Question

Rank the following compounds in order of increasing acidity and explain in detail your choice of order.

Short answer

Conjugate base of compound C

Least acidic

Conjugate base of compound D

Most acidic

Conjugate base of compound E

Intermediate acidic

The increasing order of acidity of the above compounds is as follows:

Compound D < Compound E < Compound C

Compound C is the most acidic, followed by compound E. The compound D is the least acidic.

Step-by-step solution

Factors affecting the strength of an acid

The strength of an acid depends upon the inductive effect and the resonance effect.Some of the compounds are more acidic in nature due to the electron-withdrawing inductive effect of the groups attached to them.

The alkyl groups contribute towards the electron releasing inductive effect whereas the halogens provide the electron-withdrawing inductive effect.

The resonance effect is responsible for the stability of the conjugate base of an acid which, therefore increases the acidic strength of the given compound.

The increasing order of acidity of the given compounds

The structure of the given compounds as well as their conjugate bases are given below:

Conjugate base of compound C

Least acidic

Conjugate base of compound D

Most acidic

Conjugate base of compound E

Intermediate acidic

The increasing order of acidity of the above compounds is as follows:

Compound D < Compound E < Compound C

Compound C is the most acidic, followed by compound E. The compound D is the least acidic.

In case of compound C, the nitro group, as well as the carbonyl group are directly bonded to the benzene ring. As a result, the resulting conjugate base is stabilized by both the inductive as well as the resonance effect. Following are the resonating structures for compound C.

Resonating structures

In case of compound E, the carbonyl group is not directly bonded to the benzene ring. Therefore, the resonance effect cannot take place. Only the inductive effect of the nitro group helps to stabilize the conjugate base of compound E.

For compound D, the carbonyl carbon is not directly bonded to the benzene ring. Therefore, the resonance effect cannot take place. As a result, the conjugate base of compound D is least acidic.