12. Alcohols, Ethers, Epoxides and Thiols

(•••) Suggest the appropriate reagents to carry out the following transformations.

(a) <IMAGE>

 (••••) Another method for converting alcohols to chloroalkanes makes use of chlorotrimethylsilane (TMSCl) and DMSO. Suggest a mechanism for this reaction to form (a) a 1° chloroalkane and (b) a 3° chloroalkane. [The reaction begins by the reaction of DMSO and TMSCl and is analogous to the Swern oxidation.]

<IMAGE>

Predict the product of the following reactions.

(b) <IMAGE>

(•••) Predict the product of the following reactions.

(c) <IMAGE>

(••) Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (vii) HCl; (viii) HBr. If no reaction occurs, write 'no reaction.'  

(a) <IMAGE>

(•••) Predict the product of the following reactions.

(b) <IMAGE>

(•••) Suggest the appropriate reagents to carry out the following transformations.

(d) <IMAGE>

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) <IMAGE of reaction>

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) <IMAGE> HBr, heat, (SN2 or SN1)—> <IMAGE> + H2O

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?