Question

The strongest acid is:
(a)- Acetic acid
(b)- Trichloroacetic acid
(c)- Dichloroacetic acid
(d)- Monochloroacetic acid

Answer 1

Hint When any electron-donating substituent is attached with aliphatic acid then the acidity decreases because they decrease the stability of the carboxylate ion. When any electron-withdrawing substituent is with aliphatic acid then the acidity increases because they increase the stability of the carboxylate ion.

Complete answer:
When the electron-withdrawing group that attracts the electrons towards itself is attached with carboxylic acid like nitro ($N{{O}_{2}}$), chloro ($Cl$), fluoro ($F$ ) then the acidity increases.
When the electron-donating group that donates the electrons towards the acid is attached with carboxylic acid like methyl ($C{{H}_{3}}$), hydroxyl ($OH$) then the acidity decreases.
When there is 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 then it is called inductive effect or I-effect.
–I-Effect is seen when the substituent attached to the end of the carbon chain is electron-withdrawing.
+I-Effect is seen when the attached to the end of the carbon chain is electron-donating.
The negative charge on the carboxylate ion intensifies when there is +I-Effect thereby making the carboxylate ion less stable, which decreases the acidity of the carboxylic acid. The negative charge on the carboxylate ion will disperse when there is –I-Effect thereby making the carboxylate ion more stable, which increases the acidity of the carboxylic acid.
So, from the options given, trichloroacetic acid will be the strongest acid because there are three chlorine atoms present on the same carbon which will disperse the negative charge the most and make the most stable carboxylate ion.

Therefore, the correct answer is an option (b)- Trichloroacetic acid.

Note: In the same way as the number of electron-donating groups or methyl groups increases on the carboxylic acid, the acidity will decrease. This effect of substituents is the same as benzoic acid.