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.
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- à HCHO + 2Cl- + H2O.
The initial SN2 attack on dichloromethane is slow and gives chloromethanol:
CH2Cl2 + OH- à HOCH2Cl + Cl-.
Chloromethanol then reacts with hydroxide ion in a fast E2 step to give methanal, water and chloride ion.
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 à HOH + CCl3- (fast)
CCl3- à 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 à CO + 2HCOO- + 8H+ + 6Cl- .
The overall reaction of trichloromethane with hydroxide ions is therefore
3CHCl3 + 3 OH- + 2H2O à 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 C-Cl bond length is 175 pm, Si-Cl is 206 pm (1).
- The C-Cl bond dissociation enthalpy is 327 kJ mol-1 in CCl4, although the average is given as 397 ± 29 kJ mol-1 (1); Si-Cl bond dissociation enthalpy in SiCl4 is 406 kJ mol-1.
- The C-Cl electronegativity difference is 0.5, Si-Cl is 1.2. This corresponds to an approximate ionic character of 6% in C-Cl, 30% in Si-Cl (2).
- Chlorines covalent radius is 99 pm, carbons 77 pm, silicons 118 pm.
- The silicon atom has accessible empty d-orbitals, whereas carbon does not.
The combined effect of these is:
- The large size of the chlorine atom means that there is considerable steric hindrance on the incoming hydroxide ion with tetrachloromethane and even more in the 5-co-ordinate transition state which SN2 produces. The energy barrier to reaction is thus much larger than in the case of chloromethane with its small hydrogen atoms. Tetrachloromethane is therefore difficult to hydrolyse.
- The C-Cl bond is weaker than the Si-Cl bond despite the greater length of the latter. This is due firstly to the higher degree of ionic character in the case of silicon tetrachloride, and probably also to non-bonded repulsions between the chlorine atoms in tetrachloromethane. This latter view is supported by the weakness of C-Cl in tetrachloromethane compared with the average value for this bond.
- The silicon atom being larger than carbon means that there is less of a problem with steric hindrance in SiCl4, so the necessary 5-coordinate transition state can be formed. It may be a true 5-covalent intermediate, an sp3d hybrid.
- The d-orbitals of silicon are available to accept the lone electron pair from the attacking hydroxide ion, so the Si-Cl bond does not have to break first; in any case this is easier for Si-Cl compared with C-Cl given the much higher polarity.
The net result is the much faster hydrolysis of the silicon halide.
(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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