Heteroatom bond polarisation

The differentiation between the coordination polymerisation of heterocyclic and heterounsaturated monomers and their nucleophilically initiated anionic polymerisation lies in the covalent nature of the metal-heteroatom bond (but being polarised as in the Mt 5+— Xs bond) in the coordination catalyst, which activates the monomer by its coordination, enhancing the nucleophilicity of the metal substituent simultaneously, and the ionic character of the metal-heteroatom bond in the nucleophilic initiators. 

Only the C-Br bond is polarised to any significant extent, and it is polarised towards the bromine atom. Hence, it is likely that this is the bond that will be broken most readily. Write down the first step of the SN1 reaction in which the carbon/heteroatom bond is broken, using curly arrows to indicate the movement of electrons. Identify the products. 

The sulphur that occupies a terminal position is in a similar configuration to one of the oxygens. However, it has two covalent homoatomic bonds to the central sulphur and so its oxidation number is zero. The central sulphur has four heteroatomic bonds to oxygen atoms and in each case they are polarised away from the sulphur, and so its oxidation number is +4. Thus, instead of each sulphur being given the same oxidation number of +2, as was the result above, a distinction is drawn between them which reflects the different chemical environments they occupy. 

The strong polarisation of the single covalent bond found in the H-F, H-Cl and H-0 bonds means that the hydrogen is quite positive. In each of these cases, there is a lone pair of electrons that exists on the other atom, i.e. the heteroatom. (A heteroatom is any other atom apart from hydrogen or carbon.) A lone pair of electrons is a region of space that is rich in electrons. Suggest how in the case of HC1, there may be an interaction between two different molecules of HC1. 

Anisotropic double bonds have been scored on the basis of each C=N contributing 1.5, each C=0 scoring +2 (and also N=N scoring +2). This reflects the heteroatom s increasing ability to polarise the double bonds. 

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