Among the following structures, which will show the most stable enamine formation? (Where Me is $$-CH_3$$)

For any carbonyl compound the sequence for enamine formation is : (1) nucleophilic addition of a secondary amine on the $$C=O$$ carbon, (2) proton transfers, and (3) loss of water accompanied by removal of an $$\alpha$$-hydrogen to give the C=C-N system. The position of equilibrium

$$\text{carbonyl compound}\;+\;\text{(sec. amine)} \rightleftharpoons \text{enamine}+H_2O$$

depends on the stability of the enamine that is produced. A more stable enamine pulls the equilibrium to the right and therefore “forms most readily”. The stability of an enamine is governed by the same factors that stabilise ordinary alkenes:

• greater alkyl substitution on the C=C bond (hyperconjugation and +I effect)
• conjugation of the C=C bond with another $$\pi$$-system (aromatic ring, C=C, C≡C, etc.)

All alkyl groups are simple methyl/ethyl groups, so the resulting C=C bond is only modestly substituted and not conjugated with any other $$\pi$$-system.

The situation is similar to Case A; no extra substitution and no conjugation.

The carbonyl carbon is directly attached to a phenyl ring. When the $$\alpha$$-proton is removed, the resulting C=C bond becomes

$$\text{Ph}-C(=C)-N$$

This double bond is in conjugation with the benzene ring. Resonance delocalises the enamine’s double bond into the aromatic system, giving several resonance contributors such as

$$\text{Ph}^{\ominus}\!-\!C-C^{\oplus}\!=\!N \longleftrightarrow \text{Ph}-C^{\oplus}\!=\!C-N^{\ominus}$$

Conjugation with the aromatic ring therefore provides a large extra stabilisation on top of the normal alkyl-substitution stabilisation.

Here again the C=C bond produced in the enamine is only alkyl-substituted and not conjugated with any extended $$\pi$$-system.

Comparing all four cases, the enamine derived from Case C is the only one that enjoys resonance conjugation with an aromatic ring in addition to normal alkyl substitution. That additional delocalisation lowers its energy the most, so Case C gives the most stable enamine and therefore shows the easiest / most favourable enamine formation.

Hence the correct choice is:
Option C which is: the carbonyl compound having the phenyl ring directly attached to the carbonyl carbon.