Question

Ethylene may be obtained by dehydration of which of the following is concentrated \[{H_2}S{O_4}\] at \[160 - 170^\circ C\]?
(a) \[{C_2}{H_5}OH\]
(b) \[C{H_3}OH\]
(c) \[C{H_3}C{H_2}C{H_2}OH\]
(d) \[{(C{H_3})_2}CHC{H_2}OH\]

Answer 1

Hint: Dehydration reactions are the reaction that involves the removal of water (\[{H_2}O\]) molecules upon heating the molecular compounds. The best example of dehydration reaction is the elimination reaction where the formation of a more stable alkene occurs by the abstraction of a beta-proton via base.

Complete Step by Step Answer:
As we know the dehydration reaction involves the removal of water \[{H_2}O\] molecules.
In a dehydration reaction, a water molecule is generated by the removal of a beta-hydrogen atom and hydroxyl group (\[ - OH\]).

Image: Ethanol

The dehydration of alcohol can be done by the following two methods.
1. By allowing the vapours of alcohols over heated alumina (\[A{l_2}{O_3}\]).
2. By heating alcohols with conc. Sulfuric acid (\[{H_2}S{O_4}\]) and phosphoric acid (\[{H_3}P{O_4}\]).

Image: Dehydration of alcohol
Sometimes anhydrous zinc chloride (\[ZnC{l_2}\]) is also used for the dehydration of alcohols.
The dehydration trends of alcohols can be represented as tertiary alcohols > secondary alcohols> primary alcohols.

The tertiary alcohols and secondary alcohols can also be dehydrated by diluting sulfuric acid (\[{H_2}S{O_4}\]).

In the question, option (b), (c), and (d) will be the incorrect option because the dehydration of \[C{H_3}OH\] , \[C{H_3}C{H_2}C{H_2}OH\]and \[{(C{H_3})_2}CHC{H_2}OH\]do not produce ethylene. For the formation of ethylene, the alcohol must have two carbon atoms.
Therefore, from the above discussion, it is quite clear that option (a) will be the correct answer.

Note: During the dehydration of alcohols in the presence of acids the number of carbon atoms remains the same. Sulfuric acid (\[{H_2}S{O_4}\]) is a colourless and corrosive liquid. It is highly miscible with water.