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Consider the acid strength of the carboxylic acids.
$\mathrm{~ I . ~ P h C O O H ~ I I . ~ o - N}$
III. $p-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH} \quad$ IV. $m-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH}$
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$\mathrm{~ I . ~ P h C O O H ~ I I . ~ o - N}$
III. $p-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH} \quad$ IV. $m-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH}$
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Verified Answer
The correct answer is:
II $>I I I>I V>I \quad$
Due to $-I$ and $-R$-effects of $-\mathrm{NO}_{2}$ 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 $-\mathrm{NO}_{2}$ group increase the acidity, while in $m$-nitrobenzoic acid only the weaker $-I$-effect increases the acidity.
Hence, the order of acidic strength is $o$-nitrobenzoic acid $>p$-nitrobenzoic acid
$>m$-nitrobenzoic acid $>$ benzoic acid.
In $p$-nitrobenzoic acid both $-R$-effect and $-I$-effect of the $-\mathrm{NO}_{2}$ group increase the acidity, while in $m$-nitrobenzoic acid only the weaker $-I$-effect increases the acidity.
Hence, the order of acidic strength is $o$-nitrobenzoic acid $>p$-nitrobenzoic acid
$>m$-nitrobenzoic acid $>$ benzoic acid.
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