Leaving groups halides

What are the implications of the effects of the various halide leaving groups Because the halogen with the weakest bond to carbon reacts fastest Ingold concluded that the carbon-halogen bond breaks m the rate determining step The weaker the carbon-halogen bond the easier it breaks... 

Next in what amounts to an intramolecular Williamson ether synthesis the alkoxide oxygen attacks the carbon that bears the halide leaving group giving an epoxide As m other nucleophilic substitutions the nucleophile approaches carbon from the side oppo site the bond to the leaving group... 

Overall the stereospecificity of this method is the same as that observed m per oxy acid oxidation of alkenes Substituents that are cis to each other m the alkene remain CIS m the epoxide This is because formation of the bromohydrm involves anti addition and the ensuing intramolecular nucleophilic substitution reaction takes place with mver Sion of configuration at the carbon that bears the halide leaving group... 

The generally accepted mechanism for nucleophilic aromatic substitution m nitro substituted aryl halides illustrated for the reaction of p fluoromtrobenzene with sodium methoxide is outlined m Figure 23 3 It is a two step addition-elimination mechanism, m which addition of the nucleophile to the aryl halide is followed by elimination of the halide leaving group Figure 23 4 shows the structure of the key intermediate The mech anism is consistent with the following experimental observations... 

Nitro substituted aromatic compounds that do not bear halide leaving groups react with nucleophiles according to the equation... 

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

Elimination bimolecular (E2) mechanism (Section 5.15) Mechanism for elimination of alkyl halides characterized by a transition state in which the attacking base removes a proton at the same time that the bond to the halide leaving group is broken. 

Yields are always almost quantitative within 1-2 min, irrespective of chain length and the nature of the halide leaving groups. 

As one of the most reactive groups of carboxylic acid derivatives, acyl halides are very useful substrates for the preparation of the other classes of derivatives. For example, anhydrides may be synthesized by the reaction of carboxylic acid salts with an acyl halide. In this reaction, the carboxylate anion acts as the nucleophile, eventually displacing the halide leaving group. 

Halodiazines react readily with nucleophiles with displacement of the halide leaving group. This follows what we have seen with halopyridines (see Section 11.4.1), but the halodiazines are more... 

Figure 5-83. The formation of a macrocyclic complex by the formation of S-S disulfide bonds. The co-ordination of the halide leaving groups to the metal is common in reactions of this type.

Internal cyclization to the oxirane (7) rather than to the substitution product (8) is found in the alkaline epoxidation of tetraoyanoethylene (Linn et al., 1965). This may be due to a longer life-time of the intermediate (6) with the cyano leaving group, compared to carbanions with halide leaving groups. 

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