Question

The powerful electron-withdrawing property of a carbonyl group increases the acidity of α-protons (i.e., protons attached to a carbon that is adjacent to the carbonyl group). The effect varies depending on what else is attached to the carbonyl, and whether there is a second carbonyl group attached to the α-carbon. Rank the following structures from highest acidity (lowest pKa value) to lowest acidity (highest pKa value).

Answer 1

Concepts and reason

A series of compounds are given which needs to be arranged in decreasing order of acidic strength of to the carbonyl group. Acidic strength depends on how easily proton can be released or abstracted from the parent compound. Generally, electron withdrawing group increases the acidic strength and electron donating group decreases the acidic strength.

Fundamentals

Acidic strength depends on the following factors:

• Presence of electron withdrawing and electron donating groups.

Consider the following compound:

• Stability of conjugate base: Conjugate base is formed after the removal of acidic proton. The stability of the conjugate base is directly related to the stability of carbanion.

The acidic strength of a proton can be estimated by its value. Mathematically, is defined as the negative logarithm of . value is inversely related to the acidic strength of the proton, that is, the most acidic proton should have lowest value and vice versa. The proton having the lowest value gets abstracted first.

Draw the structure of the conjugate base formed by the given compounds as shown below:

Arrange the given compounds in decreasing order of the acidic strength based on the presence of electron withdrawing/donating groups present and the stability of conjugate base as follows:

Stability of conjugate base:

Decreasing order of acidic strength is as follows:

Ans:

Hence, the correct order for decreasing acidic strength of the given compounds is as follows:

Answer 2