15.12:
Reactions of α-Halocarbonyl Compounds: Nucleophilic Substitution
α-halocarbonyl compounds undergo nucleophilic substitution at their α-position via an SN2 pathway.
With α-halo ketones, the nucleophiles used must be less basic since strongly basic nucleophiles result in enolate formation.
On the contrary, α-halo acids react well with strongly basic nucleophiles via the SN2 pathway.
Under basic condition, α-halo acids first ionize to its conjugate-base anion. Further deprotonation of the α hydrogen to form the dianion does not take place and instead, the anionic species undergoes a substitution reaction.
SN1 reactions are unfavorable for α-halocarbonyl compounds, as they form less stable carbocations at the α-position.
The resonance structure of the intermediate carbocation involves electron-deficient oxygen, making it less resonance-stabilized.
Electrostatic interaction of the carbocation with the bond dipole of the carbonyl group also destabilizes the α-carbonyl carbocation.
15.12:
Reactions of α-Halocarbonyl Compounds: Nucleophilic Substitution
Nucleophilic substitution in α-halocarbonyl compounds can be achieved via an SN2 pathway. The reaction in α-haloketones is generally carried out with less basic nucleophiles. The use of strong basic nucleophiles leads to the generation of α-⁠haloenolate ions, which often participate in other side reactions.
However, α-haloacids undergo SN2 reactions with strong basic nucleophiles. Under this condition, the base abstracts the acidic proton of the acid forming its conjugate base. The anion further participates in a substitution reaction, and the final acidification results in α-substituted acids.
In α-halocarbonyl compounds, nucleophilic substitution via an SN1 pathway is forbidden, as it generates less stable α-carbocation intermediate.