Question

The acid strength of active methylene group in given compound decreases as:
(i)- $C{{H}_{3}}COC{{H}_{2}}COO{{C}_{2}}{{H}_{5}}$
(ii)- $C{{H}_{3}}COC{{H}_{2}}COC{{H}_{3}}$
(iii)- ${{C}_{2}}{{H}_{5}}OOCC{{H}_{2}}COO{{C}_{2}}{{H}_{5}}$

(a)- i > ii > iii
(b)- iii > i > ii
(c)- i > iii > ii
(d)- ii > i > iii

Answer 1

Methylene group is an electron-donating group, an electron-donating group decreases the acidity of the molecule. This is explained based on +I effect, the functional group which will have more +I effect will be less acidic.

Complete step by step answer:
When the organic molecule is substituted by electron-withdrawing groups like nitro ($N{{O}_{2}}$) the acidity increases.
When the organic molecule is substituted by electron-donating groups like hydroxyl ($OH$) the acidity decreases.
And we know that a methylene anion is an electron-donating group, an electron-donating group decreases the acidity of the molecule. This is explained based on +I-effect, the functional group which will have more +I-effect will be less acidic. So, we have to check which molecule has the more functional group that has more +I-effect.
The displacement of $\sigma -electrons$ along the saturated carbon chain whenever an electron-withdrawing or electron-donating group is present at the end of the carbon chain is called inductive effect or I-effect.
–I-Effect is when the substituent attached to the end of the carbon chain is electron-withdrawing.
+I-Effect is when the substituent attached to the end of the carbon chain is electron-donating.

So, the ketone has less +I-effect than the ester group. So, the compounds that have more ketone groups will be more acidic. Therefore, (ii) have two ketones, (i) have one ketone, and (iii)- doesn’t have any ketone group. So the order will be: (ii) > (i) > (iii).
So, the correct answer is “Option D”.

Note: Not only the I-effect affects the acidity of the molecule but R-effect also does.
The flow of electrons from one part of the conjugated system to the other creating centers of low and high electron density due to the phenomenon of resonance is called resonance effect or R-effect.
Groups that donate electrons to the double bond are said to have +R-effect.
Groups which withdraw electrons from the double bond are said to have –R-effect
-R increases the acidity whereas +R decreases the acidity.