12. Alcohols, Ethers, Epoxides and Thiols

Suggest the appropriate reagents to carry out the following transformations.

(d)

Propose mechanisms for the following reactions. In most cases, more products are formed than are shown here. You only need to explain the formation of the products shown, however.

(a)

Propose a mechanism for the reaction of

(a) 1-methylcyclohexanol with HBr to form 1-bromo-1-methylcyclohexane.

When cis-2-methylcyclohexanol reacts with the Lucas reagent, the major product is 1-chloro-1-methylcyclohexane. Propose a mechanism to explain the formation of this product.

(a) The reaction of butan-2-ol with concentrated aqueous HBr goes with partial racemization, giving more inversion than retention of configuration. Propose a mechanism that accounts for racemization with excess inversion.

(b) Under the same conditions, an optically active sample of trans-2-bromocyclopentanol reacts with concentrated aqueous HBr to give an optically inactive product, (racemic) trans-1,2-dibromocyclopentane. Propose a mechanism to show how this reaction goes with apparently complete retention of configuration, yet with racemization. (Hint: Draw out the mechanism of the reaction of cyclopentene with Br₂ in water to give the starting material, trans-2- bromocyclopentanol. Consider how parts of this mechanism might be involved in the reaction with HBr.)

Protonation converts the hydroxy group of an alcohol to a good leaving group. Suggest a mechanism for each reaction.

(a)

What product would be formed if the four-membered ring alcohol in Problem 70 were heated with an equivalent amount of HBr rather than with a catalytic amount of H₂SO₄?

Propose a mechanism for each of the following reactions:

c.