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Question #45091

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The Question

Consider the first-order solvolysis of the following tertiary alkyl chlorides in 80% aqueous ethanol at 298 K: (I) 2-chloro-2-methylpropane (II) 3-chloro-3-ethylpentane (III) 3-chloro-3-isopropyl-2,4-dimethylpentane Let $E_{a(I)}$ , $E_{a(II)}$ , and $E_{a(III)}$ be their respective activation energies for the rate-determining heterolytic \ce{C-Cl} bond cleavage. Which of the following correctly represents the order of activation energies and identifies the dominant factor dictating this trend?

Options

A.
$E_{a(III)} > E_{a(II)} > E_{a(I)}$ ; Dominant factor: Hyperconjugative stabilization of the carbocation intermediate.
B.
$E_{a(I)} > E_{a(II)} > E_{a(III)}$ ; Dominant factor: Relief of ground-state steric strain (B-strain) in the transition state.
C.
$E_{a(I)} > E_{a(III)} > E_{a(II)}$ ; Dominant factor: Inductive electron donation (+I effect) by the alkyl groups.
D.
$E_{a(III)} > E_{a(I)} > E_{a(II)}$ ; Dominant factor: Steric hindrance to solvation of the transition state.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Bond Cleavage Energetics|Reaction Intermediates Stability|Steric Hindrance (B-Strain)

Why This Question Is Tricky

Common Mistakes

Students typically equate the activation energy of heterolytic bond cleavage solely with the electronic stability of the resulting carbocation (hyperconjugation/inductive effects).

The trap is failing to account for 'B-Strain' (Back-Strain).

In highly branched systems like triisopropylmethyl chloride, the ground state is severely destabilized by steric hindrance.

The transition state, which approaches a planar $sp^2$ geometry, relieves this strain.

This ground-state destabilization effectively lowers the activation energy barrier, making the reaction significantly faster despite having fewer alpha-hydrogens for hyperconjugation.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Organic Chemistry – Some Basic Principles and Techniques - Class 11 Part 2
Concept	Bond Cleavage Energetics
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free