Substituent groups halogenation

A halogen atom directly attached to a benzene ring is usually unreactive, unless it is activated by the nature and position of certain other substituent groups. It has been show n by Ullmann, however, that halogen atoms normally of low reactivity will condense with aromatic amines in the presence of an alkali carbonate (to absorb the hydrogen halide formed) and a trace of copper powder or oxide to act as a catalyst. This reaction, known as the Ullmant Condensation, is frequently used to prepare substituted diphenylamines it is exemplified... 

Category Ic cyclizations involve formation of the C3-C3a bond and require aniline derivatives with a nitrogen substituent appropriate for such reaction. Some, but not all, such cyclizations also require an o-substituent, frequently halogen. The rctrosynthetic transformations corresponding to the most important of this group of syntheses are shown in Scheme 4.1... 

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... 

The effect of the substituent group can be seen in the reactions of the following senes of halogenated esters Methyl 2,3 dichloro-3-fluoro-2-methylpropionate and chlorine monofluoride first form methyl 2-chloro-3,3-difluoro-2-methylpro-pionate An additional equivalent of chlorine monofluoride produces methyl 2 methyl-2,3,3-trifluoropropionate (equation 14)... 

Studies of chlorination and bromination of 2//-cyclopenta[reactivity differences dependent on substituents and halogenation conditions. In monochlorination the unsubstituted compound was more reactive than its 2-methyl and 2-phenyl derivatives, the reactivity ratio being 7.1 1.7 1 [78H(11)155]. Chlorination occurred most readily in the 5- and 7-positions of the cyclopentadienyl moiety, but once all three positions had been substituted, NCS attacked the methyl group... 

A variety of alternative reactions occur when there is a substituent para to the nitro-group. Halogen [77] and carboxyl groups [78] are eliminated while the methyl... 

The effects of other substituent groups on the reactivity of ring halogen atoms are predictable. Electron-donating substituents make nucleophilic substitution more difficult while electron-withdrawing substituents facilitate the process. The effect of an electron-withdrawing substituent which is so positioned that it can participate in resonance stabilization of the intermediate adduct can be considerable. 

There have been several further examples of the replacement of hydroxyl or oxo substituents with halogens in 1,7-phenanthrolines using phosphorus halides or oxyhalides.96,169-171,215 In 4,10-dioxo-l,4,7,10-tetrahydro-1,7-phenanthroline (100) there are marked differences in the ease with which the two oxo groups are replaced by halogen.171 With 1 mole of phosphorus pentachloride, the product is 4-chloro-10-hydroxy -... 

On the other hand, a proton directly bound to the 1,2,4-triazine ring can be replaced by halogen. This is an electrophilic substitution reaction at carbon, and, as expected for such a heavily aza-substituted ring system, it needs considerable activation by electron-donating substituents. The halogenation reaction is best known in bromination at the 6-position. From the published data it seems that either an amino group in the 3-position or an oxo group in the 5-position is necessary. The formation of 6-halo-substituted compounds has been reported for l,2,4-triazin-5-ones, l,2,4-triazine-3,5-diones, 3-amino-l,2,4-triazines and 3-amino-l,2,4-triazin-5-ones. 

Structures are arranged in this section according to the kind of atom attached to the cyclopropane ring. Substituted derivatives with an sp3, sp2 and sp carbon atom are followed by those with halogens, oxygen, nitrogen etc. Mixed derivatives are discussed under the substituent group of our primary interest. 

When a hydrocarbon is substituted with other than alkyl groups a new problem arises, which can be illustrated by CH3CH2C1. This substance can be called either chloroethane or ethyl chloride, and both names are used in conversation and in print almost interchangeably. In the IUPAC system, halogens, nitro groups, and a few other monovalent groups are considered to be substituent groups on hydrocarbons and are named as haloalkanes, nitro-alkanes, and so on. 

Electronegativities have also been estimated for various common substituent groups. They are arranged in Table 3.2B in clusters of related residues. The values for the individual halogens are from Table 3.2A. 

Mixed-substituent polyphosphazenes are also accessible by utilizing the different reactivities of halogens according to their location in the polymer. For example, the chlorine atoms in the side groups of phosphazo polymers (3.5 and 3.6) are more reactive than those linked directly to the main chain hence, the side-group halogens react first,... 

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