synthesis of alkenes from alcohols-cyclohexene from cyclohexanol (ch.10)

What type of reaction is used to convert cyclohexanol to cyclohexene?

An elimination reaction (dehydration under acidic conditions).

What is lost during the dehydration of cyclohexanol?

Water (H₂O) is removed.

What acid is used as the catalyst in this reaction?

Phosphoric acid (H₃PO₄)

Why is the hydroxyl group in alcohols a poor leaving group?

Because OH⁻ is a very strong base, making it unstable as a leaving group.

How is the poor leaving group (OH⁻) converted into a better leaving group?

It is protonated by an acid to form water (H₂O), which is a good leaving group.

What type of carbocation is formed from cyclohexanol in this reaction?

A secondary carbocation.

What happens after the carbocation forms?

A β-hydrogen is removed, electrons shift to form a double bond, producing cyclohexene.

Why is distillation used during the reaction?

To continuously remove cyclohexene, driving the equilibrium toward product (Le Châtelier's Principle).

Why does cyclohexene distill out of the reaction mixture?

It has a lower boiling point (83°C) than cyclohexanol (161°C).

What is the purpose of the iodine test?

To detect alkenes—a positive test occurs when the purple iodine color fades to clear or light pink.

What is the purpose of the potassium permanganate test?

To detect alkenes—a positive test occurs when purple KMnO₄ becomes brown (MnO₂).

Why is the reaction reversible?

The reverse reaction is acid-catalyzed hydration of alkenes, which can convert cyclohexene back to cyclohexanol.

How do we prevent the reverse (hydration) reaction from occurring?

By distilling off cyclohexene as it forms, preventing it from reacting with water.

What is the key evidence used to confirm cyclohexene formation in this lab?

Gas chromatography (GC) results.