Alkanes are saturated hydrocarbons containing only single covalent (sigma) bonds. Their general formula is \( C_nH_{2n+2} \). They are also known as paraffins because of their low reactivity.

1. Saturated Compounds: They contain only single bonds and do not readily undergo addition reactions.

2. Non-polar: They are insoluble in water but soluble in organic solvents like benzene.

3. Low Reactivity: Because of strong C–C and C–H bonds, they only undergo substitution and combustion reactions.

4. Combustion: They burn in air or oxygen to form CO₂ and H₂O.

5. Boiling Point Trend: Boiling points increase with molecular mass due to stronger van der Waals forces.

6. Homologous Series: They differ from each other by a CH₂ group.

7. Tetrahedral Geometry: Carbon atoms in alkanes are sp³-hybridized with bond angle approximately 109.5°.

Methane is the first member of the alkane series with molecular formula CH₄. It is the simplest hydrocarbon and is also known as marsh gas because it is commonly released from swamps, marshes, stagnant water bodies and decaying organic matter under anaerobic conditions.

Methane is a saturated hydrocarbon consisting of one carbon atom bonded tetrahedrally to four hydrogen atoms. It belongs to the homologous series called alkanes.

Sources of Methane:

• Natural gas (main component, about 80–95%)
• Biogas from decomposition of organic waste
• Swamps and marshes (marsh gas)
• Digestive tracts of ruminants (cows, goats, sheep)
• Volcanic emissions
• Coal mines

Structure of Methane:

      H
      |
  H - C - H
      |
      H

The carbon atom is sp³ hybridized, forming a tetrahedral molecule with bond angles of approximately 109.5°.

Method 1: Heating a mixture of sodium ethanoate and sodium hydroxide (soda-lime).

Equation

CH₃COONa + NaOH → CH₄ + Na₂CO₃
  

Calcium oxide (CaO) is added to keep the mixture dry and porous.

Method 2: Preparation from Aluminium Carbide

Equation:

Al₄C₃ + 12H₂O → 4Al(OH)₃ + 3CH₄
  

• It is a colourless, odourless gas.
• It is slightly soluble in water but highly soluble in organic solvents.
• It is lighter than air.
• It burns with a non-sooty flame.
• It is a major greenhouse gas.
• It burns with a pale blue flame.
• It has a melting point of –182°C and boiling point of –164°C.

1. Combustion

Methane burns completely in excess oxygen to form carbon dioxide and water:

$$ CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O $$

2. Substitution Reaction with Chlorine (Halogenation)

This occurs in the presence of sunlight (UV light). Hydrogen atoms are substituted one after another by chlorine.

(a) Formation of Chloromethane

$$ CH_4 + Cl_2 \xrightarrow{} CH_3Cl + HCl $$

(b) Formation of Dichloromethane

$$ CH_3Cl + Cl_2 \xrightarrow{} CH_2Cl_2 + HCl $$

(c) Formation of Trichloromethane (Chloroform)

$$ CH_2Cl_2 + Cl_2 \xrightarrow{} CHCl_3 + HCl $$

(d) Formation of Tetrachloromethane (Carbon Tetrachloride)

$$ CHCl_3 + Cl_2 \xrightarrow{} CCl_4 + HCl $$

3. Reaction with Steam (Steam Reforming)

$$ CH_4 + H_2O \xrightarrow{} CO + 3H_2 $$ This reaction is carried out in the presence of Nickel as catalyst.

4. Cracking

At high temperatures, methane decomposes:

$$ CH_4 \rightarrow C + 2H_2 $$

• Used as domestic fuel (cooking gas)
• Used in power generation
• Used in the production of hydrogen (steam reforming)
• Used to manufacture methanol
• Raw material for producing carbon black
• Used as fuel in industrial furnaces