Why are certain compounds called hydrocarbons? Write the general formula for homologous series of alkanes, alkenes and alkynes and also draw the structure of the first member of each series. Write the name of the reaction that converts alkenes into alkanes and also write a chemical equation to show the necessary conditions for the reaction to occur.

Why Compounds are Called Hydrocarbons

Compounds that are composed entirely of only carbon and hydrogen atoms are called hydrocarbons.

General Formulas and First Members of Homologous Series

1. Alkanes (Saturated Hydrocarbons):

• General Formula: $C_nH_{2n+2}$
• First Member: Methane ($CH_4$)
• Structure:

``

H

H-C-H

H

`

1. Alkenes (Unsaturated Hydrocarbons with a double bond):

• General Formula: $C_nH_{2n}$ (where $n \ge 2$)
• First Member: Ethene ($C_2H_4$)
• Structure:

`

H H

\ /

C=C

/ \

H H

`

1. Alkynes (Unsaturated Hydrocarbons with a triple bond):

• General Formula: $C_nH_{2n-2}$ (where $n \ge 2$)
• First Member: Ethyne ($C_2H_2$)
• Structure:

`

H-C≡C-H

``

(Note: Methene and methyne do not exist because a carbon-carbon double or triple bond requires at least two carbon atoms.)

Conversion of Alkenes into Alkanes

• Name of the Reaction: Hydrogenation Reaction (or Addition Reaction).
• Chemical Equation and Conditions:

Alkenes react with hydrogen gas ($H_2$) in the presence of catalysts like finely divided Nickel ($Ni$), Palladium ($Pd$), or Platinum ($Pt$) at an elevated temperature to form saturated alkanes.

$$\text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni catalyst, } \Delta} \text{CH}_3-\text{CH}_3$$