CBSE Class 10 Science 2017 · Paper 31/1/3 · Question 8

${\text{C}}_4{\text{H}}_8$ is the one most likely to show an addition reaction. Justification: ${\text{C}}_4{\text{H}}_8$ fits the general formula of the alkenes, ${\text{C}}_n{\text{H}}_{2n}$ ($n=4$), so it is most likely an unsaturated hydrocarbon (a butene) with a carbon–carbon double bond across which atoms can add. ${\text{C}}_3{\text{H}}_8$ fits ${\text{C}}_n{\text{H}}_{2n+2}$ ($n=3$): it is propane, a saturated alkane with only single bonds, which reacts by substitution rather than addition. Equation (addition of hydrogen, nickel catalyst): ${\text{CH}}_2=\text{CH}-{\text{CH}}_2-{\text{CH}}_3+{\text{H}}_2\stackrel{\text{Ni catalyst}}{\to }{\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$, that is, ${\text{C}}_4{\text{H}}_8+{\text{H}}_2\to {\text{C}}_4{\text{H}}_{10}$ (butane).