Question

Consider the acid strength of the carboxylic acids: (i) ${PhCOOH}$ (ii) $o - { NO_2C_6H_4COOH}$ (iii) $p- {NO_2C_6H_4COOH}$ (iv) $m- {NO_2C_6H_4COOH}$

• A. $i > ii > iii > iv$
• B. $iv > iii > ii > i$
• C. $ii > iii > iv > i$
• D. $ii > iv > iii > i$

Answer 1

Answer: (C)

$ii > iii > iv > i$

Answer 2

Due to -I and -R effects of -N02 group, nitrobenzoic acids are stronger acids than benzoic acid. Due to ortho-effect, o-nitrobenzoic acid is the strongest acid.
In p-nitrobenzoic acid, both - R effect and -I effect of the $-NO_2$ group increase the acidity while in m-nitrobenzoic acid only the weaker - I effect increases the acid_ity. Hence, o-nitrobenzoic acid > p-nitrobenzoic acid > m-nitrobenzoic acid > benzoic acid.