Question

Outline a mechanism for the dehydration of 2-methylcyclohexanol into 1-methylcyclohexene catalyzed by acid. (include all arrows, charges, and intermediates – you can assume H2O behaves as the base)

Answer 1

Dear Student,

An alcohol is dehydrated and an alkene is formed.

2- methylcyclohexanol is reacted with phosphoric acid, which results into three different products .

This means that any single molecule of 2-methylcyclohexanol can form into 3- Methylcyclohexene , 1-methylcyclohexene and methylenecyclohexane.

Here the phosphoric acid acts as a catalyst.

The full mechanism follows.

The dehydration of 2-methylcylcohexanol begins with the protonation of the (-OH) by the phosphoric acid which acts as catalyst in this mechanism , transforming it from a poor leaving group (-OH) to a good leaving group (-OH²⁺ ).

(-OH²⁺ ) group leaves as a water molecule, forming a secondary carbocation.

Secondary carbocation can form two of the products with the elimination of a (H) on an adjacent carbon, 1-methylcyclohexene and 3-methylcyclohexene.

Alternatively, the secondary carbocation can undergo a rearrangement forming a more stable tertiary carbocation.

Tertiary carbocation can form two products methylenecyclohexane and 1-methylcyclohexene.

The product distribution can be under either kinetic or thermodynamic control. Typically the conditions in this lab favour thermodynamic control.

Following diagram shows the mechanism of 2-methylcyclohexanol into 1-methylcyclohexene.