Aliphatic halogen compounds elimination reactions

Reaction CXXXIll. Action of Aromatic Halogen Compounds on Ammonia or Amino Compounds.—Aliphatic halides readily react with ammonia and amines according to Reaction CXXXII. Aromatic halides are less reactive, unless negative groups are also present. The addition of copper powder or cuprous halide greatly accelerates the elimination of halogen hydride. 

Volume 9 deals with the majority of addition and elimination reactions involving aliphatic compounds. Chapter 1 covers electrophilic addition processes, mainly of water, acids and halogens to olefins and acetylenes, and Chapter 2 the addition of unsaturated compounds to each other (the Diels-Alder reaction and other cycloadditions). This is followed by a full discussion of a-, y- and S-eliminations (mainly olefin and alkyne forming) and fragmentation reactions. In Chapter 4 carbene and carbenoid reactions, and in Chapter 5 alkene isomerisation (including prototropic and anionotropic, and Cope and Claisen rearrangements), are discussed. 

The organization is fairly classical, with some exceptions. After an introductory chapter on bonding, isomerism, and an overview of the subject (Chapter 1), the next three chapters treat saturated, unsaturated, and aromatic hydrocarbons in sequence. The concept of reaction mechanism is presented early, and examples are included in virtually all subsequent chapters. Stereoisomerism is also introduced early, briefly in Chapters 2 and 3, and then given separate attention in a fuU chapter (Chapter 5). Halogenated compounds are used in Chapter 6 as a vehicle for introducing aliphatic substitution and elimination mechanisms and dynamic stereochemistry. 

In unsaturated aliphatic systems the most important reactions are those of czirbonyl compounds of the type —COX, in which X is a good leaving group such as halogen. In general, most displacement reactions of anionic nucleophiles on the carbonyl carbon atom of acyl halides involve an addition-elimination mechanism " (e.g. equation 16). In such reactions bond-formation is in advance of bond-rupture... 

An attractive pathway with a lot of potential uses the transition metal mediated reaction of organic halides with carbon monoxide. Suitable substrates are organic halides capable of oxidative addition to low-valent transition metal compounds. Insertion of carbon monoxide and reductive elimination of an acid halide will complete the catalytic cycle. In tins way it was shown tiiat allyl chloride yields butenoic acid chloride in >80% yield accor g to equation 22)P As well as palladium, rhodium and iridium also act catalytically. It is of no surprise that allylic halides, benzylic halides and aryl halides in particular are readily converted to acid halides. Simple aliphatic halid undergo the oxidative addition step more slowly and, if they cany hydrogen atoms on an sf hybridized C atom in the -position to the halogen atom, may give alkenes via 3-hydrogen elimination. Alkenes can also be converted to acid halides widi carbon monoxide in the presence of transition metal catalysts in solvents such as methylene chloride or tetrachloromethane. ... 

A considerable number of ring-fluorinated diazines undergoes various nucleophilic aromatic substitution reactions. Nucleophilic aromatic substitution reactions follow the well-established two-step addition-elimination mechanism via a Meisenheimer intermediate. The destabilization of sp -C bound fluorine by p-Jt repulsion activates fluorinated aromatic compounds toward nucleophilic attack and subsequent substitution. The susceptibility of the carbon center toward nucleophiles is also enhanced by the negative inductive (-la) effect of fluorine. Therefore the ease of nucleophilic halogen replacement - F>Cl>Br>I - is in the opposite order to that for aliphatic... 

As noted in Chapter 9, conventional nucleophilic substitution reactions of the type described for aliphatic compounds don t occur at sp hybrid carbon atoms. The formation of aryl cations is disfavored because they are unstable, and attack from the backside of the C-halogen bond is rendered impossible by the ring structure. So the substitution of aryl halides is much more difficult, and the mechanism of the S).j2Ar reaction is quite different, involving addition followed by elimination—a two-step process. For example, although substitution of chlorobenzene by hydroxyl ion is possible, conditions are harsh (350 °C, high pressure), and even then, yields are low. The reaction (Figure 13.13) probably involves a benzyne intermediate (see Sections 10.9 and 13.6). 

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