Question

Which out of the following hydrocarbons undergo addition reactions?
${C_2}{H_6}$ ​, ${C_3}{H_8}$​, ${C_3}{H_6}$​, ${C_2}{H_2}$​ and $C{H_4}$​
A.${C_2}{H_6}$​​ and ${C_3}{H_8}$
B.${C_3}{H_6}$​​ and ${C_2}{H_2}$
C.$C{H_4}$​ and ${C_2}{H_6}$
D.${C_3}{H_8}$​​ and ${C_2}{H_2}$

Answer 1

The unsaturated compounds having one or more carbon-carbon double bonds or triple bonds or both types of bonds undergo addition reactions whereas the saturated hydrocarbons do not undergo addition reactions.

Complete answer:
Only unsaturated hydrocarbons undergo addition reactions. For open chain hydrocarbons, the general molecular formula of a saturated compound (alkane category) is ${C_n}{H_{2n + 2}}$.
The general molecular formula of an aliphatic hydrocarbon with one double bond is ${C_n}{H_{2n}}$, and the general molecular formula of a hydrocarbon having one triple bond or two double bonds is ${C_n}{H_{2n - 2}}$.
${C_2}{H_6}$​ (Ethane) is a saturated hydrocarbon and thus does not undergo addition reaction.
${C_3}{H_8}$​ (Propane) is a saturated hydrocarbon and thus it also does not undergo addition reaction.
and CH4​ (Methane) is also a saturated hydrocarbon and thus it also does not undergo addition reactions.
All the above three follow the general formula of a saturated compound, i.e. ${C_n}{H_{2n + 2}}$, so they will not undergo an addition reaction.
${C_3}{H_6}$ (Propene)$= {\text{ }}{C_3}{H_{2 \times 3}}\;$ follows the general formula ${C_n}{H_{2n}}$.and ${C_2}{H_2}$​ (Ethyne/acetylene)$= {\text{ }}{C_2}{H_{2 \times 2}}$ also follows the general formula ${C_n}{H_{2n - 2}}$, so they are unsaturated hydrocarbon and they both undergo addition reactions.
Hence, among the given compounds only ${C_3}{H_6}$​ (Propene) and ${C_2}{H_2}$​ (acetylene) are unsaturated Hydrocarbons.

Therefore, the correct answer is option (B).

Note: An additional reaction, in organic chemistry, in its simplest terms is an organic reaction where two or more molecules combine to form a larger molecule (the adduct). But it should be noted that only chemical compounds which contain multiple bonds can undergo an additional reaction such as a double bond or triple bond which is usually broken to form the required single bonds.