Halogen substituents main effects

In conclusion, the difference in mechanisms for the main oxidation route of 68a (DIM2 mechanism) and 68b, 68c (e-p-RRC-p mechanism with fast deprotonation of radical cations) can be easily understood, taking into account that halogenated radical cations 68b+ and 68c+ are much stronger acids than 68a+ due to inductive and mesomeric effects of halogen substituent, as revealed from the equilibrium acidity estimated in DMSO (Table 3). 

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. 

Halogen substituents inductively withdraw electron density (Section 16-1) however, they are donors by resonance. On balance, the indnctive effect wins out, rendering haloarenes deactivated. Nevertheless, the electrophilic substitution that does take place is mainly at the... 

The temperature dependence of the rate constants of radical addition (k ) is described by the Arrhenius equation (Section 10.2). At a given temperature, rate variations due to the effects of radical and substrate substituents are due to differences in the Arrhenius parameters, the frequency factor, A , and activation energy for addition, . For polyatomic radicals, A values span a narrow range of one to two orders of magnitude [6.5 < log (A /dm3 mol-1 s-1) < 8.5] [2], which implies that large variations in fcj are mainly due to variations in the activation energies, E. This is illustrated by the rate constants and Arrhenius parameters for the addition to ethene of methyl and halogen-substituted methyl radicals shown in Table 10.1. 

By the effect of ionization irradiation side substituents are abstracted from excited macromolecules in parallel to the scission of main chains. Elimination of side substitutions such as hydrogen and halogen atoms may be the main primary process. For instance, in polyethylene, although the C —H bond is stronger than the C—C bond, excited macromolecules eliminate hydrogen atoms preferentially through a C —C bond cleavage of a macromolecular backbone. Part of the... 

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