Enolate anions are bidentate nucleophiles that can undergo reaction at the O atom or the alpha C atom. In solution, the C atom is usually the preferred reaction site, but in the gas phase, the O atom is. To better understand this preference in the gas phase, we carried out density functional theory calculations at the MPWPW91/6-311++G(3df,3pd) level of theory to determine the contributions by resonance and inductive effects toward the enhanced reactivity at O over C for the S<sub>N</sub>2 reaction between the formate anion and CH<sub>3</sub>F. To separate the contribution by resonance from that by inductive effects, we applied the vinylogue extrapolation method. Our results suggest that the dominant factor is the loss of resonance from the enolate nucleophile upon going to the transition state. That loss of resonance serves to disfavor both C attack and O attack, but C attack is disfavored significantly more than O attack.

Why does the acetaldehyde enolate favor reaction at the O atom during gas-phase nucleophilic substitution? Contributions by resonance and inductive effects

249th National Meeting (2015)

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