Alkyl halides, elimination

Section 5 17 In the absence of a strong base alkyl halides eliminate by an El mech anism Rate determining ionization of the alkyl halide to a carbocation is followed by deprotonation of the carbocation... 

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... 

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. 

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

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. 

A DIFFERENT MECHANISM FOR ALKYL HALIDE ELIMINATION THE El MECHANISM... 

A Different Mechanism for Alkyl Halide Elimination The El Mechanism... 

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.)... 

Section 5.18 In the absence of a strong base, alkyl halides eliminate by an El mechanism. 

Sections 12.1 and 12.2 described the E2 reaction of acyclic alkyl halides. Elimination also occurs with cyclic halides, but the conformational demands of the ring must be taken into account (see Chapter 8, Section 8.5). The E2 reaction must proceed by an anti transition state, which imposes additional conformational demands on ring compounds. 

At equilibrium, there is a very small amount of enolate, but there is a lot of ketone and a lot of hydroxide present. So, if we introduce some alkyl halide into the reaction flask, we run into a major obstacle. The excess hydroxide can react with the alkyl halide (elimination or substitution), which creates competing side reactions that generate a mixture of undesired products. 

Lafond V., Mutin P.H., Vioux A. Non-hydrolytic sol-gel routes based on alkyl halide elimination toward better mixed oxide catalysts and new supports Application to the preparation of a Si02-Ti02 epoxidation catalyst. J. Mol. Catal. A 2002 182-183 81-88 Licoccia S., Traversa E., Di Vona M.L. Spectroscopic monitoring of sol-gel syntheses of heterometal-lic ceramics. J. Sol-Gel Sci. Tech. 2003 26 119-124 Linacero R., Rojas-Cervantes M.L., Lopez-Gonzalez J.D.D. Influence ofthe synthesis methods on the physical and structural characteristics of 0.6Ti02-0.4Al203 catalytic supports. J. Mater. Sci. 2000 35 3269-3278... 

Depending on the reaction system, that is, precursor and solvent, several aprotic condensation reactions have been established (Scheme 2.3). Alkyl halide elimination involves the condensation between a metal alkoxide and a metal halide (Scheme 2.3a), ester elimination occurs between metal carboxylates and metal alkoxides, for example, by transesterification (Scheme 2.3b), and ether... 

Scheme 2.3 Condensation mechanisms (a) alkyl halide elimination (b) ester elimination and (c) ether elimination.

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