Explain the following observations: When tert -butyl bromide is treated with sod

Question

Explain the following observations:
When tert-butyl bromide is treated with sodium methoxide in a mixture of methanol and water, the rate of formation of tert-butyl alcohol and tert-butyl methyl ether does not change appreciably as the concentration of sodium methoxide is increased. However, increasing the concentration of sodium methoxide causes a marked increase in the rate at which tert-butyl bromide disappears from the mixture.

1- tert-Butyl alcohol and tert-butyl methyl ether are formed via an __SN1_____ mechanism. The rate of this reaction is Entry field with INDEPENDENT of the concentration of methoxide ion (from sodium methoxide).

2-

a Another competing reaction also causes tert-butyl bromide to disappear. This E2 reaction is one in which methoxide ion reacts with tert-butyl bromide. Since this reaction is dependent on the concentration of methoxide ion, increasing the methoxide ion concentration causes an increase in the rate of disappearance of tert-butyl bromide.

Explanation / Answer

tert-butyl bromide is a tertiary alkyl halide and as such will not undergo SN2 nucleophilic reaction; the formation of tert-butyl alcohol and tert-butyl methyl ether takes place via SN1 pathways, which are dependent on the concentration of the base, sodium methoxide. However, the observations tells us clearly that the concentrations of tert-butyl alcohol and tert-butyl methyl ether do not increase significantly on increasing the concentration of sodium methoxide. This observation is contrary to what we expect from a SN1 reaction; therefore, a competing reaction must be taking place.

tert-butyl bromide, being a tertiary alkyl halide, shows a strong propensity to undergo elimination reaction via the E1 pathway. The elimination reaction proceeds via the E1 pathway because the alkyl halide can generate a stable carbocation on reaction with sodium ethoxide. The E1 pathway depends on the concentration of tert-butyl bromide as well as sodium methoxide. The size and basic nature of the base/nucleophile used influences the E1/SN1 pathways significantly.

Sodium methoxide is a strong base and a poor nucleophile. A strong base always promotes elimination reaction and reduces nucleophilic substitution. Moreover, sodium methoxide is a bulky base and the nucleophilic reaction involves strong steric interactions. Therefore, E1 elimination occurs preferentially, giving isobutylene. Thus, more tert-butyl bromide is consumed and we will have a higher concentration of isobutylene over time. The concentrations of tert-butyl alcohol and tert-butyl methyl ether will remain steady over time and both of these are minor products in the reaction.

Therefore, option (b) is the correct explanation of the observation.

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