Animated Solution for Chemistry - Organic Chemistry: The correct order of their reactivity towards hydrolysis at room temperature is

The reactivity of acid derivatives towards nucleophilic acyl substitution (like hydrolysis) is a classic and highly conceptual topic in organic chemistry. It beautifully demonstrates how competing electronic effects—inductive and resonance—dictate the chemical behavior of a molecule.

Nucleophilic Attack

Imagine you are a nucleophile, like a water molecule or a hydroxide ion. You are rich in electrons and looking for a place to attack. Where would you go? Naturally, you'd seek out an electron-deficient center.

In acid derivatives, the carbonyl carbon is your target. The oxygen atom, being highly electronegative, pulls electron density away from the carbon, creating a partial positive charge ($\delta +$). The rate of hydrolysis is directly proportional to the magnitude of this $\delta +$ charge. The more positive the carbon, the faster and more aggressive the nucleophilic attack.

Inductive vs Resonance Effects

The leaving group attached to the carbonyl carbon, let's call it 'L', plays a pivotal role in determining the magnitude of this $\delta +$ charge. It engages in a molecular tug-of-war through two distinct electronic effects:

1. The Inductive Effect ($-I$): Through the sigma bond, an electronegative leaving group pulls electron density away from the carbonyl carbon. This effect increases the $\delta +$ charge, making the molecule more reactive. 2. The Resonance Effect ($+R$): If the leaving group has lone pairs, it can donate them into the pi system of the carbonyl group. This delocalization of electrons decreases the $\delta +$ charge, stabilizing the molecule and making it less reactive.

The ultimate reactivity of the acid derivative depends on which of these two effects dominates.

Let's analyze our four contenders one by one:

1. Acid Chloride ($R-CO-Cl$) Chlorine is highly electronegative, exerting a very strong $-I$ effect. However, its lone pairs reside in a larger 3p orbital. This 3p orbital overlaps poorly with the smaller 2p orbital of the carbonyl carbon. Consequently, its $+R$ effect is extremely weak. The strong electron withdrawal completely overpowers the weak donation, leaving the carbonyl carbon highly electron-deficient. Thus, acid chlorides are the most reactive.

2. Acid Anhydride ($R-CO-O-CO-R$) Here, the central oxygen atom donates its lone pair via resonance. However, this oxygen is sandwiched between two carbonyl groups. Its lone pair is cross-conjugated, meaning it must divide its electron donation between both carbonyl carbons. Because its attention is split, the $+R$ effect experienced by any single carbonyl carbon is only moderate.

3. Ester ($R-CO-O-R$) In an ester, the alkoxy oxygen also donates its lone pair through resonance. Unlike the anhydride, this lone pair is fully committed to a single carbonyl group. This focused and strong $+R$ effect significantly quenches the positive charge on the carbonyl carbon, making esters less reactive than anhydrides.

4. Amide ($R-CO-N{H}_2$) Finally, we have the amide. Nitrogen is less electronegative than oxygen. Because it holds onto its electrons less tightly, it is a far superior electron donor. The $+R$ effect of the amino group is exceptionally strong. It donates so much electron density that it almost completely neutralizes the $\delta +$ charge on the carbonyl carbon. This makes amides the least reactive of the acid derivatives.

By evaluating the delicate balance between inductive withdrawal and resonance donation, we can confidently rank the reactivity of these acid derivatives. The acid chloride, with its massive $\delta +$ charge, takes the crown. It is followed by the anhydride, then the ester, and finally the highly stable amide.

Therefore, the correct order of reactivity towards hydrolysis is (A) > (B) > (C) > (D).