Explain the given reaction with examples:

(a) Hydrogenation
(b) Oxidation
(c) Substitution
(d) Combustion

(a)Addition of hydrogen to unsaturated hydrocarbon to obtain a saturated hydrocarbon is called hydrogenation reaction. Hydrogenation is used for converting unsaturated vegetable oil into fat. Unsaturated compounds undergo addition reactions in the presence of catalysts like nickel and palladium to give saturated compounds.
This can be shown by following equation:

${\text{CR}}_2={\text{CR}}_2+{\text{H}}_2\to {\text{CR}}_2\text{H}-{\text{CR}}_2\text{H}$


(b) When oxygen is added to alcohol to make carboxylic acid, this is called Oxidation reaction.
Ethanol is oxidised to ethanoic acid in presence of alkaline ${\text{KMnO}}_4$ on heating.

${\text{CH}}_3{\text{CH}}_2\text{OH}\to {\text{CH}}_3\text{COOH}$


(c) Saturated hydrocarbons are fairly unreactive, but in the presence of sunlight, chlorine substitute hydrogen one by one. Alkanes combine with halogens to undergo substitution reactions in the presence of sunlight and form haloalkanes.

${\text{CH}}_4+{\text{Cl}}_2\to {\text{CH}}_3\text{Cl}+\text{HCl}$


(d) Hydrocarbons undergo combustion reactions to form carbon dioxide and water vapour along with the heat and light.

${\text{CH}}_4+2{\text{O}}_2\to {\text{CO}}_2+2{\text{H}}_2\text{O}+\text{light + heat}$