The correct acidity order of phenol (I), 4-hydroxybenzaldehyde (II) and 3-hydroxybenzaldehyde (III) is

• A

  $I \lt II \lt III$
• Correct answer
  B

  $I \lt III \lt II$
• C

  $II \lt III \lt I$
• D

  $III \lt II \lt I$

Solution

$\begin{array}{llll}\text { pKa } & 9.95 & 7.61 & 8.98\end{array}$ Acidic Strength $II \gt III \gt I$ -CHO is electron withdrawing group which increases acidic strength of phenolic compound. Also withdrawing effect of -CHO is more through para position as compound to meta position.

Hence, the correct answer is option: 'B'.