Tert-Butyl bromide nucleophilic substitution

Suggest a structure for the product of nucleophilic substitution obtained on solvolysis of tert-butyl bromide in methanol and outline a reason able mechanism for its formation... 

This type of substitution is called an SN1 reaction, for Substitution, Nucleophilic, unimolecular. The term unimolecular means there is only one molecule involved in the transition state of the rate-limiting step. The mechanism of the SnI reaction of tert-butyl bromide with methanol is shown here. Ionization of the alkyl halide (first step) is the rate-limiting step. 

Another nucleophilic aliphatic substitution is the reaction between sodium hydroxide and tert-butyl bromide (TBB) ... 

They found that the rate of hydrolysis depends only on the concentration of tert-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. 

Just as there are two important nucleophilic substitution reactions—SnI and Sn2—there are two important elimination reactions El and E2. The reaction of tert-butyl bromide with hydroxide ion is an example of an E2 reaction E stands for elimination and 2 stands for bimolecular. The product of an ehmination reaction is an alkene. 

The reaction rate depends only on the concentration of tert-butyl bromide. Just as Hughes and Ingold interpreted a second-order rate law in terms of a bimolecular rate-determining step, they saw first-order kinetics as evidence for a unimoZecM/ar rate-determiiung step—one that involves only the alkyl halide and is independent of both the concentration and identity of the nucleophile. Like the mechanism for the reaction of alcohols with hydrogen halides (Section 4.8), this pathway is classified as SnI (substitution-nucleophilic-unimolecular) and is characterized by the formation of a carbocation in the rate-determining step. 

For example, when 2-bromo-2-methylpropane (tert-butyl bromide) is mixed with water, it is rapidly converted into 2-methyl-2-propanol (tert-butyl alcohol) and hydrogen bromide. Water is the nucleophile here, even though it is poor in this capacity. Such a transformation, in which a substrate undergoes substitution by solvent molecules, is called solvolysis, such as methano-lysis, ethanolysis, and so on. When the solvent is water, the term hydrolysis is applied. 

This solvolysis is a substitution because methoxide has replaced bromide on the tert-butyl group. It does not go through the SN2 mechanism, however. The SN2 requires a strong nucleophile and a substrate that is not too hindered. Methanol is a weak nucleophile, and ferf-butyl bromide is a hindered tertiary halide—a poor SN2 substrate. 

---