F 2-Bromo-2-methylpropane

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.

Finally, despite use of solvents (or mixtures of solvents) to solvate the intermediate carbocation and the anion, the structure of the substrate continues to play the criticalrole and,for example,while,as noted above,at50°C,2-bromo-2-methylpropane (f-butyl bromide [(CH3)3C-Brj) dissociates approximately 105 times faster than 2-bromopropane (isopropyl bromide [(CH3)2CHBrj) in water at 50°C, the difference is only slightly diminished (from 105 to 104) in 60% ethanol 40% water (v/v) at 55°C. 

Predict the major substitution product that would result from mixing 2-bromo-2-methylpropane with concentrated aqueous ammonia. (Caution Although ammonia in water forms ammonium hydroxide according to the equation NHy -f H2O NHj OH, the for this process is very small. Thus the concentration of hydroxide is quite low.)... 

Match the following NMR spectra (pp 360-61) with the following compounds (a) ethyl bromide (b) 1,1-dibromoethane (c) 1,2-di-bromo-2-methylpropane (d) 1,1,2-tribro-moethane (e) ethyl alcohol and (f) p-(I-butyl)toluene. The number in a circle near a set of peaks refers to the relative area for those peaks. 

Figure 6.7. The and spectra of l-bromo-2-methylpropane [isobutyl bromide, (CH3)2CHCH2Br] and 2-bromo-2-methylpropane [f-butyl bromide, (CHjjjCBr].

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... 

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