Halogens as Substituents

Attempts to explore the curious reactivity sequences for the halogens as substituents have prompted the examination of many substitution reactions (Tables 13-16). 

A spiro[5,5]undecane skeleton characterizes the chamigranes which formaUy originate from linking the bonds C-l-C-6 and C-6-C-11 of famesane. More than 50 naturally occurring representatives with halogens as substituents are predominantly... 

When an alkane is substituted with one or more halogen atoms, then the halogens are specified as substituents, and the halo substituents take precedence in numbering the chain over alkyl substituents. Halogens as substituents are named as chloro-, bromo-, etc. Thus, 2.23 is 3-bromo-heptane, and 2.24 is 2-chloro-7-methyloctane. 

Substitutive nomenclature of alkyl halides treats the halogen as a halo—(fluoro chloro bromo or lodo ) substituent on an alkane chain The carbon chain is numbered m the direction that gives the substituted carbon the lower number... 

Although members of the class are commonly called alkyl halides, they are named systematically as haloalkanes (Section 3.4), treating the halogen as a substituent on a parent alkane chain. There are three steps ... 

It is also possible to prepare FOSS species with halogen-terminated substituents using hydrosilylation in good-to-excellent yields (see Table 7). For example, Liu and Dare have prepared a wide range of bromo- and chloro-terminated compounds with long alkyl chains using H2PtCl6 or Ft/C as catalysts (Table 7, entries 2-7). Dare et al. have studied the hydrosilylation of a chlorohexyne however, two isomers were obtained which could not be separated (Table 7, entry 8). 

Halogens (F, Cl, Br, I) are usually not named in the suffix of a compound. They get named as substituents, which we will see later on. 

Halogens are named as substituents in the following way fluoro, chloro, bromo, and iodo. Essentially, we add the letter 0 at the end to say that they are substituents. If there are multiple substituents of the same kind (for example, if there are five chlorine atoms in fhe compound), we use fhe same prefixes fhaf we used earlier when classifying fhe number of double and friple bonds ... 

No conformational dependence (NCD). Groups of this type include monatomic substituents such as hydrogen and the halogens cylindrical substituents such as the ethynyl and cyano groups, and tetracoordinate symmetric top substituents such as the methyl, trifluoromethyl and silyl groups. 

Stereocontrol in intermolecular cyclopropanation also depends on the structure of the unsaturated substrate. Early work concerning the influence of substrate on stereoselectivity has been summarized by Doyle2. In general, cyclopropanation of ciy-disubstituted alkenes results in higher stereoselectivity than with monosubstituted alkenes and the steric bulk of the olefinic substituent enhances the stereoselectivity. However, the stereocontrol appears not simply to be caused by a steric factor. In comparable cases, the presence of halogen as an alkene substituent may cause a reversal of the normal stereoselectivity. A few examples which illustrate these effects are shown in equations 124167 172, 74. 

Arenediazonium tetrafluoroborates react with PhS- ions in DMSO to give diaryl sulphides by the S l mechanism284. However, when the aromatic ring has halogen as a substituent, as in 233, the monosubstitution product 234 and the disubstitution product 235 were obtained depending on the halogen and its position (equation 154). 

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