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

Hint: 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.