Question

The origin of acidity and basicity in organic compounds is of great interest and provides an extensive comparison. Among hydrocarbons % s character is taken into account while deciding the acidity. In simple aliphatic acids, more the number of alkyl groups (+$I$ effect) less is the acidity and more the (-$I$ effect) alkyl groups larger the acidity and vice versa in the case of simple aliphatic bases. Benzoic acid is stronger than aliphatic amines as the electron pair is less available in case of aromatic amines. The presence of solvent also plays a very important role and at times governs the order too.
Which of the following orders regarding acid strength is correct?
(A) $HCOOH$ $<$ $C{{H}_{3}}COOH$ $<$ $PhCOOH$
(B) $HCOOH>PhCOOH>C{{H}_{3}}COOH$
(C) $HCOOH>C{{H}_{3}}COOH>PhCOOH$
(D) $C{{H}_{3}}COOH>HCOOH>PhCOOH$

Answer 1

Hint: methyl group has a positive inductive effect. Electron withdrawing group leads to an increase in acidity and electron-donating group leads to a decrease in acidity. Phenyl groups increase acidity and Hydrogen does not show any inductive effect.
Complete solution step by step:
 Substituents attached to carboxylic acid have an effect on the stability of the conjugate base and thus have an effect on the acidity of carboxylic acids.
-Acidity is increased due to electron-withdrawing groups as they tend to stabilize and acidity is decreased due to electron-donating groups.
-Due to greater electronegativity of $s{{p}^{2}}$ carbon atom of phenyl attached to a carboxyl carbon atom, phenyl group increases acidity.
- Due to positive inductive effect, methyl group decreases acidity and hydrogen does not show any inductive effect, Hydrogen is more electronegative than Methyl group.

So, (B) $HCOOH>PhCOOH>C{{H}_{3}}COOH$ is the correct order of acid strength.

Note: Inductive effect affects the acidity of carboxylic acid, methyl group has a positive inductive effect and it decreases acidity. A phenyl group is more electronegative than the methyl group, due to more electronegative $s{{p}^{2}}$ carbon atom, so it increases acidity and hydrogen group does show any inductive effect.