Before You Begin

Question #44940

Difficulty: 8/10 | 🟢 Free

The Question

Consider two aromatic amines: 2,6-dimethylaniline ( $\textbf{P}$ ) and 4-methylaniline ( $\textbf{Q}$ ). Let $k_P$ and $k_Q$ be their respective second-order rate constants for nucleophilic acyl substitution with acetyl chloride ( $\ce{CH3COCl}$ ) in anhydrous $\ce{CH2Cl2}$ at 298 K. Let $PA_P$ and $PA_Q$ be their respective gas-phase proton affinities. Which of the following correctly relates their kinetic and thermodynamic parameters?

Options

A.
$k_P < k_Q$ and $PA_P > PA_Q$
B.
$k_P > k_Q$ and $PA_P > PA_Q$
C.
$k_P < k_Q$ and $PA_P < PA_Q$
D.
$k_P > k_Q$ and $PA_P < PA_Q$

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Ortho-Effect in Anilines|Esterification Reversibility|Nucleophilic Acyl Substitution

Why This Question Is Tricky

Common Mistakes

This problem tests the decoupling of basicity and nucleophilicity caused by the 'Ortho-effect' (Steric Inhibition of Resonance).

The chemical trap lies in the assumption that a localized lone pair (which increases thermodynamic proton affinity) automatically translates to higher nucleophilicity.

Students fail to account for the 'dual-threat' of the ortho-substituents: while they electronically isolate the amine (increasing basicity), they simultaneously create a massive physical steric shield.

Because the rate-determining step of acylation requires the formation of a sterically demanding tetrahedral intermediate, the kinetic rate drops precipitously, even as the thermodynamic proton affinity rises.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Amines - Class 12
Concept	Ortho-Effect in Anilines
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free