Which one the following compounds will readily react with dilute NaOH ?

Solution

Chemistry ma’am · 3 steps

Dilute NaOH is a base, so look for the acid: only phenol is acidic enough to hand over its proton.

1. What NaOH does. Dilute NaOH does not replace the -OH group, because OH⁻ is a poor leaving group. It can only pull off the O-H proton, and it does this fully only when the compound is more acidic than water.
2. Why phenol is acidic. When phenol loses H⁺, the negative charge on the phenoxide oxygen spreads into the benzene ring by resonance. In an alcohol the -OH sits on an sp³ carbon, so the charge on the alkoxide ion stays on oxygen alone.

   C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O

   Compound	-OH attached to	Anion stabilised by resonance?	Reacts with dilute NaOH?
   C₆H₅CH₂OH	CH₂ (sp³ carbon)	No, the CH₂ cuts O off from the ring	No
   C₂H₅OH	sp³ carbon	No	No
   C₆H₅OH	ring carbon (sp²)	Yes, charge spreads into the ring	Yes, forms sodium phenoxide
   (CH₃)₃COH	sp³ carbon	No, and three CH₃ groups push electrons onto O	No

3. So. Only phenol, option (c), reacts readily with dilute NaOH, giving sodium phenoxide and water.

The idea to keep. Alcohols are too weakly acidic to react with aqueous NaOH, but phenol is not: its phenoxide ion is resonance-stabilised, so it forms a salt. This is the classic test that tells phenols from alcohols.

Answer from the NTA final answer key. Solution by PrepWiser.