I have question about hetero-cycles: Why do we use (the most common hetero-cyclic elements) nitrogen, oxygen, sulfur as hetero compounds most of time?
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A possible reason for this is that heterocycles with O, N or S are often easier to obtain and more stable than the corresponding compounds which contain other main group elements as heteroatoms. Phosphole, for example, is less stable than furan, pyrrole and thiophene because of its lower aromaticity. Borole, the structural analog with the lighter heteroatom B, has not yet been isolated in unsubstituted form. Ring strain and the stability of X-C and X-H bonds (X = heteroatom) also need to be considered. Bond lengths tend to increase with heavier heteroatoms. With decreasing electronegativity of X in relation to C, the X-C bond becomes less covalent and is more prone to be cleaved by hydrolysis, for example.