Question

Order or reactivity of ${C_2}{H_6}$ , ${C_2}{H _4}$ and ${C_2}{H_2}$ is:
(A) ${C_2}{H_6} > {C_2}{H_4} > {C_2}{H_2}$
(B) ${C_2}{H_2} > {C_2}{H_6} > {C_2}{H_4}$
(C) ${C_2}{H_4} > {C_2}{H_2} > {C_2}{H_6}$
(D) All are equally reactive

Answer 1

Hint: Compounds containing carbon and hydrogen are called hydrocarbons. These are mainly of two types- saturated and unsaturated. Unsaturated compounds have double or triple bonds in the molecule. Such compounds are also commercially important. These are obtained from coal and petroleum.

Complete answer: Hydrocarbons are the compounds containing carbon and hydrogen. There are two main types-saturated hydrocarbons and unsaturated hydrocarbons. A saturated hydrocarbon has only a carbon-carbon single bond. Unsaturated hydrocarbons consist of double or triple bonds in them.
Saturated hydrocarbons constitute a family of straight or branched chain compounds called alkanes. Their general molecular formula is given by ${C_n}{H_{2n + 2}}$ where $n$ is the number of carbon atoms.
The first member of the alkane series is methane. It has a carbon atom and three hydrogen are attached to carbon by a single bond. It can be represented as $C{H_4}$ . Now the alkane containing two carbon atoms is ethane. Its molecular formula is ${C_2}{H_6}$ .
Unsaturated compounds are further divided into two classes- alkenes and alkynes.
Alkenes have double bonds. Their general formula is ${C_n}{H_{2n}}$ . The first member of the alkene series is ${C_2}{H_4}$ . Alkynes have triple bonds. Their general formula is ${C_n}{H_{2n - 2}}$ . The first member of the alkyne series is ${C_2}{H_2}$ .
Unsaturated compounds are more reactive than saturated compounds. Alkynes and alkenes are more reactive than alkenes. The pi bonds in unsaturated compounds can donate the electrons. This is not possible in case of alkanes.
In case of unsaturated compounds alkenes are more reactive than alkynes because the triple bond in alkynes is stronger and more stable. So it cannot be attacked easily like alkenes.
So the order of reactivity is alkenes $ > $ alkynes $ > $ alkanes.
The given compounds have the same number of carbon atoms each. Their order of reactivity can be given as- ${C_2}{H_4} > {C_2}{H_2} > {C_2}{H_6}$ .

The correct option is (C).

Note: Alkanes have two hydrogen atoms more than the corresponding alkene having the same number of carbon atoms. And alkenes have two hydrogen atoms more than the corresponding alkyne having the same number of carbon atoms. Alkenes and alkynes undergo additional reactions whereas alkanes undergo substitution reactions due to the absence of pi bonds.