11.3 Saturated Hydrocarbons—Alkane

Definition:

• Alkanes are hydrocarbons that have carbon-carbon single bonds in their carbon chain.

General Formula:

• The common formula for alkanes is CnH2n+2, where “n” represents the number of carbon atoms.

• For example:

• Methane (n = 1) = CH4

• Ethane (n = 2) = C2H6

• Propane (n = 3) = C3H8

• Butane (n = 4) = C4H10

Chemical Characteristics:

• Alkanes are chemically inactive because it is difficult to break the carbon-carbon and carbon-hydrogen bonds. This is why they are referred to as “paraffins,” meaning “lacking affinity.”

• Alkanes typically only participate in reactions under high temperatures, high pressures, or with the presence of strong catalysts.

Naming of Alkanes (IUPAC System):

1. Straight-Chain Alkanes:

• 1 carbon: Methane (CH4)

• 2 carbons: Ethane (C2H6)

• 3 carbons: Propane (C3H8)

• 4 carbons: Butane (C4H10)

2. Naming Convention:

• Alkanes are named by adding the suffix “-ane” to the root name based on the number of carbon atoms.

• For example:

• Methane (1 carbon), Ethane (2 carbons), Propane (3 carbons), Butane (4 carbons).

Alkyl Group:

• An alkyl group is a single-valent radical formed by removing one hydrogen atom from an alkane.

• The general formula for alkyl groups is R-H, where “R” represents the alkyl group.

• Alkyl radicals are named by replacing “-ane” with “-yl” in the name of the alkane.

• For example:

• Methyl (CH3-) from Methane (CH4)

• Ethyl (C2H5-) from Ethane (C2H6)

• Propyl (C3H7-) from Propane (C3H8)

• Butyl (C4H9-) from Butane (C4H10)

Table 11.01: Carbon Numbers, Names, and Formulas of Alkanes

Name Formula

Methane CH4

Ethane C2H6

Propane C3H8

Butane C4H10

Pentane C5H12

Hexane C6H14

Heptane C7H16

Octane C8H18

Nonane C9H20

Decane C10H22

Preparation of Alkanes:

1. From Carbon Dioxide:

• Methane and water are produced when hydrogen reacts with carbon dioxide at 250°C in the presence of a nickel catalyst.

• Reaction:

CO2 + 4H2 → CH4 + 2H2O

2. From Alkene and Alkyne:

• Ethane is produced when hydrogen reacts with ethene (C2H4) or ethyne (C2H2) at temperatures of 180°C to 200°C, in the presence of a nickel catalyst.

• Reaction (from Ethene):

C2H4 + H2 → C2H6

• Reaction (from Ethyne):

C2H2 + 2H2 → C2H6

3. From Decarboxylation Reaction:

• Methane and sodium carbonate are produced when sodium ethanoate (CH3COONa) is heated with sodium hydroxide (NaOH) in the presence of calcium oxide (CaO).

• Reaction:

CH3COONa + NaOH → CH4 + Na2CO3

Properties of Alkanes:

1. Physical Properties:

• Boiling Point & State:

• Alkanes’ physical properties depend on the number of carbon atoms.

• Alkanes with 1-4 carbon atoms have boiling points below room temperature, remaining gaseous.

• Alkanes with 5-15 carbon atoms have boiling points above room temperature, remaining liquid.

• Alkanes with more than 16 carbon atoms are solid.

• Example: Pentane (C5H12) has a boiling point of 36.1°C.

2. Chemical Properties:

• Alkanes are largely chemically inert but can participate in certain reactions under specific conditions.

• Reaction with Chlorine (Halogenation):

• Alkanes react with chlorine in the presence of UV light, forming chlorinated methane derivatives.

• This occurs in several steps:

• Methane (CH4) reacts with chlorine (Cl2) in the presence of UV light, producing:

1. Chloro Methane (CH3Cl)

2. Dichloro Methane (CH2Cl2)

3. Trichloro Methane (CHCl3)

4. Tetrachloro Methane (CCl4)

• Combustion Reaction with Oxygen:

• Methane reacts with oxygen to produce carbon dioxide, water, and heat. This reaction is commonly used for cooking.

• Reaction:

CH4 + 2O2 → CO2 + 2H2O + Heat