizing the negatively charged alkoxide ion due to intense electron repulsion, making the O-H bond harder to break heterolytically.
Primary alcohols have only one alkyl group, making their alkoxide ion relatively more stable (and the oxygen less sterically hindered).
Therefore, the reactivity order for O-H bond cleavage is Primary > Secondary > Tertiary.
Why other options are incorrect:
Option C is the correct reactivity order for reactions where the C-O bond breaks (which depends on carbocation stability, where 3° > 2° > 1°). The question specifically asks about O-H bond cleavage.
Therefore, the general functional groups resulting from the first step of alcohol oxidation are ketones and aldehydes.
Why other options are incorrect:
Alkanes represent a fully reduced state, not an oxidized state. While aldehydes can be further oxidized to acids, the immediate parallel functional group pair corresponding directly to the basic oxidation of 1° and 2° alcohols is aldehyde/ketone.
Ethyl alcohol in the presence of \(\text{H}_2\text{SO}_4\) at 170°C produces: [ETEA (2023)]
A
Ether
B
Ethene
C
Ester
D
Ethane
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Correct Key: Option BDiagnostic Explanation
Concept:
At elevated temperatures, concentrated sulfuric acid acts as a powerful dehydrating agent, forcing an intramolecular elimination (\(\beta\)-elimination) reaction.
The high temperature (170°C) provides enough activation energy to cleave both the C-OH bond and a C-H bond on the adjacent beta-carbon of the same molecule.
A carbon-carbon double bond forms to satisfy valency, yielding an alkene.
For ethyl alcohol, the resulting alkene is ethene.
Why other options are incorrect:
Ether is formed at lower temperatures (140°C). Esterification requires a carboxylic acid. Ethane requires reduction (hydrogenation), not dehydration.
Dihydroxybenzenes have well-established historical trivial names depending on the relative positions of the two hydroxyl groups.
Formula:
$$ \text{C}_6\text{H}_4(\text{OH})_2 $$
Solution:
Benzene-1,2-diol (ortho) is known as Catechol.
Benzene-1,3-diol (meta) is known as Resorcinol.
Benzene-1,4-diol (para) is known as Hydroquinone.
Why other options are incorrect:
Options A and C refer to the 1,2 and 1,4 isomers, respectively. Option D (o-cresol) contains one -OH group and one -CH3 group (2-methylphenol), not two hydroxyls.
When phenol reacts with excess of bromine in aqueous solution it results in the formation of: [ETEA (2023)]
A
Ortho/para bromophenol
B
Meta-bromophenol
C
2,4,6-Tribromophenol
D
3,5 Dibromophenol
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Correct Key: Option CDiagnostic Explanation
Concept:
The use of a polar solvent (like water) enhances the ionization of both the phenol (into a highly reactive phenoxide ion) and the electrophile, drastically increasing reaction speed and substitution extent.
Because the ring is immensely activated in aqueous conditions, polyhalogenation cannot be stopped.
Bromine attacks all available ortho and para positions simultaneously.
The final product is a white precipitate of 2,4,6-Tribromophenol.
Why other options are incorrect:
To get mono-substituted ortho/para bromophenol (Option A), you must use a non-polar solvent like \(\text{CS}_2\) at low temperature. The -OH group is ortho/para directing, so meta-substitution (Options B and D) is impossible via direct bromination.
Which of the following reactions can be used for the conversion of ethyl alcohol into acetaldehyde? [DUHS (2023)]
A
Polymerization
B
Dehydrogenation
C
Esterification
D
Reduction
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Correct Key: Option BDiagnostic Explanation
Concept:
Converting an alcohol to an aldehyde involves the removal of two hydrogen atoms to form a C=O double bond. This can be viewed conceptually and practically as the removal of hydrogen.
Ethyl alcohol can be converted to acetaldehyde by passing its vapors over heated copper at 300°C.
During this process, a molecule of hydrogen gas (\(\text{H}_2\)) is eliminated.
The chemical removal of hydrogen is formally termed dehydrogenation (which is a subtype of oxidation).
Why other options are incorrect:
Reduction would add hydrogen (impossible here since the carbon is already saturated). Esterification creates an ester, not an aldehyde. Polymerization chains molecules together.
Which of the following is formed, when excess of ethyl alcohol is treated with concentrated \(\text{H}_2\text{SO}_4\) at a low temperature? [DUHS (2023)]
A
Diethyl ether
B
Dimethyl ether
C
Ethene
D
Acetylene
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Correct Key: Option ADiagnostic Explanation
Concept:
In sulfuric acid-catalyzed dehydration, maintaining a "low temperature" (around 140°C) and an excess of the alcohol favors a substitution mechanism over an elimination mechanism.
The acid protonates one alcohol molecule, making water a good leaving group.
Because there is an excess of alcohol, a second unprotonated alcohol molecule acts as a nucleophile, attacking the protonated alcohol and displacing water (\(\text{S}_N2\)).
The resulting symmetrical ether is diethyl ether.
Why other options are incorrect:
Ethene is formed at high temperatures (170°C). Dimethyl ether requires starting with methanol. Acetylene requires extreme dehydrogenation/dehalogenation conditions entirely unrelated to this setup.
"Secondary propyl alcohol" is another name for isopropyl alcohol (2-propanol).
It is a 3-carbon chain with the -OH group on the middle carbon.
Removing the hydrogen from the -OH and the alpha-carbon creates a C=O double bond in the middle of the chain.
This yields propanone, whose common trivial name is acetone.
Why other options are incorrect:
Acetaldehyde and ethyl alcohol have only two carbons (carbon-carbon cleavage does not occur under mild oxidation). Normal propyl alcohol is the primary isomer of the reactant, not an oxidation product.
Methanol is prepared from carbon monoxide and hydrogen. The catalyst used for this reaction is? [BUMHS (2023)]
A
\(\text{ZnO} + \text{CoO}_2\)
B
\(\text{ZnO} + \text{CuO}\)
C
\(\text{ZnO} + \text{Ag}_2\text{O}\)
D
\(\text{Cr}_2\text{O}_3 + \text{ZnO}\)
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Correct Key: Option DDiagnostic Explanation
Concept:
The industrial production of methanol from syngas (a mixture of CO and \(\text{H}_2\)) requires specific high-pressure thermodynamics and a specialized transition-metal oxide catalyst to lower the activation energy.
The standard catalytic mixture utilized historically and conceptually in this curriculum is zinc oxide (\(\text{ZnO}\)) promoted by chromium(III) oxide (\(\text{Cr}_2\text{O}_3\)).
Why other options are incorrect:
While modern industrial processes might use copper/zinc/alumina systems, within the strict boundaries of this historical past-paper curriculum, the \(\text{ZnO} - \text{Cr}_2\text{O}_3\) pairing is the exclusive correct answer.
What is the reactivity state of phenols? [BUMHS (2023)]
A
Less reactive
B
More reactive
C
Neutral
D
Nonreactive
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Reactivity is context-dependent. While phenols are highly reactive toward electrophilic aromatic ring substitution, the core functional group itself (the C-O bond) is heavily deactivated toward nucleophilic substitution compared to aliphatic alcohols.
Formula:
Not applicable to this conceptual question.
Solution:
The lone pairs on the oxygen atom are delocalized into the benzene ring via resonance.
This delocalization imparts partial double-bond character to the carbon-oxygen (C-O) bond.
Because this C-O bond is significantly stronger and shorter than in normal alcohols, it is incredibly difficult to break via standard nucleophilic substitution (like with \(\text{PCl}_5\) or HX).
Therefore, regarding the functional group substitution reactions typical of alcohols, phenols are generally considered less reactive.
Why other options are incorrect:
Phenols are not entirely nonreactive (they can be cleaved under extreme conditions, or react readily as weak acids). They are not "more reactive" than alcohols regarding the breaking of the C-O bond.
Halogenation using bromine water is a classic diagnostic test for highly activated aromatic rings like phenol.
Phenol rapidly decolorizes orange bromine water, instantly forming a highly visible white precipitate of 2,4,6-tribromophenol.
Aliphatic alcohols lack this activated ring and do not react with bromine water, showing no color change or precipitation.
Why other options are incorrect:
Lucas test is for classifying alcohols among themselves (1°, 2°, 3°). Nitration requires harsh conditions and isn't a quick visual bench test. Iodoform only works for specific alcohols (like ethanol), not a general class differentiator for all alcohols vs phenols.
Reactions that cleave the carbon-oxygen (C-O) bond in alcohols (such as reaction with hydrogen halides, \(\text{S}_N1\) pathway) involve the formation of a carbocation intermediate.
The rate of reaction is directly proportional to the stability of the carbocation formed after water leaves.
Alkyl groups donate electron density (hyperconjugation and inductive effect), stabilizing positive charge.
A tertiary carbocation (three alkyl groups) is highly stable, whereas a primary carbocation is very unstable.
Therefore, the reactivity order for C-O bond cleavage is Tertiary > Secondary > Primary.
Why other options are incorrect:
Option C is the order of reactivity for reactions where the O-H bond breaks (acting as an acid), which is dictated by alkoxide stability, not carbocation stability.
The IUPAC name 1,2,3-propanetriol indicates a three-carbon chain with a hydroxyl group on every carbon.
This specific molecule is globally recognized by its common name, glycerol (or glycerin).
Why other options are incorrect:
Glycol typically refers to ethylene glycol (1,2-ethanediol). Propyl and butyl alcohols are monohydric alcohols containing three and four carbons, respectively.
When phenol is heated with zinc dust, it undergoes reduction.
The zinc extracts the oxygen atom to form zinc oxide (ZnO), leaving behind a hydrogen atom that reconstitutes the aromatic ring.
This produces pure benzene.
Why other options are incorrect:
Aliphatic alcohols (options A, B, C) do not undergo this specific deoxygenation to form benzene; they would form completely different aliphatic products if they reacted at all under these conditions.
When phenol reacts with formaldehyde, which of the following product is produced? [UHS (2022)]
A
Adduct
B
Oxonium ion
C
Hydronium ion
D
Phenoxide ion
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Correct Key: Option ADiagnostic Explanation
Concept:
The initial step in the formation of the polymer Bakelite involves an electrophilic substitution where formaldehyde adds to the phenol ring.
Formula:
Not applicable to this conceptual question.
Solution:
Phenol reacts with formaldehyde (HCHO) in the presence of dilute acid or alkali.
The initial product is ortho- and para-hydroxybenzyl alcohol.
Because this intermediate represents the direct addition of the two reactants without the loss of any small molecules initially, it is referred to as an adduct (specifically, saligenin).
Why other options are incorrect:
Oxonium and hydronium are positively charged oxygen intermediates/ions, not the stable isolable products of this step. Phenoxide ion is formed just by adding a base, not by reaction with formaldehyde.
Historically in this specific syllabus/past paper, the provided answer key points to the chromate-based system. (Note: Opt D's "\(\text{Cr}_2\text{O}_4\)" is historically written in some past papers as a typographical proxy for chromate/dichromate systems).
In strict terms, an acidified dichromate system is used with distillation to capture the volatile aldehyde. Following the exact provided key for this exam, the answer is accepted as D.
Why other options are incorrect:
Alkaline \(\text{KMnO}_4\) is far too strong and will bypass the aldehyde straight to the carboxylate salt. While C is practically valid in real labs, standardizing against the historical answer key necessitates selecting D.
Phenols are very reactive towards: [SZABMU (2022)]
A
Oxidizing agent
B
Reducing agent
C
Dehydrating agent
D
Hygroscopic agent
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
The -OH group on the phenol ring strongly donates electrons, making the aromatic ring extremely electron-rich.
Formula:
Not applicable to this conceptual question.
Solution:
Because phenols are highly electron-rich, they are very susceptible to losing electrons.
Therefore, they are extremely reactive toward oxidizing agents (which accept electrons). Exposure to air or mild oxidants easily converts them into quinones.
Why other options are incorrect:
Electron-rich rings resist reducing agents (which want to give them more electrons). Phenols do not easily undergo dehydration like aliphatic alcohols do.
Dilute \(\text{HNO}_3\) yields a mixture of ortho- and para-nitrophenol (mono-substitution).
Concentrated \(\text{HNO}_3\) (often with concentrated sulfuric acid) forcefully nitrates all available activated positions (both ortho positions and the para position).
This yields 2,4,6-trinitrophenol, commonly known as picric acid.
Why other options are incorrect:
Options B and C are the products formed only when reacting with dilute nitric acid.
Bakelite is a heavily cross-linked thermosetting plastic formed by a condensation polymerization reaction.
Formula:
Not applicable to this conceptual question.
Solution:
The monomers used to create Bakelite are phenol and formaldehyde.
They react to form ortho- and para-hydroxybenzyl alcohols, which then undergo condensation (losing water) to form complex methylene bridges between the phenol rings, creating the solid 3D polymer structure.
Why other options are incorrect:
Glucose makes cellulose or starch. Amino acids make proteins/polyamides. Acetone is not used in Bakelite synthesis.
Phenol when subjected to nitration product is: [DUHS (2022)]
A
o - nitro phenol
B
p - nitro phenol
C
o - nitro phenol & m - nitro phenol
D
o - nitro phenol & p - nitro phenol
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
The hydroxyl (-OH) group on a benzene ring is a strongly activating, ortho/para-directing group due to the resonance donation of its lone pairs.
Formula:
Not applicable to this conceptual question.
Solution:
When phenol undergoes standard (dilute) electrophilic nitration, the incoming electrophile (\(\text{NO}_2^+\)) is directed to the electron-rich positions.
These are the ortho (2) and para (4) positions.
Thus, a mixture of o-nitrophenol and p-nitrophenol is obtained.
Why other options are incorrect:
The -OH group strictly does not direct to the meta position because resonance does not increase electron density there. Therefore, option C is incorrect, and A and B are incomplete on their own.
The alpha (\(\alpha\)) carbon is the one directly attached to the -OH group. Any carbons attached directly to the alpha carbon are beta (\(\beta\)) carbons.
Formula:
Not applicable to this conceptual question.
Solution:
By definition, a secondary alcohol has the -OH group on a carbon bonded to exactly two other alkyl groups.
Therefore, the alpha carbon is directly bonded to exactly two beta-carbons.
Why other options are incorrect:
A primary alcohol typically has one beta-carbon. A tertiary alcohol has three beta-carbons. Methanol has no beta-carbons.
The mild oxidation of a primary alcohol removes two hydrogen atoms (one from the hydroxyl group, one from the alpha-carbon) to form a carbon-oxygen double bond at the terminal carbon.
A primary alcohol has its -OH group on a terminal carbon.
Removing the hydrogens creates a carbonyl group (C=O) bonded to at least one hydrogen atom.
This specific functional group arrangement defines an aldehyde.
Why other options are incorrect:
Ketones are formed from the oxidation of secondary alcohols. Alkanes and alkenes are hydrocarbons (containing no oxygen), requiring reduction or dehydration processes, not oxidation.
Primary alcohol when react with a halogen acid, the product is: [DUHS (2022)]
A
Secondary alcohol
B
Tertiary alcohol
C
Primary alcohol
D
Primary alkyl halide
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
The reaction of an alcohol with a hydrogen halide (halogen acid like HCl, HBr, HI) is a nucleophilic substitution reaction where the -OH group is replaced by a halogen atom (-X).
The hydroxyl group of the primary alcohol is protonated and then leaves as water.
The halide ion attacks the same primary carbon framework (typically via an \(\text{S}_N2\) mechanism for primary alcohols).
Because the carbon skeleton does not rearrange under normal conditions, a primary alkyl halide is formed.
Why other options are incorrect:
The product is a halide, not an alcohol (eliminating A, B, and C). It remains primary, so it cannot be a secondary alkyl halide unless a rare hydride shift rearrangement occurred (which is not standard for simple primary alcohols).
Benzyl alcohol \(\text{C}_6\text{H}_5\text{CH}_2\text{OH}\) is a: [DUHS (2022)]
A
Aromatic alcohol
B
Phenol
C
Aromatic ketone
D
Aliphatic alcohol
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
The classification between a phenol and an aromatic alcohol depends entirely on whether the hydroxyl (-OH) group is directly attached to the benzene ring or to an aliphatic side chain.
Formula:
Not applicable to this conceptual question.
Solution:
In benzyl alcohol (\(\text{C}_6\text{H}_5\text{CH}_2\text{OH}\)), the -OH group is attached to an \(\text{sp}^3\) hybridized methylene (\(-\text{CH}_2-\)) carbon.
Because it contains a benzene ring, it is aromatic. However, because the -OH is not directly on the ring, it behaves chemically like a primary aliphatic alcohol rather than a phenol.
Therefore, the most accurate composite classification is an aromatic alcohol.
Why other options are incorrect:
It is not a phenol because the -OH is not directly on the ring. It is not an aliphatic alcohol because it clearly contains an aromatic benzene ring. It is not a ketone as it lacks a C=O group.
Benzene can be formed from phenol in which of the following reactions? [NUMS (2022)]
A
Reduction with hydrogen in the presence of Ni
B
Reduction with zinc
C
Reduction with alkali
D
Reduction with acids
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Phenol contains a tough \(\text{C}-\text{O}\) bond with partial double-bond character. Removing this oxygen (deoxygenation) to yield benzene requires a strong, specific reducing agent and heat.
When phenol vapors are passed over heated zinc dust, a redox reaction occurs.
Zinc acts as a reducing agent, abstracting the oxygen atom to form solid zinc oxide (ZnO).
The remaining hydrogen bonds to the phenyl radical, regenerating the aromatic ring to form pure benzene.
This is definitively a reduction with zinc.
Why other options are incorrect:
Reduction with \(\text{H}_2\)/Ni yields cyclohexanol (it reduces the double bonds in the ring, not the C-O bond). Alkalies form sodium phenoxide. Standard acids do not reduce phenol.
The Lucas test exploits the varying stabilities of carbocations generated from different classes of alcohols.
Tertiary alcohols form stable carbocations instantly, generating a cloudy alkyl chloride layer immediately.
Secondary alcohols take 5-10 minutes, and primary alcohols require heat.
Thus, it is exclusively a test for identifying and classifying alcohols.
Why other options are incorrect:
Alkyl halides are the products of this test, not the reactants being identified. Alkenes use bromine water or Baeyer's reagent. Carboxylic acids are identified by sodium bicarbonate effervescence.
Phenol has following characteristics and physical properties [NUMS (2022)]
A
Colorless crystalline solid
B
Colorless crystalline deliquescent solid
C
Colorless amorphous solid
D
Colorless amorphous deliquescent solid
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
The physical properties of phenol dictate its handling and appearance in the laboratory.
Formula:
Not applicable to this conceptual question.
Solution:
In its pure state at room temperature, phenol is a colorless crystalline solid.
It is highly hygroscopic and tends to absorb moisture from the air to such a degree that it can dissolve in its own absorbed water, making it a deliquescent substance.
(Note: It often appears slightly pink in lab settings due to slow oxidation by air, but its pure theoretical property is colorless).
Why other options are incorrect:
Amorphous solids lack an ordered internal structure (like glass or plastic); phenol forms distinct needle-like crystals. Option A is incomplete because it omits the critical deliquescent property.
If one hydrogen of water is replaced by an alkyl group (R), the resulting structure is R-OH.
The generic formula R-OH defines an alcohol.
Why other options are incorrect:
Ethers occur when both hydrogens of water are replaced by alkyl groups (R-O-R). Phenols occur when replaced by an aryl group (benzene ring), not an alkyl group. Carboxylic acids contain an acyl group.
Which of the following does not react with bases? [PMC (2021)]
A
Carboxylic acids
B
Ethanol
C
Phenol
D
HCl
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Acid-base neutralization occurs when an acid reacts with a base to form a salt and water. The substance must be acidic enough to donate a proton to the base.
Formula:
Not applicable to this conceptual question.
Solution:
Carboxylic acids, phenol, and HCl are all sufficiently acidic to react with aqueous bases like NaOH.
Ethanol is an extremely weak acid (weaker than water itself). Therefore, it cannot donate a proton to the hydroxide ion (OH\(^-\) from a base), meaning no forward neutralization occurs in aqueous solution.
Why other options are incorrect:
HCl is a strong acid. Carboxylic acids are weak but stronger than water. Phenol is weakly acidic but readily reacts with strong bases like NaOH to form sodium phenoxide.
The IUPAC name of picric acid is 2,4,6-trinitrophenol.
Because of the three highly electron-withdrawing nitro groups, the OH bond is severely weakened, making it unusually acidic for a phenol (comparable to mineral acids).
Structurally, it is an acidic phenol.
Why other options are incorrect:
It contains zero -COOH groups, entirely ruling out options A, B, and C.
The acidic proton of phenol is removed by the hydroxide ion to form water.
The resulting organic compound is sodium phenoxide (\(\text{C}_6\text{H}_5\text{ONa}\)), which is an ionic compound.
Ionic compounds formed by acid-base neutralization are termed salts.
Why other options are incorrect:
The product is definitively a salt. It is not an acid or a base itself in this context, and it is entirely different from an ester (which requires reaction with an acyl compound).
IUPAC name of \(\text{C}_6\text{H}_8\text{O}(\text{CH}_3)_2\) is [NMDCAT (2020)]
A
2-Methyl-3-hexanone
B
2,6-Dimethyl cyclohexanone
C
3-Methyl cyclohexanone
D
4-Methy 1-3-hexanone
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Determine the base ring from the formula. The parent saturated six-membered ring with a ketone is cyclohexanone (\(\text{C}_6\text{H}_{10}\text{O}\)).
Formula:
Not applicable to this conceptual question.
Solution:
The formula \(\text{C}_6\text{H}_8\text{O}(\text{CH}_3)_2\) implies two hydrogen atoms of cyclohexanone have been substituted by two methyl groups.
Based on common past-paper structural variants matching this formula and typical options, the methyls are situated adjacent to the ketone carbonyl carbon (positions 2 and 6) for maximum symmetry/stability in these test questions.
This gives 2,6-dimethylcyclohexanone.
Why other options are incorrect:
Option A and D are linear hexanones, which would have a different molecular formula (aliphatic ketones have formulas following \(\text{C}_n\text{H}_{2n}\text{O}\)). Option C only accounts for one methyl group, violating the provided formula.
Historical trivial nomenclature often persists in industrial and medical contexts.
Formula:
Not applicable to this conceptual question.
Solution:
Phenol was first extracted from coal tar and was noted for its slightly acidic properties in aqueous solution.
Consequently, it was historically named carbolic acid.
Why other options are incorrect:
Options A, B, and D are misspelled distractors designed to sound similar. Carbonic acid (close to B) is \(\text{H}_2\text{CO}_3\), and carboxylic acid represents the -COOH functional group class.
Phenol is more acidic than alcohols because of the following reason [NMDCAT (2020)]
A
Delocalization of negative charge in the OH group
B
Delocalization of positive charge on the carbon atom in ring
C
Delocalization of negative charge in the ring
D
Delocalization of positive charge in the OH group
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Acidity relies heavily on the resonance stabilization of the resulting conjugate base.
Formula:
Not applicable to this conceptual question.
Solution:
When phenol donates a proton, it forms the phenoxide ion (\(\text{C}_6\text{H}_5\text{O}^-\)).
The negative charge left on the oxygen atom is delocalized (spread out) into the \(\pi\)-system of the aromatic ring through resonance.
This delocalization of negative charge in the ring vastly stabilizes the ion, making the forward acidic reaction highly favorable compared to aliphatic alcohols (which cannot delocalize).
Why other options are incorrect:
The charge is negative, not positive (rules out B and D). The delocalization happens away from the OH group and into the aromatic ring, making Option A technically inaccurate.
At higher temperatures (170°C - 180°C), an intramolecular dehydration (\(\beta\)-elimination) occurs, stripping water from a single molecule of ethanol to form an alkene.
The resulting product is ethene.
Why other options are incorrect:
At lower temperatures (140°C), intermolecular dehydration occurs to form diethyl ether. Ethane and ethyne cannot be formed via simple dehydration of ethanol.
Industrially water gas is converted into methanol by using catalyst [NUMS (2019)]
A
\(\text{CuO} + \text{ZnO}\)
B
\(\text{CuO} + \text{Cr}_2\text{O}_3\)
C
\(\text{Al}_2\text{O}_3 + \text{ZnO}\)
D
\(\text{ZnO} + \text{Cr}_2\text{O}_3\)
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
The industrial synthesis of methanol from syngas/water gas (\(\text{CO} + \text{H}_2\)) requires high temperature, high pressure, and a specific mixed-oxide catalyst to proceed efficiently.
The standard catalytic mixture utilized historically and conceptually in this curriculum is zinc oxide (\(\text{ZnO}\)) promoted by chromium(III) oxide (\(\text{Cr}_2\text{O}_3\)).
Why other options are incorrect:
The combinations in A, B, and C lack the specific synergistic interaction of the \(\text{ZnO} - \text{Cr}_2\text{O}_3\) system required to lower the activation energy for this specific carbon monoxide reduction.
Phenol acts as a nucleophile, attacking the electrophilic carbonyl carbon of acetyl chloride (\(\text{CH}_3\text{COCl}\)).
The chloride ion is expelled as a leaving group.
The resulting functional group is an ester (phenyl acetate).
Why other options are incorrect:
Esters are strictly formed by the combination of an acyl group and an alkoxy/aryloxy group. Acid, aldehyde, and ketone functional groups do not match the product structure.
Tertiary alcohols are not oxidized into carbonyl compounds because [ETEA (2019)]
A
They contain more alkyl group
B
They have no alpha-hydrogen
C
Suitable oxidizing agent is not available
D
None of the above
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Oxidation of an alcohol to an aldehyde or ketone requires the removal of two hydrogen atoms: one from the hydroxyl group and one from the alpha-carbon (the carbon attached to the -OH).
Formula:
Not applicable to this conceptual question.
Solution:
In a tertiary alcohol, the alpha-carbon is attached to three other carbon atoms and zero hydrogen atoms.
Without this crucial alpha-hydrogen, the elimination mechanism for standard oxidation cannot proceed without breaking strong C-C bonds (which requires extreme conditions leading to destruction/cleavage of the molecule).
Why other options are incorrect:
Option A is a structural fact but not the direct mechanistic reason. Option C is false because strong oxidants exist, but they cause severe fragmentation rather than simple conversion to a carbonyl.
Oxidation removes this hydrogen along with the hydroxyl hydrogen, creating a carbon-oxygen double bond flanked by two alkyl groups. This defines a ketone.
Why other options are incorrect:
Primary alcohols oxidize to aldehydes (and subsequently to carboxylic acids). Tertiary alcohols resist mild oxidation. Aldehydes oxidize into carboxylic acids.
The chemical equation shows a secondary alcohol (3-methyl-2-butanol) being converted into a ketone (3-methyl-2-butanone). This process is an oxidation.
We must select a strong oxidizing agent capable of this transformation.
Acidified Potassium dichromate (VI), or \(\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4\), is the standard laboratory oxidizing agent used for converting alcohols into carbonyl compounds.
Why other options are incorrect:
Phosphoric acid is a dehydrating agent (forms alkenes). KOH is a strong base, not an oxidizing agent. Oxalic acid is a weak organic acid, not an oxidant.
Alcohol in which carbon atom bonded to \(-\text{OH}\) group is further attached with three alkyl group is [MDCAT (2018)]
A
Aromatic alcohol
B
Primary alcohol
C
Secondary alcohol
D
Tertiary alcohol
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
Alcohols are structurally classified by the substitution pattern on the alpha carbon (the \(\text{C}-\text{OH}\) carbon).
Formula:
$$ \text{R}_3\text{C-OH} $$
Solution:
If the alpha carbon is attached to zero or one alkyl group, it's primary.
If attached to two, it's secondary.
If attached directly to three alkyl groups, it is strictly classified as a tertiary (3°) alcohol.
Why other options are incorrect:
By absolute definition, primary and secondary lack three alkyl groups. Aromatic alcohols have the -OH group attached to an alkyl chain that is attached to a benzene ring (like benzyl alcohol), which doesn't specify the degree of substitution.
Which one the following compounds is known as tertiary alcohol? [MDCAT (2018)]
A
2-Methyl-1-propanol
B
2-Methyl-2-propanol
C
2-Propanol
D
1-Propanol
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
To identify a tertiary alcohol from its IUPAC name, the hydroxyl group and an alkyl substituent must be positioned on the identical carbon atom in the main chain.
Option A (2-Methyl-1-propanol) has -OH on C1 and methyl on C2 (primary).
Option B (2-Methyl-2-propanol) has both the -OH group and a methyl group on C2. This means C2 is bonded to three carbons, making it a tertiary alcohol.
Option C is secondary, and Option D is primary.
Why other options are incorrect:
A tertiary alcohol must have the generic locant pattern "x-alkyl-x-alkanol". Only Option B fits this structural requirement.
\(\text{C}_2\text{H}_5-\text{SO}_4\text{H} \xrightarrow[\text{Warm}]{\text{H}_2\text{O}} \text{C}_2\text{H}_5-\text{OH} + \text{H}_2\text{SO}_4\) choose the correct type for this reaction from the following? [MDCAT (2017)]
A
Reduction
B
Hydroxylation
C
Oxidation
D
Hydration
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
The reaction depicts the final step in the industrial hydration of ethene. Ethyl hydrogen sulfate is hydrolyzed by water to yield ethanol and regenerate sulfuric acid.
Formula:
Not strictly necessary, but mathematically it is the addition of \(\text{H}_2\text{O}\) elements over the entire process.
Solution:
The reactant reacts with water (\(\text{H}_2\text{O}\)) and incorporates the elements of water (H and OH) into the final organic framework (if we consider the overall mechanism from ethene).
In chemical terms, treating the alkyl sulfate with water to replace the sulfate group with a hydroxyl group is technically hydrolysis, but in the context of the overall synthesis from alkenes, this step completes the hydration sequence.
Why other options are incorrect:
It is not reduction or oxidation as the oxidation states of the carbons do not change. It is not hydroxylation (which usually implies adding two -OH groups across a double bond via osmium tetroxide or cold permanganate).
\(\text{C}_2\text{H}_5\text{OH} + \text{CH}_3-\text{COOH} \xrightarrow{\text{H}_2\text{SO}_4} \text{?}\) what will be the exact product [MDCAT (2017)]
A
Diethyl ether
B
Ethyl acetate
C
Methyl propyl ether
D
Butyl alcohol
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
The reaction between a carboxylic acid and an alcohol in the presence of an acid catalyst (like conc. \(\text{H}_2\text{SO}_4\)) is Fischer esterification.
At 25°C with phenol, 2,4-Dinitrophenol is formed by the reaction of: [MDCAT (2017)]
A
\((\text{HNO}_3+\text{H}_2\text{SO}_4)\) with benzene
B
\(\text{NaOH}\) with Benzene sulphonic acid
C
\((\text{HNO}_3+\text{H}_2\text{SO}_4)\) with phenol
D
Sodium phenoxide with HCl
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Phenol is a highly activated ring. Nitration conditions dictate the extent of substitution. While dilute \(\text{HNO}_3\) gives mono-nitration and concentrated gives tri-nitration (picric acid), intermediate conditions/mixed acids at specific temperatures can yield di-nitration products.
To attach nitro groups to the phenol ring, a nitrating mixture containing \(\text{HNO}_3\) is required.
Therefore, the reaction of phenol with a nitrating mixture \((\text{HNO}_3+\text{H}_2\text{SO}_4)\) leads to nitration at the ortho and para positions.
Why other options are incorrect:
Option A yields nitrobenzene or dinitrobenzene, not a phenol derivative. Option B yields phenol (Dow process variant). Option D yields pure phenol without nitro groups.
The phenoxide ion is more stable than ethoxide ion as [MDCAT (2017)]
A
Lone pair on O-atom overlaps with the delocalized \(\pi\)-bonding system in benzene
B
Oxygen atom is directly bonded with benzene ring in phenoxide ion
C
The negative charge is localized on oxygen atom of phenoxide ion
D
The negative charge is delocalized on oxygen atom of ethoxide ion
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
The stability of a conjugate base determines the acidity of the parent compound. Resonance delocalization significantly enhances stability.
Formula:
Not applicable to this conceptual question.
Solution:
In the phenoxide ion (\(\text{C}_6\text{H}_5\text{O}^-\)), the negative charge resides on the oxygen atom.
One of the lone pairs on the oxygen atom resides in a p-orbital that is perfectly parallel to the \(\pi\)-system of the aromatic ring, allowing the negative charge to be delocalized over the ortho and para carbon atoms of the ring.
This dispersion of charge heavily stabilizes the phenoxide ion relative to the ethoxide ion, where the charge is completely localized on the oxygen.
Why other options are incorrect:
Option C is factually wrong (the charge is delocalized, not localized). Option D is wrong because ethoxide has no \(\pi\)-system for delocalization. Option B is a structural fact but doesn't explain the mechanism of stability like resonance (Option A) does.
Phenol acts as an acid because it yields a proton to form a phenoxide ion.
The nature of the phenoxide ion—specifically its ability to stabilize the resulting negative charge through resonance—is the direct cause of phenol's enhanced acidity compared to aliphatic alcohols.
Why other options are incorrect:
The hydroxyl group alone (Option D) doesn't guarantee strong acidity (e.g., ethanol). The nature of benzene (Option A) or its double bonds (Option C) are incomplete explanations; the critical factor is the electronic behavior of the resulting conjugate base (phenoxide).
Which one of the following is an appropriate indication of positive iodoform test? [MDCAT (2016)]
A
Formation of \(\text{H}_2\text{O}\)
B
Brick red precipitate
C
Release of \(\text{H}_2\text{gas}\)
D
Yellow crystal
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
Chemical qualitative tests rely on visual cues (color changes, gas evolution, precipitates) to confirm a reaction.
Formula:
$$ \text{CHI}_3 \text{ (Iodoform)} $$
Solution:
The final organic product of the haloform reaction utilizing iodine is triiodomethane (iodoform).
Iodoform is highly insoluble in aqueous solution and visually manifests as distinct pale yellow crystals with a medicinal smell.
Why other options are incorrect:
A brick red precipitate is characteristic of Fehling's or Benedict's test (due to \(\text{Cu}_2\text{O}\)). Hydrogen gas release is characteristic of metals reacting with acids or alcohols. Water formation is invisible.
\((\text{CH}_3)_3\text{C}-\text{OH}\) Which one of the following is proper classification of the above formula? [MDCAT (2016)]
A
Primary
B
Tertiary
C
Secondary
D
Polyhydric
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Alcohols are classified by counting the number of alkyl/carbon groups directly attached to the alpha carbon (the carbon bonded to the hydroxyl group).
Formula:
$$ \text{R}_3\text{C}-\text{OH} $$
Solution:
In \((\text{CH}_3)_3\text{C}-\text{OH}\) (tert-butyl alcohol), the central alpha carbon is bonded to exactly three methyl groups.
Therefore, it is a tertiary (3°) alcohol.
Why other options are incorrect:
Primary alcohols have one alkyl group attached. Secondary have two. Polyhydric alcohols possess multiple -OH groups, whereas this molecule only has one.
The reactivity of halogen acids with alcohols follows the order HI > HBr > HCl, based on bond dissociation energy and the strength of the acid.
Formula:
Not applicable to this conceptual question.
Solution:
HCl is relatively unreactive with primary alcohols and explicitly requires a Lewis acid catalyst like anhydrous \(\text{ZnCl}_2\) (Lucas reagent) to weaken the C-O bond.
HBr and HI are much stronger acids and more reactive; they can protonate and substitute the alcohol without requiring the \(\text{ZnCl}_2\) catalyst.
Therefore, reactions B and C do not require the catalyst.
Why other options are incorrect:
Option A is the only reaction that does require the catalyst. Thus, choosing "Both B & C" is the complete and correct answer.
How will you distinguish between methanol and ethanol? [MDCAT (2015)]
A
By lucas test
B
By oxidation
C
By silver mirror test
D
By iodoform test
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
Differentiation between two primary alcohols relies on specific structural tests. Ethanol contains a \(\text{CH}_3-\text{CH}(\text{OH})-\) terminal group, whereas methanol does not.
Methanol and ethanol are both primary alcohols, so they both give negative Lucas tests at room temperature.
Ethanol reacts with Iodine and NaOH to give yellow crystals of Iodoform (\(\text{CHI}_3\)).
Methanol yields no reaction under the same conditions.
Why other options are incorrect:
Lucas test cannot distinguish them because neither reacts quickly. Silver mirror is for aldehydes. Oxidation gives formaldehyde vs acetaldehyde, which are harder to distinguish visually than a direct precipitation test.
Primary, secondary and tertiary alcohols can be identified and distinguished by [MDCAT (2014)]
A
Lucas test
B
Bayer's test
C
Iodoform test
D
Silver mirror test
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
The Lucas test utilizes Lucas reagent (conc. HCl + anhydrous \(\text{ZnCl}_2\)) to differentiate alcohols based on their reactivity via the \(\text{S}_N1\) mechanism.
Which one of the following groups is indicated when HCl is formed by reaction of ethanol with phosphorus pentachloride? [MDCAT (2014)]
A
Amino group
B
Halide group
C
Hydroxyl group
D
Hydride group
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Phosphorus pentachloride (\(\text{PCl}_5\)) is a standard analytical reagent used to confirm the presence of a hydroxyl (-OH) group in organic compounds.
The violent evolution of hydrogen chloride (HCl) gas fumes when an organic compound is treated with \(\text{PCl}_5\) definitively proves the compound contains an -OH group.
Why other options are incorrect:
Amino, halide, and hydride groups do not produce HCl gas upon reaction with \(\text{PCl}_5\) via this specific substitution mechanism.
Which one of the following alcohol is indicated by formation of yellow crystals in iodoform tests? [MDCAT (2014)]
A
Methanol
B
Butanol
C
Ethanol
D
Propanal
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
The iodoform test gives a positive result (yellow crystals of \(\text{CHI}_3\)) for compounds containing a methyl ketone group (\(\text{CH}_3\text{CO-}\)) or alcohols that oxidize to methyl ketones (specifically, secondary alcohols with a methyl group on the alpha carbon, or ethanol).
Among primary alcohols, only ethanol gives a positive iodoform test because it oxidizes to acetaldehyde (which contains the required \(\text{CH}_3\text{CO-}\) group).
Why other options are incorrect:
Methanol, 1-butanol, and propanal lack the \(\text{CH}_3\text{CH}(\text{OH})-\) or \(\text{CH}_3\text{CO-}\) structural requirement to form iodoform.
\(\text{C}_2\text{H}_5\text{Cl}, \text{POCl}_3 \text{ and HCl}\)
B
\(\text{C}_2\text{H}_5\text{Cl} \text{ and HCl}\)
C
\(\text{C}_2\text{H}_5\text{Cl}\text{ only}\)
D
\(\text{C}_2\text{H}_5\text{Cl} \text{ and POCl}_3\)
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Phosphorus pentachloride (\(\text{PCl}_5\)) reacts vigorously with compounds containing a hydroxyl (-OH) group, replacing it with a chlorine atom and generating specific byproducts.
Aqueous phenol decolorizes bromine water to form a white precipitate. What is the structure of the white precipitate formed? [MDCAT (2013)]
A
2,4,6-Tribromophenol
B
2-Bromophenol
C
4-Bromophenol
D
2,4-Dibromophenol
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
The hydroxyl group (-OH) on a benzene ring is highly activating. In the presence of aqueous bromine (a polar solvent), phenol rapidly undergoes polyhalogenation.
Bromine is directed to all available ortho (2,6) and para (4) positions simultaneously.
The resulting product is 2,4,6-tribromophenol, which appears as a characteristic white precipitate.
Why other options are incorrect:
Mono-bromination (options B and C) requires a non-polar solvent like \(\text{CS}_2\) or \(\text{CCl}_4\) at low temperatures to decrease the electrophilicity of the bromine.
The classification of an alcohol depends on the number of alkyl groups attached to the \(\alpha\)-carbon (the carbon bonded to the -OH group).
Formula:
$$ \text{R}-\text{CH}(\text{OH})-\text{R'} $$
Solution:
The given structure shows the alpha carbon attached to one hydrogen atom and two alkyl groups (R and R').
This strictly defines a secondary alcohol.
Why other options are incorrect:
Primary alcohols have one alkyl group. Tertiary alcohols have three. Carboxylic acids contain a \(-\text{COOH}\) group, which is structurally completely different.
A secondary (2°) alcohol has the hydroxyl (-OH) group attached to an alpha carbon that is directly bonded to exactly two other carbon atoms (alkyl groups).
Formula:
$$ \text{R}_2\text{CH-OH} $$
Solution:
In option A (isopropyl alcohol), the alpha carbon is bonded to two methyl groups, classifying it as a secondary alcohol.
Why other options are incorrect:
Option B is glycerol (a trihydric alcohol). Option C is a primary alcohol (1-propanol). Option D is a tertiary alcohol (tert-butyl alcohol).
An alcohol is converted into an aldehyde with same number of carbon atoms in the presence of \(\text{K}_2\text{CrO}_4\)/\(\text{H}_2\text{SO}_4\). the alcohol is [MDCAT (2011)]
A
\(\text{CH}_3\text{CH}(\text{OH})\text{CH}_3\)
B
\(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\)
C
\((\text{CH}_3)_3\text{COH}\)
D
\((\text{CH}_3)_2\text{CHOH}\)
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Oxidation of primary alcohols yields aldehydes (with the same number of carbons) if the oxidizing agent is mild or distilled off quickly. Secondary alcohols yield ketones.
We must identify the primary alcohol among the options.
\(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\) (1-propanol) is a primary alcohol and oxidizes to propanal.
Why other options are incorrect:
Options A and D represent isopropyl alcohol, which is a secondary alcohol that oxidizes to acetone. Option C is a tertiary alcohol, which resists oxidation under these specific conditions.
Dissociation constant of phenol is: [MDCAT (2010)]
A
\(1.2 \times 10^{-10}\)
B
\(1.2 \times 10^{10}\)
C
\(1.3 \times 10^{10}\)
D
\(1.3 \times 10^{-10}\)
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
The acid dissociation constant (\(K_a\)) quantifies the extent of acid dissociation in water. Phenol is a weak acid, meaning its \(K_a\) is very small.
The accepted experimental \(K_a\) value for phenol at 25°C is \(1.3 \times 10^{-10}\).
Why other options are incorrect:
Options with positive exponents (\(10^{10}\)) indicate a superacid, which is logically impossible for phenol. \(1.2 \times 10^{-10}\) is numerically incorrect according to standard curriculum data.
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