Ques. 1AIPMT 2015 (Cancelled)
Given : [figure] Which of the given compounds can exhibit tautomerism?
- II and III
- I, II and III
- I and II
- I and III
Answer
(2) I, II and III
Solution
Keto–enol tautomerism needs a hydrogen that can shift to the carbonyl oxygen: an α-H, or in an α,β-unsaturated ketone also a γ-H (through the conjugated C=C–C=O system).
Number the ring: C-1 = C=O, C-2=C-3 double bond.
I: C-6 is a CH2 group next to C=O ⇒ α-H present ⇒ enolises.
II: C-6 is a CH2 group ⇒ α-H present ⇒ enolises.
III: C-6 carries two CH3 groups (no α-H there), but C-4 (CH2) is the γ-carbon of the conjugated enone ⇒ its H can shift to O through the C=C–C=O system to give a dienol ⇒ tautomerism possible.
∴ All three, I, II and III, can show tautomerism.
Answer: (2)