The influence of a-halogen groups

This well-known effect has been extensively investigated in the Raman field [20]. It is applicable to all types of carbonyl absorption, and Hampton [21] and Hartwell, Richards and Thompson [12] have observed a similar effect for esters. Studies were first concentrated on the sterol series by Woodward [22], Djerassi and Scholz [23], Dickson and Page [113], and especially by Jones et al. [9, 10,63,83, 119]. In these systems the individual rings are rigidly locked in chair or boat forms, so they are particularly well suited to the study of the stereochemical effects of different halogen configurations. 

In general, they found that substitution of an a-bromo-group results in a rise in the C=0 frequency of about 20cm . Thus non-conjugated 3-keto-steroids absorb at 1719—1715 cm , whereas 

2- or -4-bromo-3-keto-steroids absorb near 1735 cm . Similarly, A -3-keto-steroids absorb at 1684—1680 cm due to the conjugation, and this is raised to 1697 cm in A -2- or -4-bromo- 

This displacement of the CO band is increased by halogen substitution on both sides of the carbonyl group, so that 2 4-dibromo-3-ketones absorb near 1760cm , the second frequency increment being the same as the first. However, the substitution of two bromines on the same a-carbon atom has very little more effect than one, and 2-bromocholestanone-3, and 2 2-dibromocholestanone-3 both absorb at 1739—1735 cm . This 

Thus bromine substituted at an a-equatorial position in a ring with a chair configuration will raise the frequency of an adjacent CO group by 20 cm . On the other hand, substitution in a polar position does not result in any alteration in the frequency [63]. Similarly a C20 carbonyl group has its frequency increased by 20 cm by bromine substitution at C21, but the frequency remains unchanged when bromine is substituted at Ci 7 a. 

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Electronic effects that modify the polarity of a bond have an influence on chemical shifts. If the position of a methyl group in an alkyl halide CH3-X is compared to that of methane, the shift in signal increases with the electronegativity of the halogen atom (Table 9.3). 

The exclusive ortho selectivity observed in the above chlorosulfonation followed earlier observations54 that the para directive influence of a halogen on sulfonation exceeds that of an alkyl group. 

The introduction of a halogen at the a-position in an acceptor aldehyde influences the course of the cross-aldol reaction by imposing steric constraints and by activating the carbonyl group towards electrophilic attack [144]. As a result, a-haloaldehyde preferentially reacts as an acceptor in L-proline-catalyzed reactions, affording aldols with rzwft-stereoselectivity. 

Almost comparable with the activating influence of a nitro group is that of the ring nitrogen atom in pyridine, and many examples of the introduction of amino groups by nucleophihc displacement of halogen atoms from the a- and y-positions are known (reaction 55) Cor-... 

Low rotational viscosities, which are necessary for outdoor or video applications of liquid crystal displays, are obtained in mixtures with unpolar substances having short side chains such as, for example, dialkylcy-clohexylphenyls or the corresponding alkenyl compounds [83, 84]. Lateral substitUT ents lead to a viscosity increase as well as the substitution of hydrogen atoms at the benzene ring by halogens. The effect is small for fluorine atoms and increases in the sequence Flateral cyano group is more pronounced. 

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