Alkyl halides 2- chloro-2-methylbutane

The best alkylating agents for silyl enol ethers are tertiary alkyl halides they form stable carbocations in the presence of Lewis acids such as TiCl4 or SnCl4. Most fortunately, this is just the type of compound that is unsuitable for reaction with lithium enolates or enamines, as elimination results rather than alkylation a nice piece of complementary selectivity. Below is an example the alkylation of cyclopentanone with 2-chloro-2-methylbutane. The ketone was converted to the trimethylsilyl enol ether with triethylamine and trimethylsilylchloride we discussed this step on p. 466 (Chapter 20). Titanium tetrachloride in dry dichloromethane promotes the alkylation step. 

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. 

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. 

Alkyl Group. Tertiary alkyl halides may be used in solvolyses with confidence that the reactions are proceeding by the Sj jl mechanism. Two compounds that allow you to examine the possible effects of the structure of the alkyl group on reaction rates are 2-chloro-2-methylpropane (ferf-butyl chloride) and 2[Pg.484]

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