2- Methylbutan alkyl halide from

Different reagents such as HX and PX3 may be used to prepare alkyl halides from primary and secondary alcohols. However, because elimination reactions predominate when tertiary alcohols are treated with phosphorous trihalides, preparing tertiary alkyl halides from tertiary alcohols proceeds with good yields only if concentrated hydrogen halides, HX, are used. The reaction of 2-methyl-2-butanol with hydrochloric acid to produce 2-chloro-2-methylbutane (Eq. 14.17) illustrates this transformation. 

Under second-order conditions (strong base/nucleophile), S 2 and E2 reactions may occur simultaneously and compete with each other. Show what products might be expected from the reaction of 2-bromo-3-methylbutane (a moderately hindered 2° alkyl halide) with sodium ethoxide. 

A carbocation intermediate is formed in an SnI reaction. In Section 4.6, we saw that a carbocation will rearrange if it becomes more stable in the process. If the carbocation formed in an SnI reaction can rearrange, SnI and Sn2 reactions of the same alkyl halide can produce different constitutional isomers as products, since a carbocation is not formed in an Sn2 reaction and therefore the carbon skeleton cannot rearrange. For example, the product obtained when HO is substituted for Br in 2-bromo-3-methylbutane by an SnI reaction is different from the product obtained by an Sn2 reaction. When the reaction is carried out under conditions that favor an SnI reaction, the initially formed secondary carbocation undergoes a 1,2-hydride shift to form a more stable tertiary carbocation. 

To synthesize 2-methyl-2-butene from 2-bromo-2-methylbutane, you would use Sn2/E2 conditions (a high concentration of HO in an aprotic polar solvent) because a tertiary alkyl halide gives only the elimination product under those conditions. If Sn1/E1 conditions were used (a low concentration of HO in water), both elimination and substitution products would be obtained. 

Stereoelectronic effects are also important in the dehydrohalogenation of acyclic alkyl halides by an E2 pathway. Again, the most favorable arrangement for the hydrogen and the halide being lost is anti coplanar. In the formation of 2-methyl-2-butene from 2-bromo-2-methylbutane shown on page 208, the elimination of HBr occurs readily from the conformation on the left but not from the one on the right. 

This result confirmed the earlier work of Whitmore, F. C. Johnston, F. /. Am. Chem. Soc. 1933, 55,5020, who had found that the addition of HCl to 3-methyl-l-butene without solvent, in a sealed reaction tube for 7 weeks, gave both 2-chloro-3-methylbutane and 2-chloro-2-methylbutane. This result contradicted the suggestion of earlier investigators that the 3° alkyl halide was formed by rearrangement of the 2° alkyl halide formed from the addition reaction. Hammond, G. S. Collins, C. H. /. Am. Chem. Soc. 1960, 82, 4323 found that addition of HCl to 1,2-dimethylcyclopentene produced Tchloro-frans-l,2-dimethylcyclopentane as the major (perhaps only) addition product, but it isomerized to l-chloro-c/s-l,2-dimethylcyclopentane. 

Many substrates have nonequivalent /3-protons, so a 1,2-elimination may produce more than one alkene. For example, ethoxide-promoted elimination of 2-iodo-3-methylbutane produced 82% of 2-methyl-2-butene and 18% of 3-methy 1-1-butene (equation 10.34). The generalization that 1,2-elimination reactions of alkyl halides usually give the more substituted alkene is known as the Saytzeff rule. Saytzeff observed that the regiochemistry of elimination could be correlated with removal of a hydrogen atom from the... 

The first item in Table 8.7 is the toluenesulfonic acid ester (toluenesulfonate, tosylate,-OTs) of 3- Hi-3-methyl-2-butanol, which is permitted to undergo solvolysis in an acetic acid solvent. As might be anticipated (and as shown in Scheme 8.76), the secondary tosylate group leaves and the secondary carbocation suffers a hydride migration (in this case, a deuteride, i.e., rather than H) to generate the more stable tertiary carbocation. The latter is then captured by the nucleophilic solvent. The acetic acid ester of 2-methyl-2-butanol (i.e., 2-acetoxy-2-methylbutane) results. Hydride and alkyl migrations from a less stable carbocation to a more stable carbocation have been used before (Chapter 7) to explain product mixtures resulting from solvolysis of alkyl halides. 

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