Steric halogen effects

Further examples of these steric halogen effects are the hydrolyses in soil degradation half-life (DT50) of the herbicidal nitriles bromoxynil (8 1962, Brominal , May Baker) [26], ioxynil (9 1962, Actril , May Baker) [27], and diclobenil (10 1960, Casoron , Philips Duphar) [28] (Fig. 35.5a). 

Similarly, the beneficial steric halogen effects of both the fluorine in the 2-position and that in the 6-position on the inhibition of insecticidal chitin syn-... 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

Substituents with -I,+M effects such as halogen show a similar orientation effect to that of alkyl groups. If the substituent is in the 2-position, substitution occurs at C-5 if it is in position 3, substitution occurs preferentially at C-2 or if steric requirements of the group or electrophile intervene, then substitution occurs at C-5. 

The reaction is quite susceptible to steric effects since hindered secondary hydroxyl groups were found to be unreactive. The method can therefore be used to selectively replace a primary hydroxyl group by halogen in the presence of more hindered secondary hydroxyl groups in the same molecule. An example (70) is the reaction of 52 with triphenylphosphite methiodide which affords the 6-deoxy-6-iodo derivative 53 (60%) in which the C-2 hydroxyl group remains intact. 

It was postulated that the rate decreased as the basicity of the halogen decreased and/or steric compression increased in f-BuX, and as the polarizability of halogen in MeX increased. The objective of the present research was to extend this model study to isobutylene polymerization systems, in particular to investigate the effect of reagent addition sequence and that of the nature of the halogen in f-BuX and MeX on the polymerization rate and PIB yield using Me3 Al coinitiator. 

It is also important to note that trans-alkenes are often more stable than cis-alkenes due to diminished steric hindrance (p. 190), but this is not always the case. It is known, for example, that c -l,2-difluoroethene is thermodynamically more stable than tra j-l,2-difluoroethene. This appears to be due to delocalization of halogen lone-pair electrons and an antiperiplanar effect between vicinal antiperiplanar bonds. 

An investigation of the competing halogen transfer from BrCCl3 and CCl45, 79 has shown that steric effects are also of importance in atom transfer reactions to alkyl and aryl radicals. Giese80 investigated very carefully the temperature depen-... 

The addition of halogens and halogen acids to alkenes has been shown to be predominantly trans and where the results do not agree, explanations have been given in terms of steric factors. Dewar has proposed that in all electrophilic addition reactions where a classical carbocation is formed, cis addition is the rule and where there is the preponderance of the trans product, the effect is due to steric factors. 

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