Fert-Butyl bromide

Subsequent reactions of the cations with bromide ion give fert-butyl bromide and isobutyl bromide. In the usual way of running these additions, the product is very pure tert-butyl bromide. 

The products obtained in a reaction subject to kinetic control are not necessarily the same as those obtained under equilibrium control. Indeed, the equilibrium constant for interconversion of fert-butyl bromide and isobutyl... 

Only perfluoro-tert-butyl copper has been prepared by this method. The compound was isolated in 72% yield from the reaction of m-(tri-fluoromethyl)phenylcopper with perfluoro-fert-butyl bromide in ether-dioxane at 0°C(34). Exchange and cross-coupling reactions occur simultaneously in many reactions between organocopper reagents and various organic halides (69, 97, 174, 217, 220, 266, 297). However, the method is only of limited use as the new reagent [R Cu in Eq. (16)] and its precursor are capable of reacting with both species of halide, RX and R X, present in the mixture. 

The rate of an E2 reaction depends on the concentrations of both fert-butyl bromide and hydroxide ion. It is, therefore, a second-order reaction (Section 10.2). 

The rate law tells us that both fert-butyl bromide and hydroxide ion are involved in the rate-determining step of the reaction. The following mechanism agrees with the observed second-order kinetics ... 

They found that the rate of hydrolysis depends only on the concentration of fert-butyl bromide. Adding the stronger nucleophile hydroxide ion, moreover, causes no change in the rate of substitution, nor does this rate depend on the concentration of hydroxide. Just as second-order kinetics was interpreted as indicating a bimolecular rate-determining step, first-order kinetics was interpreted as evidence for a unimolecular rate-determining step—a step that involves only the alkyl halide. 

PROBLEM 8.8 Suggest a structure for the product of nucleophilic substitution obtained on solvolysis of fert-butyl bromide in methanol, and outline a reason- I able mechanism for its formation. J... 

Since the starting material given is fert-butyl bromide, a practical synthesis is ... 

We choose the first approach because it is an S 2 reaction on the unhindered substrate, methyl bromide. The second approach requires an Sj 2 reaction on a hindered tertiary alkyl halide, a very poor choice. Indeed, we would expect that the reaction of methoxide ion with fert-butyl bromide could not give any ether at all but would proceed entirely by E2 elimination ... 

The rate of reaction is fast with MeBr and slower with secondary halides. Tertiary halides such as fert-butyl bromide, Mc3C—Br, do not undergo substitution via this mechanism at any reasonable rate. 

Energy diagram illustrating the SnI mechanism for hydrolysis of fert-butyl bromide. 

An S l reaction is illustrated by the solvolysis reaction of 2-bromo-2-methylpropane (fert-butyl bromide) in methanol to form 2-methoxy-2-methylpropane terthutyl methyl ether). You may notice that the second step of the mechanism is identical to the second step of the mechanism for the addition of hydrogen hahdes (H—X) to alkenes (Section 5.3A) and the acid-catalyzed hydration of alkenes (Section 5.3B). 

Dibromopropane from 2-bromopropane l-Bromo-2-propanol from 2-propanol l-Bromo-2-methyl-2-propanol from fert-butyl bromide... 

Kinetic studies show that this reaction is first order in fert-butyl bromide and 2ero order in water. The overall rate is determined by the slow step, so water does not appear in the rate law, which is... 

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