Carboxylic Acids and Derivatives: Question 5

Worked solution

Comparing the two mechanisms

Both an acyl chloride and (in principle) a carboxylic acid could react with water at the same electrophilic carbonyl carbon by nucleophilic addition-elimination. What actually differs is how good the leaving group is and how electrophilic the carbonyl carbon is:

• In butanoyl chloride, the leaving group is $\text{Cl}^-$, a weak base and an excellent leaving group. The chlorine atom is also highly electronegative, so it inductively withdraws electron density from the carbonyl carbon, making it strongly electrophilic and very open to attack by the lone pair on a water molecule’s oxygen. Once water adds, chloride leaves easily, and the intermediate loses a proton to re-form a C=O, giving butanoic acid and HCl, fast enough to be complete within seconds, with the HCl escaping as steamy white fumes.
• In butanoic acid, the group attached to the carbonyl carbon is $-\text{OH}$. Hydroxide, $\text{OH}^-$, is a poor leaving group (a much stronger base than $\text{Cl}^-$), so the addition-elimination pathway is far too slow to proceed at room temperature. In addition, the oxygen of the $-\text{OH}$ group can donate electron density into the carbonyl by resonance, which makes the carbonyl carbon less electrophilic than in the acyl chloride. With no good leaving group and a less reactive carbonyl carbon, the acid shows no observable reaction with water.

This same combination, better leaving group and greater carbonyl electrophilicity, is why acyl chlorides also react far more readily than carboxylic acids with alcohols (to form esters) and with amines (to form amides), without needing a catalyst or reflux.

Why the other options are wrong

• B is incorrect: reactivity here is about a chemical transformation (bond breaking and forming), not simple solubility or dissolving.
• C is factually wrong: the carbonyl carbon in both an acyl chloride and a carboxylic acid is $sp^2$ hybridised (planar, trigonal), not $sp^3$.
• D wrongly claims no hydrogen bonding occurs with butanoyl chloride and wrongly claims hydrogen bonding with butanoic acid prevents all reaction; hydrogen bonding is not the reason the acid fails to react by addition-elimination.

Final answer

A. The acyl chloride reacts rapidly because $\text{Cl}^-$ is a good leaving group and chlorine’s electronegativity makes the carbonyl carbon more electrophilic; the carboxylic acid has no good leaving group ($\text{OH}^-$ is a poor one) and a less electrophilic carbonyl carbon, so it does not react with water under these conditions.