Question

Arrange the following in the increasing order of their acidic strength:

A ) III < I < IV < II
B ) II < I < IV < III
C ) I < III < IV < II
D ) II < III < I < IV

Answer 1

Hint: Determine the nature and number of different types of substituents. Here, type of substituent refers to its electron donating nature or electron withdrawing nature. Identify the inductive effect and resonance effect present.

Complete step by step answer:
Electron withdrawing groups show -I effect and electron releasing groups show +I effect. Here, ‘I’ represents an inductive effect. When electron withdrawing groups are present, the acid strength increases and when electron donating groups are present, the acid strength decreases. With increase in the number of electron withdrawing groups, the acid strength increases. With increase in the strength of -I effect of electron withdrawing groups, the acid strength increases.
Compound IV is most acidic among the given compounds. Because, the tertiary carbon atom has three strong electron withdrawing, carbonyl groups. When this tertiary carbon atom loses an electron, it forms a carbanion in which the negative charge is delocalised through resonance with three carbonyl groups. Greater is the resonance, greater is the stabilisation of the carbanion and greater is the acid strength.

After compound IV, the compound I is most acidic among the given compounds. Because, one of the secondary carbon atoms has two strong electron withdrawing, carbonyl groups. When this carbon atom loses an electron, it forms a carbanion in which the negative charge is delocalised through resonance with two carbonyl groups. Greater is the resonance, greater is the stabilisation of the carbanion and greater is the acid strength.

Among the compounds II and III, the more acidic compound is compound III as it has -I effect of carbonyl group and ether group. However in compound II, the ether group is far away, so it exerts less of -I effect.

Hence, the option D ) II < III < I < IV is the correct answer.

Note:
A strong acid can easily donate a proton. The carbanion obtained by donation of a proton is stabilised due to inductive and resonance effects. Greater is the stabilisation of the carbanion through inductive and resonance effects, greater is the acid strength of the molecule.