When methyl ketones are treated with a halogen in the presence of base, the three hydrogen atoms on the methyl carbon are replaced to give a CX₃-substituted ketone. This product is not stable under the basic conditions and proceeds to react with hydroxide, ultimately furnishing the carboxylic acid (as its conjugate base) and a molecule of HCX₃, which has the common name haloform (i.e., chloroform, bromoform, and iodoform, for X 5 Cl, Br, and I, respectively). For example: 

Propose a series of mechanistic steps to convert the tribromoketone into the carboxylate. What is the leaving group? Why do you think this species is capable of acting as a leaving group in this process?

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:0: || RCCBR3 + :0: II.. RCO: :OH HCBR3