Question

What happens when tertiary butyl alcohol is passed over heated copper at $300{}^\circ C$?
(A) Secondary butyl alcohol is formed
(B) $\text{2-methylpropene}$ is formed
(C) $\text{1-butene}$ is formed
(D) Butanal is formed

Answer 1

Hint: Normally the reaction would have been an oxidation reaction. But things change when a tertiary alcohol is present. Formation of double-bond will take place.

Complete step by step answer:
Let us first know about the reaction that is mentioned in the question. Generally when primary or secondary alcohol vapours are passed over heated copper at $300{}^\circ C$ then dehydrogenation or in other words oxidation takes place. It converts the primary alcohol into aldehyde and the secondary one into ketone. The reaction is as below:
- Primary alcohol

- Secondary alcohol

This is not the case with tertiary alcohols as they are difficult to oxidise. In their case, dehydration takes place rather than dehydrogenation. The dehydration takes a hydroxyl and hydrogen atom from adjacent carbons, which results in the formation of a double bond. The reaction is as follows:

As you can see the tertiary butyl alcohol has been dehydrated into an alkene.

So the answer to the above question is option (C) $\text{1-butene}$ is formed.

Additional information:
The formation of the double bond takes place in such a way as to ensure maximum number of hyperconjugative structures. An example is given below:

The product will only be “A” because the number of hyperconjugative structures is $6$while that in “B” is $5$. Number of hyperconjugative structures is equal to the number of hydrogen atoms that are attached to the carbon atoms adjacent to the double bond.

Note:
Tertiary alcohols are not oxidised into carbonyl compounds or acids by mild oxidising agents such as chromic oxide. When subjected to strong oxidising agents such as potassium permanganate, they convert into acids with lower number of carbon atoms.
The heated copper process which is described above is an industrial one, and is seldom used in laboratories.