Bonds to Sulphur, Phosphorus, Nitrogen and Halogens

The carbon-sulphur bond resists hydrolysis and oxidation mercaptanes are an important component of petroleum. Although it is homeolytically stable (dissociation energy 330 kJ/mol [4], the silicon-sulphur bond is sensitive to oxygen. It is easily converted to a Si—O bond by water or oxygen compounds with Ge, Sn or Pb. Analogously, the C—P bond is stable, but the Si—P bond is sensitive to oxygen. 

In spite of the fact that the Si—N, Si—F and Si—Cl bonds are reinforced, they (and also the Si—Br and Si—I bonds) can easily be changed into the Si—O bond in a solvolysis reaction. Hydrolysis or alcoholysis proceeds rapidly by an Sn2 reaction mechanism. A transition state is generated involving the vacant silicon d orbitals and thus increasing the silicon coordination number. The hydrolysis of the C—Br and C—I bonds, which are easily polarised, also proceeds by an 8 2 reaction mechanism, and is also rapid. For example, the hydrolysis constant K[CH3Br] [OH ] in an ethanol/water mixture (4 1) at 55°C is 0.0021 1-mol see [6]. 

The interpretation of a transition state with a penta-coordinated carbon atom is very difficult. It was supposed [7] that the orbital of the free electron pair in the hydroxyl ion and the backlap of the C—Br antibonding orbital might be overlapped. The HO—C bond could thus be formed before the C—Br bond was dissolved. Such an overlap is probably difficult for the C—Cl bond. CCI4, CHCI3, CHjClj and CH3CI can only be solvolysed by prolonged contact with water at 180-225 °C or a concentrated alcoholic alkali-hydroxide solution at 100°C. 

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