Chemistry 7 Solved Past Papers 2023 to 2024 Archives

Alkyl Halides Past Papers

Solved past paper MCQs for Alkyl Halides from official UHS, NUMS, SZABMU, DUHS, and KMU examinations. Includes verified distractor autopsies and step-by-step cognitive explanations.

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Official printable A4 booklet containing all 7 verified past paper MCQs with unbroken options, 5-column rapid answer key, and Propolis cognitive error autopsies.

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#1 of 7 UHS 2024

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Identify correct ascending order of reactivity of alkyl halides:

Cl, Br, I, F

Br, I, F, Cl

F, Cl, Br, I

I, F, Cl, Br

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Correct Key: Option C Diagnostic Explanation

Concept:

The reactivity of an alkyl halide primarily depends on the bond dissociation energy of the carbon-halogen (C-X) bond.

Solution:

"Ascending order" means from least reactive to most reactive.
Reactivity is inversely proportional to bond strength. A stronger bond is harder to break, making the molecule less reactive.
As halogen size increases down Group 17, the C-X bond length increases, and the bond energy decreases.
Bond Energy Order: C-F > C-Cl > C-Br > C-I.
Therefore, the Reactivity Order is: R-F < R-Cl < R-Br < R-I.

Why other options are incorrect:

Options A, B, & D: Do not correctly reflect the inverse relationship between atomic radius/bond length and bond dissociation energy.

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#2 of 7 UHS 2024

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Alkyl halides involving -C-X bond breakage and -C-Nu bond formation simultaneously would follow which one of the following mechanisms:

$ S_N1 $

$ E_1 $

$ S_N2 $

$ E_2 $

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Correct Key: Option C Diagnostic Explanation

Concept:

A concerted mechanism occurs when all bond-breaking and bond-forming steps happen in a single coordinated motion without forming an intermediate.

Solution:

The term "simultaneously" indicates a one-step reaction passing through a single transition state.
Because it specifies the formation of a Carbon-Nucleophile (-C-Nu) bond, this is a substitution reaction, not an elimination.
The single-step, concerted substitution mechanism is $ S_N2 $ (Substitution Nucleophilic Bimolecular).

Why other options are incorrect:

Option A: $ S_N1 $ is step-wise (breakage first, formation second).
Options B & D: $ E_1 $ and $ E_2 $ are elimination reactions that form pi bonds, not C-Nu substitution bonds.

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#3 of 7 SZABMU 2024

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Unimolecular nucleophilic substitution reaction involves ____.

1st order kinetics

3rd order kinetics

2nd order kinetics

zero order kinetics

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Correct Key: Option A Diagnostic Explanation

Concept:

The molecularity of a reaction corresponds to the number of species involved in the rate-determining step.

Solution:

"Unimolecular nucleophilic substitution" is the formal name for the $ S_N1 $ mechanism.
In this mechanism, the rate-determining (slowest) step is the spontaneous ionization of the alkyl halide to form a carbocation.
The nucleophile is not involved in this slow step.
Because the rate depends strictly on the concentration of only one reactant (the alkyl halide), the reaction follows 1st order kinetics.

$$ \text{Rate} = k [\text{Alkyl Halide}]^1 $$

Why other options are incorrect:

Option C: 2nd order kinetics apply to bimolecular reactions ($ S_N2 $ and $ E_2 $), where both the substrate and nucleophile/base are in the rate equation.

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#4 of 7 ETEA 2024

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Dehydrohalogenation of alkyl halide is carried out in presence of:

Alcoholic KOH

Conc. H$_2$SO$_4$

Aqueous KOH

Zn dust

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Correct Key: Option A Diagnostic Explanation

Concept:

Dehydrohalogenation is the elimination of a hydrogen atom and a halogen atom from an alkyl halide to form an alkene. This requires a strong base.

Solution:

Potassium hydroxide dissolved in an alcohol (like ethanol) generates alkoxide ions (e.g., ethoxide, $ C_2H_5O^- $).
Alkoxide ions are extremely strong bases. Because they are not strongly hydrated by the alcoholic solvent, their basicity dominates over their nucleophilicity.
They successfully abstract a $ \beta $-hydrogen, triggering the elimination of the halogen to form a double bond.

Why other options are incorrect:

Option B: Concentrated sulfuric acid is a dehydrating agent used to remove water from alcohols, not hydrohalic acids from alkyl halides.
Option C: Aqueous KOH acts as a nucleophile, leading to substitution (forming an alcohol) instead of elimination.
Option D: Zinc dust is used for dehalogenation (removing two halogens from vicinal dihalides), not dehydrohalogenation.

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#5 of 7 DUHS 2024

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The rate of $ S_N1 $ mechanism depends upon:

Concentration of nucleophile

Concentration of both substrate and nucleophile

Polar non-polar solvent

Concentration of substrate only

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Correct Key: Option D Diagnostic Explanation

Concept:

The kinetics of a multi-step reaction are determined solely by the slowest step in the sequence (the rate-determining step).

Solution:

The $ S_N1 $ reaction occurs in two steps.
The first step is the slow cleavage of the carbon-halogen bond to form a carbocation. The only molecule involved in this step is the alkyl halide (the substrate).
Because the nucleophile only attacks in the fast second step, its concentration has zero effect on the overall speed of the reaction.
Therefore, the rate law is: $ \text{Rate} = k [\text{Substrate}] $. It depends on the concentration of the substrate only.

Why other options are incorrect:

Options A & B: The nucleophile concentration dictates the rate of $ S_N2 $ reactions, not $ S_N1 $.

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#6 of 7 BUMHS 2024

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Alkyl halides IUPAC name of the following compound is:

$ CH_3-CH_2-C(Cl)(CH_3)-CH_2-CH_3 $

Hexylchloride

3-Chloromethylhexane

3-Chloro-3-ethylbutane

3-Chloro-3-methylpentane

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Correct Key: Option D Diagnostic Explanation

Concept:

IUPAC naming requires identifying the longest continuous carbon chain, numbering it to give the lowest locants to substituents, and listing substituents alphabetically.

Solution:

Step 1: Identify the longest chain.
The longest continuous horizontal chain consists of 5 carbon atoms: $ CH_3-CH_2-C-CH_2-CH_3 $. A 5-carbon chain is a pentane.
Step 2: Number the chain.
Numbering from either the left or the right places the central carbon at position 3.
Step 3: Identify substituents.
On Carbon 3, there is a chlorine atom (chloro) and a methyl group (methyl).
Step 4: Assemble alphabetically.
"Chloro" comes before "methyl". Both are at position 3.
Therefore, the full IUPAC name is 3-Chloro-3-methylpentane.

Why other options are incorrect:

Option C: Identifying the chain as "butane" means a 4-carbon chain was incorrectly selected instead of the longest 5-carbon chain.
Option B: Hexane implies a 6-carbon continuous chain, which does not exist in this molecule.

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#7 of 7 BUMHS 2023

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Which of the following statement is true about Grignard reagent (RMgX)?

RMgX is prepared in aqueous medium

RMgX is prepared in ether

RMgX does not react with water

RMgX inactive in ethanol

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Correct Key: Option B Diagnostic Explanation

Concept:

Grignard reagents ($ RMgX $) contain a highly nucleophilic and strongly basic carbanion-like carbon atom.

Solution:

Because they are incredibly strong bases, they will immediately react with any source of acidic protons (like water or ethanol) in a violent acid-base reaction, destroying the reagent to form an alkane.
To prevent this, they must be synthesized and stored in completely anhydrous (moisture-free) aprotic solvents.
Dry diethyl ether is the standard solvent because it lacks acidic protons and its oxygen lone pairs help stabilize the magnesium atom via coordination.

Why other options are incorrect:

Options A, C, & D: Aqueous mediums and ethanol provide acidic protons that will instantly quench (destroy) the Grignard reagent, forming an alkane. Therefore, it is highly reactive with water and ethanol, not inactive.

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Alkyl Halides Past Papers

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