Alkane

Alkanes are the saturated hydrocarbons having a general formula C_nH_2n+2 where n= any positive integer. The saturated hydrocarbons are also called paraffin for being less reactive (Parum affinis– little activity). All the saturated hydrocarbons have a single covalent bond between C-C and C-H.

General methods of preparation of alkane

1. From sodium salt of carboxylic acid (sodium carboxylate): Heating sodium carboxylate with soda-lime (NaOH + CaO) gives alkane. This reaction is called soda-lime decarboxylation. Decarboxylation means loss of the carboxyl group.

2. Wurtz reaction: Haloalkane is reacted with sodium metal in presence of dry ether to give alkane having double carbon atoms. This reaction is called the Wurtz reaction.

3. Catalytic Hydrogenation: Alkene or alkynes react with hydrogen in presence of Ni, Pt, or Pd catalyst to give alkane. (Hydrogenation means the addition of hydrogen).

4. Reduction of Haloalkane: Haloalkanes are reduced to alkanes in presence of reducing agents like Zn/Cu in ethanol, Ni-Al alloy in NaOH, Ni/H₂, Sn/HCl, LiAlH₄, NaBH₄, or red P₄/HI.

Physical properties:

1. The first four members are colorless and odourless gases. From C₅ to C₁₅ are colorless liquids and higher are solids.
2. They are insoluble in water but soluble in organic solvents like alcohol, ether, etc.
3. The boiling point of alkane increases with an increase in the number of carbon atoms.

Chemical Properties:

1. Substitution reaction

i. Halogenation:

a. Chlorination:

b. Bromination:

c. Iodination:

ii. Nitration:

iii. Sulphonation:

iv. Combustion/oxidation:

Reactions Summary

Important Conversions

Some Important Questions

1. What is Wurtz’s reaction? Give an example.
2. An organic compound A reacts with sodium metal in dry ether to give 2,3-dimethyl butane. Suggest the structure of A with chemical reaction.
3. What happens when 2-Bromo propane is heated with sodium metal in the presence of dry ether?
4. Give the preparation of ethane starting from:
   a. Ethene b. Bromoethane c. Sodium propanoate d. methane
5. What happens when sodium ethanoate is heated with soda lime?
6. Arrange in decreasing order of boiling point: n-hexane, iso-hexane, n-heptane.