Concept:Boiling point in organic molecules depends on molecular weight (London dispersion forces), hydrogen bonding, and surface area (branching). Branching decreases surface area, significantly lowering boiling point.
Formula:$$ \text{Boiling Point} \propto \text{Molar Mass} \propto \frac{1}{\text{Branching}} $$
Solution:- 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.
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