2- Bromo-2-methylpropane, from hydrogen

Further, formation of quaternary salts can interfere in the ready removal of hydrogen halide by tertiary amines such as dimethylaniline, pyridine, or quinoline.58 To overcome this, Hiinig and Kiessel59 successfully applied sterically hindered tertiary amines, e.g., ethyldiisopropylamine and 7V-ethyldi-(cyclohexyl)amine for instance, removing hydrogen bromide from 1,2-di-bromo-2-methylpropane by potassium hydroxide in ethylene glycol60 gave unsatisfactory yields of l-bromo-2-methyl-l-propene ... 

Scheme 6.18a. A depiction of a potential tertiary carbocation lying on the path from the reactants hydrogen bromide (HBr) and 2-methylpropene [isobutylene, (CH3)2C=CH2] to the prodnct 2-bromo-2-methylpropane [f-butylbromide, (CH3)3CBr]. The latter is the only prodnct formed in the reaction under ionic conditions.

The third example is the E2 elimination of hydrogen bromide (HBr) from f-butyl bromide (2-bromo-2-methylpropane [(CH3)3CBrj) by ethoxide in ethanol and is unremarkable save that it serves to emphasize that only elimination and no substitution occurs. Thus, f-butyl ethyl ether [(CH3)3COCH2CH3] cannot be prepared this way. Although not shown, bromoethane (CH3CH2Br) primarily undergoes... 

Because of this polar transition state, the rate of an SnI reaction increases as solvent polarity is increased. The effect is particularly striking when the solvent is changed from aprotic to protic. For example, hydrolysis of 2-bromo-2-methylpropane is much faster in pure water than in a 9 1 mixture of acetone and water. The protic solvent accelerates the SnI reaction because it stabilizes the transition state shown in Figure 7-4 by hydrogen bonding with the leaving group. Remember that, in contrast, the Sn2 reaction is accelerated in polar aprotic solvents, mainly because of a solvent effect on the reactivity of the nucleophile. 

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