Two-step carbanion mechanism

The basic hydrolysis of haloforms is first-order with respect to both haloform and hydroxide ion . Several arguments eliminate nucleophilic attack of hydroxide ion on haloform, Sn2 displacement (22), as a possible mechanism. 

KINETIC parameters FOR THE BASIC HYDROLYSIS OF HALOFORMS AND RELATED COMPOUNDS IN 66.7% DlOX AN-WATER  

The replacement of an a hydrogen of an alkyl halide by halogen decreases Sn2 reactivity. Chloroform, however, is about one thousandfold more reactive in basic hydrolysis than methylene chloride . Every bromine-containing halo-form studied (Table 7) is at least 600 times as reactive toward hydroxide ions in 66.7% aqueous dioxan as methylene bromide . Toward weakly basic nucleophiles, such as thiophenoxide ion, the predicted reactivity order is obeyed haloforms have been found to be less reactive than the corresponding methylene halides . The reaction of haloforms with sodium thiophenoxide is strongly accelerated, however, by the presence of hydroxide ions - . These observations are quite unexplainable in terms of scheme (22). 

Comparison of the resultant data reveals that the a halogen substituents facilitate carbanion formation in the order I Br Cl F, which is almost the reverse of that expected from the inductive effect. d-Orbital resonance and polarizability increase in the order F Cl Br I, and it is difficult to assess their relative importance in carbanion stabilization . 

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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