Halogen substituents molecular orbital effect

The electrophilic nature of sulfuryl chloride is validated in a study on the chlorination of anthracene derivatives the observed substituent effects conform well with molecular orbital modeling859. Chlorination of ethyl 3,4,5-trialkylpyrrole-2-carboxylates results in a substantial amount of / -side-chain chlorinated products in addition to the expected a-side-chain chlorination860. This has been ascribed to the electrophilic nature of the reagent861 and the mechanism proposed involves an allylic migration of the halogen from the a-sigma adduct to the adjacent / -side chain. 

Bonds become weaker as we move down the Periodic Table. Compare C O and C S, or the carbon-halogen bonds C—F, C—Cl, C—Br, C—1. This is a consequence of the first generalization, since bond distances must increase as we go down the periodic table because the number of inner electrons increases. However, it is noted that high-level ab initio molecular-orbital calculations confirm that the effect of alkyl substituents on R—X bond dissociation energies varies according to the nature of X (the stabilizing... 

These calculations show that, if vinyl alcohol and vinyl thiol indeed mainly exist in planar syn conformations, as previously stated, the situation can be very different when halogen substituents are present. In some cases, the anti conformation is more or less out-of-plane staggered and can become more stable than the syn one. The general trends of substituent effects can be discussed by molecular orbitals analysis and electrostatic interactions. The results are subject to experimental confirmation, at least in the case of enols since their conformers can be prepared by the action of singlet oxygen atom on substituted ethylen, and trapped in low temperature matrices. 

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