Question

The acidity of compounds $I-IV$ in water
$I.$ ethanol $II.$ acetic acid
$III.$ phenol $IV.$ acetonitrile follows the order

• A. IV < I < III < II
• B. I < II < III < IV
• C. IV < I < II < III
• D. IV < III < I < II

Answer 1

Answer: (A)

The acidity of compounds in water depends upon the ease with which it can lose $H^+$ ions. Acetic acid is the strongest acid as the negative charge on carboxylate ion (conjugate base) is delocalised over two oxygen atoms. Hence, $H^+$ ion can be easily lost. The next strongest acidic compound phenol. This is because the phenoxide ion is resonance stabilised. This easily allows the H to leave as $H^+$ ion. Among acetonitrile and ethanol, ethanol is more acidic, this is because in ethanol the $H-$atom is directly attached to more electronegative atom, $O.$ Thus, the correct order of acidity of compound I-IV in water will be $IV < I < III < II.$