Question

Dehydration of the alcohols will be in order______.

Answer 1

Dehydration:

• Dehydration means the removal of a water molecule.

Dehydration of alcohols:

• It is a type of chemical reaction in which water molecule is removed from the alcohols and the formation of alkene takes place.
• Dehydration of alcohol is an example of an elimination reaction.
• Dehydration of alcohol occurs when alcohol is heated at a certain temperature in the presence of sulphuric acid.
• Reaction:

$$\begin{gathered} {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\underset{∆}{\overset{{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }}{\mathrm{CH}}_2={\mathrm{CH}}_2+{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Ethanol}\mathrm{ethene}\mathrm{water} \end{gathered}$$

Mechanism:

• Different types of alcohols may dehydrate through different mechanism pathways.
• During the dehydration, the $(-\mathrm{OH})$ group in the alcohol donates two electrons to ${\mathrm{H}}^{+}$from the acid reagent, forming an alkyloxonium ion.
• This ion acts as a very good leaving group which leaves to form a carbocation.
• The deprotonated acid (the nucleophile) then attacks the hydrogen adjacent to the carbocation and forms a double bond.
• Primary alcohols undergo bimolecular elimination (E2 mechanism) while secondary and tertiary alcohols undergo unimolecular elimination (E1 mechanism). The relative reactivity of alcohols in dehydration reaction is ranked as the following

Methanol < primary < secondary < tertiary

Therefore, The dehydration of tertiary alcohol is faster than the dehydration of secondary alcohol, and a primary alcohol.

The dehydration of the secondary alcohol is faster than the dehydration of primary alcohol.

Hence, the dehydration of the alcohol will be in order Tertiary alcohol > Secondary alcohol > Primary alcohol.