Halogen, substituent effects upon

Substituent Effects upon Replacement of Hydrogen by Halogen... 

The magnitude and sign of substituent effects upon C chemical shifts were found to be consistent with our "chemical common sense of inductive and resonance effects. Therefore, for example, the substituent effects caused by halogens, e.g., 6(CH3l) > 6(CH3Br) > 6(CH3C1) > 6(CH3F), was considered normal. 

Up until this point, the contribution of the aforementioned workers could be characterized as the formulation of purely empirical rules based upon consideration of some, but not all, experimental evidence. The rules of Wolfe and collaborators were devoid of any theoretical basis and, thus, could never do anything more than act as basis for extrapolation from known experimental data. Thus, the variation of the anomeric effect as a function of halogen substituent alluded to before never attracted the attention of these authors simply because their rules were founded on an intuitive concept of bond polarity which deceptively seemed to be consistent with the great majority of the experimental facts encompassed by the three rules as stated. 

The effects upon C—X bonds of halogen substituents elsewhere in the molecule are particularly interesting. They will be summarized by three generalizations ... 

By far the most evidence for electrophilic sulphur is found in the sulphenyl halides (RSCl, RSBr) Although these compounds may in theory react either as sources of RS or X (X = halogen), none of the observed reactions of the sulphenyl halides indicate the latter mode of heterolysis. Kharasch et have presented good evidence for the existence of the 2,4-dinitrobenzene-sulphenium ion (Ar ) in strongly acidic media evidence has also been presented of a strong solvent effect upon the rate of reaction of 2,4-dinitroben-zenesulphenyl chloride and cyclohexene - and of a definite substituent effect in the reaction of this sulphenyl chloride with some substituted styrenes in acetic acid . Such observations are entirely consistent with an electrophilic heterolytic ad tion mechanism involving attack by the sulphenyl chloride in the sense iC -Cl. 

Locations of the halogen substituent on the carbon chain may also have some effect on the rate of reaction. The rate also may increase with increasing pH however, a rate dependence upon pH is typically not observed below a pH of 11 Rates of hydrolysis may also... 

Several 2-substituted derivatives of anthraquinones were also compared in photoactivity, subject to the nature of the light source, the amine co-synergist and the type of the monomer used. All were found to be effective triplet state initiators. The anthraquinones, however, with electron withdrawing substituents were more active. This suggests that electron transfer is also an important part of the process of initiation. In addition, when halogen substituents are present, upon irradiation they give rise to halogen radicals and further enhance the polymerization rate. ... 

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