Alkyl halides elimination reactions

The remainder of Chapter 7 is devoted to a discussion of the substitution reactions of alkyl halides. Elimination reactions are discussed in Chapter 8. 

The addition of HX (X = C1, Br, I) to an alkene, to form alkyl halides, occurs in two steps. The first step involves the addition of a proton (i.e. the electrophile) to the double bond to make the most stable intermediate carbocation. The second step involves nucleophilic attack by the halide anion. This gives a racemic alkyl halide product because the carbocation is planar and hence can be attacked equally from either face. (These addition reactions are the reverse of alkyl halide elimination reactions.)... 

Alkenes are prepared by P elimination of alcohols and alkyl halides These reactions are summarized with examples m Table 5 2 In both cases p elimination proceeds m the direction that yields the more highly substituted double bond (Zaitsev s rule)... 

The reaction is of the 8 2 type and works best with primary and secondary alkyl halides Elimination is the only reaction observed with tertiary alkyl halides Aryl and vinyl halides do not react Dimethyl sulfoxide is the preferred solvent for this reaction but alcohols and water-alcohol mixtures have also been used... 

It is a reaction of wide scope both the phosphite 1 and the alkyl halide 2 can be varied. Most often used are primary alkyl halides iodides react better than chlorides or bromides. With secondary alkyl halides side reactions such as elimination of HX can be observed. Aryl halides are unreactive. 

Better results are obtained if the alkyl halide is primary, in case of secondary and tertiary alkyl halides, elimination competes over substitution, if a tertiary alkyl halide is used, an alkene is the only reaction product and no ether is formed. For example, the reaction of CHsONa with (CHsJaC-Br gives exclusively 2-methylpropene. 

Oxidation of alkyl halides via reaction with A -triflyl hydrazides exploits the facile elimination of triflinic acid [74]. Acidification of N-H and C-H by a P-donor is essential to the elimination and the prototropic shift. 

The target is now cyclopentanol. Alcohols can be prepared from alkyl halides by reaction with hydroxide ion as the nucleophile. Again, however, the combination of a strongly basic nucleophile and a secondary alkyl halide will result in an unacceptable amount of elimination. A better plan is to treat bromocyclopentane with acetate ion in an aprotic solvent such as DM SO. followed by cleavage of the ester to cyclopentanol ... 

Figure 14.4. The H2 reaction of alkyl halides ////-elimination. Hydrogen and the leaving group, X, are as far apart as possible, in the anti relationship.

When we want the product of a substitution reaction, elimination is a nuisance to be avoided. But when we want an alkene from an alkyl halide, elimination is what we are trying to bring about. To do this, we use a solvent of low polarity, and a high concentration of a strong base concentrated alcoholic potassium hydroxide. 

Alkyl sulfonates offer a very real advantage over alkyl halides in reactions where stereochemistry is important this advantage lies, not in the reactions of alkyl sulfonates, but in their preparation. Whether we use an alkyl halide or sulfonate, and whether we let it undergo substitution or elimination, our starting point for the study is almost certainly the alcohol. The sulfonate must be prepared from 4he alcohol the halide nearly always will be. It is at the alcohol stage that any resolution will be carried out, or any diastereomers separated the alcohol is then converted into the halide or sulfonate, the reaction we are studying is carried out, and the products are examined. 

The E2 mechanism is a concerted one-step process in which a nucleophile abstracts a hydrogen ion from one carbon while the halide is leaving from an adjacent one. The Ei mechanism is two-steps and involves a carbocation intermediate formed upon departure of the halide ion in the first step. E2 reactions are bimolecular and the reaction rate depends on the concentrations of both the alkyl halide and nucleophile. E1 reaction rates depend on the slowest step, formation of the carbocation, and are influenced only by the concentration of the alkyl halide the reaction is unimolecular. E2 reactions involve anti elimination and produce a specific alkene, either cis or trans. E1 reactions involve an intermediate carbocation and thus give products of both syn and anti elimination. 

Reagents that are classified as bases can be used with both Lewis and Brpnsted-Lowry acids and one application is the elimination of alkyl halides (E2 reactions sec. 2.9.A) in the presence of a base. Elimination involves conversion of a saturated moiety containing a leaving group to a molecule with a multiple bond. An example of this latter transform is ... 

C-H bond breaks and both electrons are used to form a n bond to neighbouring carbocation. The first step of the reaction mechanism is the rate-determining step and as this is dependent only on the concentration of the alkyl halides, the reaction is first order (El = elimination first order). There is no stereospecificity involved in this reaction and a mixture of isomers can be obtained with the more stable (more substituted) alkene being favoured. 

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