Which of the following ketone will NOT give enamine on treatment with secondary amines? [where t-Bu is -C(CH₃)₃]

Solution

Chemistry ma’am · 3 steps

An enamine needs an α-hydrogen to give up, so find the ketone whose α-carbons have none.

1. How enamines form. The secondary amine R₂NH adds to the C=O carbon and then loses water, giving an iminium ion C=N⁺R₂. Nitrogen now has no H left to lose. So a proton is removed from the α-carbon instead, which gives the enamine C=C-NR₂.
2. What to check. Without an α-hydrogen that last step cannot happen. So count the H atoms on the carbons directly attached to the carbonyl carbon.

   Ketone	α-carbons	α-H	Enamine?
   (a) pentan-3-one, C₂H₅COC₂H₅	two CH₂	4	yes
   (b) butanone, CH₃COC₂H₅	CH₃ and CH₂	5	yes
   (c) di-tert-butyl ketone, (CH₃)₃C-CO-C(CH₃)₃	two quaternary C	0	no
   (d) cyclohexanone	two ring CH₂	4	yes

3. So. In (c) each α-carbon carries three methyl groups and no hydrogen, and the two bulky t-Bu groups also crowd out the amine's attack. Di-tert-butyl ketone cannot give an enamine, so the answer is (c).

The idea to keep. Primary amines give imines, secondary amines give enamines, and an enamine forms only if the carbonyl compound has an α-hydrogen. Always check the α-carbons first.

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