Halogen-bearing carbon

In the overall Wittig reaction, an alkene is formed from the aldehyde or ketone and an alkyl halide in which the halogen-bearing carbon contains at least one hydrogen ... 

Alkyl halides with an electron-withdrawing group on the halogen-bearing carbon can be dimerized to alkenes by treatment with bases. The Z group may be nitro, aryl,... 

Monohaloalkanes can be classified as primary, secondary or tertiary according to the number of alkyl groups attached to the halogen-bearing carbon atom. Remember that monohalomethanes are classed as primary monohaloalkanes. 

The size of the alkyl groups in the haloalkane is Important. This is known as a steric effect. You will recall that, in the mechanism, the nucleophile attacks the carbon atom of the C-X bond from the side opposite to the halogen atom. In the case of a tertiary haloalkane, attack from that side is likely to be sterlcally hindered because three bulky alkyl groups will limit access to the atom. Hence tertiary haloalkanes are unlikely to react with nucleophiles via an Sj. 2 mechanism. Primary haloalkanes, on the other hand, have no more than one alkyl group attached to the halogen-bearing carbon atom and so access to the atom will be much easier. This suggests that primary haloalkanes are most likely to react with nucleophiles via an S. j2 mechanism. 

The basic OH" ion initially attacks an H atom on the carbon atom adjacent to the halogen-bearing carbon atom in the haloalkane. It forms a bond with this H atom and an HO-H molecule is generated. At the same time, the pair of electrons in the C-H bond moves between the two carbon atoms on the left-hand side of the haloalkane to form a double bond. Finally, the C-Br breaks heterolytically, releasing a Br" ion. You are not required to know this mechanism, but it helps to explain why the elimination reaction is referred to as base-induced. 

The formation of adduct 40 appears to be favored with respect to attack at the halogen-bearing carbon atom. By a further detailed l3C-NMR study, the adducts of this type, as derived from 4-chIoro-2-dimethylaminopyrimidine and its 5-phenyl derivative, are reported to be the primary intermediates of a process leading to the final triazines via a number of identified ring-opening intermediates.101... 

Nucleophilic substitution may occur by two mechanisms. The SN2 mechanism is a one-step process. Its rate depends on the concentrations of substrate and nucleophile. If the halogen-bearing carbon is stereogenic, substitution occurs with inversion of configuration. The reaction is fastest for primary halides and slowest for tertiary halides. 

Two suitably positioned nitro groups make the halogen-bearing carbon atom in 2,4-dinitro-halobenzenes a favored point of reaction for nucleophilic substitution reactions. Thus, 2,4-dinitrophenyl hydrazine is produced from the reaction of 2,4-dinitrochlorobenzene with hydrazine ... 

These terms specify the substitution of the halogen-bearing carbon atom (sometimes called the head carbon). If the head carbon is bonded to one other carbon, it is primary if it is bonded to two carbons, it is secondary and if bonded to three carbons, it is tertiary, (p. 220)... 

Reaction at the meta position gives a sigma complex whose positive charge is not delocalized onto the halogen-bearing carbon atom. Therefore, the meta intermediate is not stabilized by the halonium ion structure. The following reaction illustrates the preference for ortho and para substitution in the nitration of chlorobenzene. 

As is seen from the foregoing examples, 1-halolithiumallylidene carbenoids are preferably alkylated at the halogen-bearing carbon atom, i.e. they react as cF-reagents1. ... 

If we view the Wittig reaction from an alkyl halide starting material (alkyl halide phosphonium salt phosphorus ylid alkene), the halogen-bearing carbon of an alkyl halide must contain at least one hydrogen as in 59 (for deprotonation at the phosphonium salt stage). 

The reaction of the e t/o-halo ketone, e.g. endo-, is simple, leading exclusively to the ring-contracted bicyclo[3.1.0]hex-2-ene-en(fo-6-carboxylic acid endo- l, R = H) in 97% yield, or derivatives thereof. The rearrangements occur stereospecifically with inversion of configuration at the halogen-bearing carbon. ... 

Bromine and iodine also direct lithiations, but isomerisation - halogen dance (see discussion in 17.4.2) - can be a problem, however advantage can be taken of the isomerisation (to the more stable hthio-derivative) in suitable cases. In the example below, the more stable lithio compound is that in which the formally negatively charged ring carbon is located between two halogen-bearing carbon atoms. ... 

UPAC name common name degree of halogen-bearing carbon... 

On the basis of the interactions observed in the enzyme—product complex and extensive mechanistic studies, a multistep catalytic mechanism for the 4-CBA-CoA dehalogenase-catalyzed reaction was proposed (Figure A key catalytic task for the enzyme is to activate the halogen-bearing carbon atom of the... 

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