Question

Propose a mechanism for the reaction of

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

(b) 2- cyclohexylethanol with HBr to form 1-bromo-2-cyclohexylethane.

Short answer

1-methyl cyclohexanol is tertiary alcohol; thus, it will react with HBr by the SN1 mechanism.

Protonation of 1-methyl cyclohexanol converts the hydroxy group to a good leaving group.

Water leaves, forming a carbocation.

Bromide ion attacks the carbocation to form the product 1- bromo-1-methylcyclohexane.

Mechanism of formation of 1- bromo-1-methylcyclohexane

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Step-by-step solution

Formation of 1- bromo-1-methylcyclohexane from 1-methyl cyclohexanol

1-methyl cyclohexanol is tertiary alcohol; thus, it will react with HBr by the SN1 mechanism.

Protonation of 1-methyl cyclohexanol converts the hydroxy group to a good leaving group.

Water leaves, forming a carbocation.

Bromide ion attacks the carbocation to form the product 1- bromo-1-methylcyclohexane.

Mechanism of formation of 1- bromo-1-methylcyclohexane

 Formation of 1-bromo-2-cyclohexylethane from 2- cyclohexylethanol

2 - cyclohexylethanol is the primary alcohol that reacts with HBr by the SN2 mechanism.

Protonation of 2- cyclohexylethanol converts the hydroxy group to a good leaving group.

Bromide displaces water to give the product 1-bromo-2-cyclohexylethane.

Mechanism of formation of 1-bromo-2-cyclohexylethane