Dehydrohalogenation Elimination reactions

Bordwell, P. G., Doomes, E. Driving forces for 1,3-elimination reactions. Dehydrohalogenation of 1-halo-2-thia-2,3-dihydrophenalene 2,2-dioxides in a Ramberg-Backlund reaction. J. Org. Chem. 1974, 39, 2531-2534. 

An elimination reaction, dehydrohalogenation, can occur for chloro-, bromo- and iodoalkanes. In such a reaction, the halogen, X, from one C atom and a hydrogen from an adjacent C atom are eliminated. A double bond between two carbon atoms is formed the molecule becomes more unsaturated. The net reaction is the transformation of an alkyl halide (or haloalkane) into an alkene. Dehydrohalogenation reactions usually require a strong base such as sodium hydroxide, NaOH. 

Hence, the elimination reaction (dehydrohalogenation) of cis-1,2-dichloroethene [Eq. (7-9)] is much more facile than that of the trans isomer because trans elimination can occur with the former but not the latter.32 Subsequently, chloroethyne can react with superoxide ion via nucleophilic attack to give the observed products (Table 7-1). 

Elimination teactions are the most important means for synthesizing alkenes. In this chapter we shall study two methods for alkene synthesis based on elimination reactions dehydrohalogenation of alkyl halides and dehydration of alcohols. 

The E2 Elimination Reaction Dehydrohalogenation of 2-Bromobutane to Yield 1-Butene, trans-2-Butene, and c s-2-Butene... 

Double dehydrohalogenation of gemmal dihalides (Section 9 7) An E2 elimination reaction of a gemmal dihalide yields an alkenyl halide If a strong enough base IS used sodium amide for example a second elimination step follows the first and the alkenyl halide IS converted to an alkyne... 

Other Syntheses. Acryhc acid and other unsaturated compounds can also be made by a number of classical elimination reactions. Acrylates have been obtained from the thermal dehydration of hydracryhc acid (3-hydroxypropanoic acid [503-66-2]) (84), from the dehydrohalogenation of 3-halopropionic acid derivatives (85), and from the reduction of dihalopropionates (2). These studies, together with the related characterization and chemical investigations, contributed significantly to the development of commercial organic chemistry. 

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. 

The two most common elimination reactions arc dehydroUalogenalion—the loss of HX from an alkyl halide—and dehydration—(he loss of water from an alcohol. Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base such as potassium hydroxide. For example, bromocvclohexane yields cyclohexene when treated with KOH in ethanol solution. 

Hydrogen bromide is eliminated from 10,11-dibromo-l 0,1 l-dihydrodibenz[7>,/]oxepin with potassium tert-butoxide at room temperature to give 10-bromodibenz[i,/]oxepin (17a).160161 When the elimination reaction was performed in boiling toy-butanol the yield increased from 58 to 92%.261 Dehydrohalogenation of 10-chloro-2,3-dimethoxy-10,ll-dihydrodi-benz[/),/]oxepin afforded 2,3-dimethoxydibenz[6,/]oxepin (17b) in 52% yield.162... 

As has been mentioned previously, one is most likely to find analogies to catalytic reactions on solids with acidic and/or basic sites in noncatalytic homogeneous reactions, and therefore the application of established LFERs is safest in this field. Also the interpretation of slopes is without great difficulty and more fruitful than with other types of catalysts. The structure effects on rate have been measured most frequently on elimination reactions, that is, on dehydration of alcohols, dehydrohalogenation of alkyl halides, deamination of amines, cracking of the C—C bond, etc. Less attention has been paid to substitution, addition, and other reactions. 

Alkyl halides typically undergo elimination reactions when heated with strong bases, typically hydroxides and afkoxides, to generate alkenes. Removal of a proton and a halide ion is called dehydrohalogenation. Any base in the reaction mixture (H2O, HS04 ) can remove the proton in the elimination reaction. 

Some elimination reactions which yield ynamines involve the dehalogenation or dehydrohalogenation of substituted amines or amides as shown in Eq. (21). 

The elimination reactions involving dehalogenation, dehydrohalogenation, and dehydration are often laborious compared to the more recent techniques involving dehalogenation of gem-dihalocyclopropanes [10a, b]. However, the availability of the starting materials is the deciding factor. 

Benzeneselenenyl trichloride, 27 Platinum-Titanium, 251 Titanium(IV) chloride-Zinc, 310 Dehydrohalogenation (see Elimination reactions)... 

Roberts et al. (1993) have provided an in-depth analysis of QSARs for dehydrohalogenation reactions of polychlorinated and polybrominated alkanes. The QSARs were developed based on a dataset of 28 polychlorinated and polybrominated compounds in aqueous solution at 25° C. The QSARs are for the OH- and water-mediated second-order elimination reactions (E2), as Equation (25) shows. The first QSAR is for base-promoted dehydrochlorination and is based on the inductive parameter o, (Equations (26) and (27)) ... 

Nucleophilic Catalysis. The presence of other ions in solution can influence the rates of substitution and elimination reactions in two principal ways. First, various anions may increase the rates of substitution reactions in proportion to their respective nucleophilicities (22.32). an effect referred to here as "nucleophilic catalysis." This phenomenon has been documented in a number of laboratory studies of nucleophilic substitution and dehydrohalogenation reactions which employed a phosphate buffer to maintain constant pH in solution (5.331. Using the hydrolysis of a halo-organic compound by HPO42- as an example, this mechanism may be portrayed as follows (3.341 ... 

Elimination reactions of alkyl halides (dehydrohalogenations) are a useful method of synthesising alkenes. For good yield, a... 

Dehydrohalogenation, another abiotic reaction of importance in the subsurface, is an elimination reaction involving halogenated alkanes in which a halogen is removed from one carbon atom, with removal of a hydrogen atom from an adjacent carbon atom and formation of a double bond, as [25]... 

In the elimination (a dehydrohalogenation), the reagent (BT) reacts as a base, abstracting a proton from the alkyl halide. Most nucleophiles are also basic and can engage in either substitution or elimination, depending on the alkyl halide and the reaction conditions. 

The mechanism of the above elimination reaction is similar to the E2 mechanism of dehydrohalogenation. The reaction proceeds by nucleophilic attack of a base on silicon, simultaneous elimination of halide ion and formation of double bond (Scheme 4.4). 

Cheleotropic fragmentation a fragmentation reaction in which two a bonds to a single atom are broken Dehydrohalogenation an elimination reaction in which the elements of a hydrogen halide molecule are eliminated from adjacent atoms leaving a n bond... 

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