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  4. pKa increases in benzoic acid when substituent x is bonded at para-position, then x is

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Q. $pK_a$ increases in benzoic acid when substituent "x" is bonded at para-position, then "x" is

AIIMSAIIMS 2012Aldehydes Ketones and Carboxylic Acids

Solution:

Larger the value of pKa, smaller will be its acidity. Out of the four groups, -COOH, -NO2 and -CN are e- withdrawing which makes benzoic acid more acidic whereas -OCH3 is e- donating which reduces the acidity (makes H+ less easily available). pKa value increases if -OCH3 is present at para-position of benzoic acid.