Halogenation of the a-Carbon

The reaction of an aldehyde or ketone with Cl2, Br2, or I2, under either acidic or basic conditions, results in the replacement of a hydrogen on the a-carbon with a halogen. 

The presence of the halogen retards enolization, so it is possible to stop the reaction after the addition of a single halogen. This reaction can provide a useful way to add a halo- 

Under basic conditions, it is the enolate anion that acts as the nucleophile. 

The cleavage reaction does not occur unless there are three halogens on the carbon, so only methyl groups are removed in this manner. The reaction with 12 (iodoform reaction) has been used as a test for methyl ketones. The formation of iodoform (CHI3), 

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Rapid halogenation of the a carbon atom takes place when an enolate ion is generated m the presence of chlorine bromine or iodine... 

Haloform Reaction Section 20.2 Halogenation of an Arene Section 17.5 Halogenation of the a-Carbon of a Ketone in Acid Section 20.2... 

Halogenation of the a-carbon of carboxylic acids the Hell-Volhard-Zelinski reaction (Section 19.5). 

The halogenation of the a-carbon of carbonyl structures is just one example of a vast field of substitution reactions that commence from enols and enolates. This field encompasses a large portion of organic synthetic procedures, and it is best left to a text devoted to that discipline. However, a few points about enolate alkylations are worth mentioning here, because the common rationalizations for many experimental observations are strongly derived from physical organic chemistry principles. 

Halogenation of the a-carbon of aldehydes and ketones (Section 18.4). The mechanisms are shown on pages 859 and 860. 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

Section 19 16 Halogenation at the a carbon atom of carboxylic acids can be accom plished by the Hell-Volhard-Zehnsky reaction An acid is treated with chlorine or bromine m the presence of a catalytic quantity of phospho rus or a phosphorus trihalide... 

The facilitation of the C-halogen bond reduction by a neighboring carbonyl group is due to the imposed donor character of the a-carbon, and by extension, the heightened acceptor character of the halogen atom. 

The comparative effect of the F atom in HF elimination under the same experimental conditions reveals that on increasing the number of halogens at the a-carbon, the energy of activation tends to increase, while the rate coefficient decreases in the following sequence ... 

Figure 23.3 summarizes the three possible outcomes of halogenation at the a carbon, depending on the chosen reaction conditions. 

Organolithiums bearing a halogen on the a-carbon are termed carbenoids. Bromine and chlorine are sufficiently acidifying to allow preparation of the halocarbenoids, but fluorine requires additional stabilization, for example by phosphorus (yide supra). Lithium halocarbenoids are thermally unstable species whose chemistry must be performed at low temperatures and this feature has hindered their use in preparative synthetic organic chemistry. 

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