BrCl addition to methyl acrylate and the methyl crotonates gave both the α-bromo-β-chloro and α-chloro-β-bromo regioisomers, with the latter being the minor product. 1 We anticipated that the carbonyl group in the ketones and aldehydes might accelerate α-attack by chloride in the intermediate halonium ions giving a greater amount of the α-chloro-β-bromo regioisomer. (See reference 1 for a discussion of the effect of carbonyl groups on S N 2 reactions.
[Thier, Ricarda; Mueller, Michael; Taylor, John B.; Pemble, Sally E.; Ketterer, Brian; Guengerich, F. Peter Chemical Research in Toxicology, 1995 , vol. 8, # 3 p. 465 - 472]