Organic Chemistry Basics 2 Question 78
78. Match the following with their $K_{a}$ values
| Benzoic acid | $4.2 \times 10^{-5}$ |
|---|---|
| $p$-nitrobenzoic acid | $3.3 \times 10^{-5}$ |
| p-chlorobenzoic acid | $6.4 \times 10^{-5}$ |
$$ \begin{array}{ll} \text { p-methylbenzoic acid } & 36.2 \times 10^{-5} \ \text { p-methoxybenzoic acid } & 10.2 \times 10^{-5} \end{array} $$
(2003)
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Solution:
- $p$-methoxy benzoic acid is the weakest and $p$-nitrobenzoic acid is the strongest acid among these acids. Chloro group has overall electron withdrawing effect on ring, therefore, increases acid strength of benzoic acid. Methyl group decreases acid strength of benzoic acid by $+I$ effect. Therefore,
| $K_{a}$ | |
|---|---|
| $p$-methoxy benzoic acid | $3.3 \times 10^{-5}$ |
| $p$-methyl benzoic acid | $4.2 \times 10^{-5}$ |
| benzoic acid | $6.4 \times 10^{-5}$ |
| $p$-chlorobenzoic acid | $10.2 \times 10^{-5}$ |
| $p$-nitrobenzoic acid | $36.2 \times 10^{-5}$ |
