Enolizable Carbonyl Compounds the Haloform Reaction

Formally, it involves an electrophilic addition to the a-carbon of an enolizable carbonyl compound. Introduction of a chlorine atom at this position helps to accelerate enolization by virtue of the increased electron-withdrawing character of Cl relative to H, so further addition of chlorine atoms is highly favored. Although simple ketones such as acetone are theoretically capable of enolization, only a vanishingly small fraction of the compound is present in the form which is reactive 

PERCENT EQUAL TO OR LESS THAN GIVEN CONCENTRATION 

Rook (1976) demonstrated that cyclic 1,3-diketones such as ninhydrin (20), dime-done (21), and 1,3-cyclohexanedione were efficient precursors of chloroform 

and Rook suggested that the 2-position of such compounds might be expected to be chlorinated readily and to react rapidly as haloform precursors. This suggestion was confirmed by an elegant experiment using isotopically labeled resorcinol 

Christman et al. (1978) also chlorinated resorcinol, showing that during chloroform formation it underwent ring contraction to produce the unusual cyclopentene-dione 25. Larson and Rockwell (1979) confirmed Rook s and Christman s observations that resorcinol derivatives were active haloform precursors, and also found that even if the 2-carbon of resorcinol was blocked by a carboxyl group (cf. 26), yields of 

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