Halogens important reactions

Fig. 3. Important reactions of PCl, where X is halogen and R is alkyl. Many of these reactions are typical for other PX as weU as POX compounds.

Other than nucleophilic addition to the carbonyl group, the most important reactions of aldehydes and ketones involve replacing an a hydrogen. A par ticularly well studied exfflnple is halogenation of aldehydes and ketones. 

An important reaction of silicon compounds containing electronegative substituents, such as the halogens or a Group 15 or 16 element, is hydrolysis, which often occurs very readily even with atmospheric moisture. The primary reaction in silicone production is the hydrolysis of halosilanes, usually the readily available chlorosilanes, to give silanols, which then undergo acid-catalyzed condensation with loss of water, as shown in Scheme 1. 

Nucleophilic substitution with heteroaryl halides is a particularly useful and important reaction. Due to higher reactivity of heteroaryl halides (e.g. 35, equation 24) in nucleophilic substitution these reactions are widely employed for synthesis of Al-heteroaryl hydroxylamines such as 36. Nucleophilic substitution of halogen or sulfonate functions has been performed at positions 2 and 4 of pyridine , quinoline, pyrimidine , pyridazine, pyrazine, purine and 1,3,5-triazine systems. In highly activated positions nucleophilic substitutions of other than halogen functional groups such as amino or methoxy are also common. 

The expression element/lithium exchange has been proposed by us in analogy to the expression halogen/lithium exchange and in order to separate this important reaction type from the numberless transmetalation reactions. 

Chlorine or bromine reacts with alkanes in the presence of light (hv) or high temperatures to give alkyl halides. Usually, this method gives mixtures of halogenated compounds containing mono-, di-, tri- and tetra-halides. However, this reaction is an important reaction of alkanes as it is the only way to convert inert alkanes to reactive alkyl halides. The simplest example is the reaction of methane with CI2 to yield a mixture of chlorinated methane derivatives. 

This isomerization reaction allows the synthesis of fluorohaloalkanes containing tri-fiuoromethyl groups, which are often not directly accessible by halogen-exchange reactions. These reactions are commercially important in the production of replacements for chloro-fiuorocarbons (CFCs) (see Vol. ElOa, p 65). Much of the work in this area has been published only in the patent literature. 

One of the more important reactions of organohalogen compounds is the formation of organometallic compounds by replacement of the halogen by a metal atom. Carbon is positive in carbon-halogen bonds and becomes negative in carbon-metal bonds, and therefore carbon is considered to be reduced in formation of an organometallic compound (see Section 11-1) ... 

The enolization process, i,e. conversion of a carbonyl compound such as 438 into the intermediate enol 439 or enolate anion 440 is an important reaction in organic chemistry because these intermediates can further react with electrophiles to undergo either protonation, halogenation, alkylation, aldo-lization, or acylation type reactions. 

One of the most important reactions in arene synthesis is the halogenation of arenes. Conventionally, these reactions are performed directly by bromine or chlorine [7]. However, on the laboratory scale chlorine is not easily manageable and is a toxic gas. Therefore, it is not often used in academic research. For iodinations normally a strong oxidizing agent is required. In halogenations, often unintentional side-chain... 

Halogen-exchange reactions provide important routes to halides. For a reaction such as ... 

Exchange involving boron-halogen bonds and boron-oxygen bonds has been observed in the synthetically important reactions for the preparation of alkyldihaloboranes from alkylboroxines and boron trihalides (165, 167, 169). 

The important reactions of alkyl halides are, (a) nucleophilic substitution in which an attacking nucleophile replaces the halogen [Following fig. (a), and (b) elimination in which the alkyl halides loses HX and gets converted to an alkene [Following fig. (b)[. 

Chemical Properties. The most important reactions which tetraorganotins undergo are heterolytic, ie, electrophilic and Kocheshkov redistribution (81—84). The tin—carbon bond in tetraorganotins is easily cleaved by halogens, hydrogen halides,... 

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