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Q8.12(i):
Arrange the following compounds in increasing order of their property as indicated: (i) Acetaldehyde, Acetone, Di-tert-butyl ketone, Methyl tert-butyl ketone (reactivity towards HCN)

Solution :

Step 1: Theoretical Basis of Nucleophilic Addition

The reaction of carbonyl compounds with $HCN$ is a classic nucleophilic addition reaction. The rate-determining step is the nucleophilic attack of the $CN^-$ ion on the electrophilic carbonyl carbon atom. The reactivity of the carbonyl compound is governed by two primary factors:

  • Electronic Factors: The presence of electron-donating groups (like alkyl groups) decreases the positive charge density on the carbonyl carbon through the $+I$ (inductive) effect, thereby reducing its electrophilicity [Per the Inductive Effect Theory].
  • Steric Factors: The presence of bulky alkyl groups around the carbonyl carbon creates steric hindrance, which obstructs the approach of the $CN^-$ nucleophile to the reaction center [Per Transition State Theory].

Step 2: Analysis of the Given Compounds

Compound Structure Steric/Electronic Environment
Acetaldehyde $CH_3CHO$ One small $H$ atom and one methyl group. Least steric hindrance.
Acetone $CH_3COCH_3$ Two methyl groups. Moderate steric hindrance.
Methyl tert-butyl ketone $(CH_3)_3CCOCH_3$ One bulky tert-butyl group. Significant steric hindrance.
Di-tert-butyl ketone $(CH_3)_3CCOC(CH_3)_3$ Two bulky tert-butyl groups. Maximum steric hindrance.

Step 3: Sequential Ordering Based on Reactivity

By comparing the substituents on the carbonyl group:

  1. Acetaldehyde: Being the least sterically hindered ($H$ and $CH_3$), it is the most reactive toward the $CN^-$ nucleophile.
  2. Acetone: Contains two methyl groups; more hindered than acetaldehyde but less than the others.
  3. Methyl tert-butyl ketone: The tert-butyl group is significantly larger than a methyl group, drastically reducing the accessibility of the carbonyl carbon.
  4. Di-tert-butyl ketone: Contains two bulky tert-butyl groups, which provide extreme steric crowding, making it the least reactive compound in the series.

Step 4: Visualizing Steric Hindrance

Acetaldehyde (Small) Di-tert-butyl ketone (Bulky) t-Bu t-Bu Increasing Steric Hindrance $\rightarrow$

Final Solution: The increasing order of reactivity towards $HCN$ is: Di-tert-butyl ketone < Methyl tert-butyl ketone < Acetone < Acetaldehyde.


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