Isotope with haloforms

A value of kH/kD = 1.4 was obtained [114] for the rate of proton transfer compared with deuteron transfer from chloroform to hydroxide ion and this result is similar to the values determined earlier for several haloforms [164, 166]. In the most recent work [171(b)] a value kH /kD = 1.11 0.05 was determined for chloroform. These values are close to those observed for reaction of cyanocarbon acids (though a different base catalyst is involved) and in Sect. 4.3 it was argued that isotope effects as low as these are expected for a transition state in which proton transfer is almost complete. The isotope effect for proton transfer from chloroform was measured using a new and useful method [114]. It can be shown that the ratio of initial rates of uptake of tritium for the first ten per cent of reaction from tritiated water into CHC13 and CDC13 is identical to the primary isotope effect for proton loss (feH /fcD). The procedure can be used for measuring isotope effects on proton transfer from carbon acids to hydroxide ion or buffer catalysts and is more convenient than other methods. Other methods which have been used, for example, involve the comparison of rates of detritiation and dedeuteration or the comparison of rates of bromination for isotopically different acids (RCH and RCD) [113]. 

Several reported kinetic studies shed light on the various steps in the Haloform reaction. Bartlett showed in 1934 that the reaction of acetone with strong alkaline solutions of hypobromite and hypoiodite involves enolization of acetone as the ratedetermining step, but the rate of the same reaction with alkaline hypochlorite involves as the slow step reaction of acetone enol with hypochlorite.15 Likewise, the mechanism of the iodoform reaction and its optimization for quantitative analysis have received much scrutiny16 including studies of acetone-1-C-l4 7 and deuterated acetone18 for isotope... 

The synthetic utility of the Haloform reaction is revealed further by a synthesis of isotopically pure (99.2%) CDCb by the reaction of trichloroacetophenone with NaOD in D20. The alkoxide-cleavage of trichloromethyl ketones has been employed to synthesize dihydropyridine esters (e.g., 24) in which ester exchange does not occur.59... 

The kinetic isotope effect of proton abstraction has been measured directly with the rapidly hydrolyzable haloforms CHCI2F (knlki,= 1.76 at 0°C and 1.52... 

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