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The nature of the attack of OH- on CH3Cl is a well-established SN2 reaction. The hydrolysis of poly-halogenoalkanes does not involve a simple extension of this reaction. It is worth looking at the other halo-substituted methane derivatives before comparing CCl4 and SiCl4 hydrolysis, a popular question illuminating the differences between carbon and silicon.

 

Dichloromethane.

The attack of hydroxide ion on this compound is initially SN2; the reaction is considerably slower than with chloromethane. The overall reaction is

CH2Cl2 + 2OH- HCHO + 2Cl- + H2O.

The initial SN2 attack on dichloromethane is slow and gives chloromethanol:

CH2Cl2 + OH- HOCH2Cl + Cl-.

Chloromethanol then reacts with hydroxide ion in a fast E2 step to give methanal, water and chloride ion.

 

 

Trichloromethane (chloroform).

Trichloromethane would be expected to be even slower than dichloromethane in its reaction with hydroxide ions. It is not. It is faster, and does not involve hydroxide ion in an SN2 mechanism. Instead a highly reactive electron-deficient intermediate CCl2 is formed after abstraction of a proton from trichloromethane, hydroxide acting as a base:

HO- + HCCl3 HOH + CCl3- (fast)

CCl3- CCl2 + Cl- (slow).

CCl2 is electrophilic and reacts with water to give either carbon monoxide and chloride ions, or methanoate ion and chloride ions. The net reaction is

3 CCl2 + 5H2O CO + 2HCOO- + 8H+ + 6Cl- .

The overall reaction of trichloromethane with hydroxide ions is therefore

3CHCl3 + 3 OH- + 2H2O CO + 2HCOO- + 8H+ + 9Cl- .

 

Tetrachloromethane and silicon tetrachloride.

Tetrachloromethane reacts with great difficulty with hydroxide ions, whereas silicon tetrachloride reacts vigorously with water alone. The differences between the molecules are:

The combined effect of these is:

The net result is the much faster hydrolysis of the silicon halide.

References:

(1) ‘Handbook of Chemistry and Physics’, 75th ed. CRC Press 1995.

(2) Stark JG & Wallace HG, ‘Chemistry Data Book’, 2nd SI edition. John Murray, 1982.


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