Animated Solution for Chemistry - Organic Chemistry: The increasing order of the rate of HCN addition to compounds A-D is (A) $\text{HCHO}$ (B) ${\text{CH}}_3{\text{COCH}}_3$ (C) ${\text{PhCOCH}}_3$ (D) $\text{PhCOPh}$

A Molecular Dance

Imagine a bustling molecular city where a highly reactive species, the nucleophile, is desperately searching for a place to land. In the world of carbonyl compounds, this landing pad is the carbonyl carbon. The addition of hydrogen cyanide ($\text{HCN}$) to a carbonyl group is a classic example of a nucleophilic addition reaction. Here, the cyanide ion (${\text{CN}}^{-}$) acts as the nucleophile, drawn to the electron-deficient (electrophilic) carbonyl carbon.

Sterics and Electronics

However, not all carbonyl carbons are equally welcoming. The speed at which this reaction occurs—the rate of nucleophilic addition—is governed by two primary guardians:

1. Steric Hindrance: Think of this as physical crowding. If the carbonyl carbon is surrounded by large, bulky groups, it becomes physically difficult for the incoming nucleophile to squeeze past them and make a successful attack. 2. Electronic Factors: The carbonyl carbon relies on its partial positive charge ($\delta +$) to attract the negatively charged nucleophile. If the groups attached to the carbonyl carbon are electron-donating (via inductive $+I$ or resonance $+R$ effects), they pump electron density towards the carbon. This quenches its positive charge, making it less attractive to the nucleophile.

Let's evaluate our four molecules based on these twin guardians:

1. Formaldehyde ($\text{HCHO}$ - Molecule A): This molecule is the most welcoming host. It has only two tiny hydrogen atoms attached to the carbonyl carbon. There is virtually zero steric hindrance. Furthermore, hydrogen atoms do not donate electrons. This leaves the carbonyl carbon highly electron-deficient and extremely reactive.

2. Acetone (${\text{CH}}_3{\text{COCH}}_3$ - Molecule B): Here, the hydrogens have been replaced by two methyl (${\text{CH}}_3$) groups. These groups are bulkier, introducing a moderate level of steric hindrance. More importantly, methyl groups exert a $+I$ (inductive) effect, pushing electron density towards the carbonyl carbon and reducing its $\delta +$ charge. Thus, acetone is less reactive than formaldehyde.

3. Acetophenone (${\text{PhCOCH}}_3$ - Molecule C): Things get significantly more crowded here. One of the methyl groups is replaced by a large phenyl ($\text{Ph}$) ring. This massive structure creates substantial steric hindrance. But the real game-changer is the electronic effect. The phenyl ring donates electrons through resonance ($+R$ effect), which is much stronger than the inductive effect. This strongly delocalizes and stabilizes the positive charge on the carbonyl carbon, drastically lowering its reactivity.

4. Benzophenone ($\text{PhCOPh}$ - Molecule D): This is the ultimate fortress. With two bulky phenyl rings, the steric crowding is at its absolute maximum. Furthermore, both rings pump electron density into the carbonyl carbon via the $+R$ effect. The positive charge is almost completely neutralized, making benzophenone the least reactive molecule in our lineup.

By combining the effects of steric hindrance and electronic stabilization, we can clearly see the trend. As we move from formaldehyde to benzophenone, both crowding and electron donation increase, leading to a sharp drop in reactivity.

The increasing order of the rate of $\text{HCN}$ addition is:

Therefore, the correct sequence is D < C < B < A.