Hydrocarbons: Methods of Preparation and Physical Properties of Alkanes (For CBSE, ICSE, IAS, NET, NRA 2022)

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Alkanes (Paraffins)

  • Alkanes are saturated hydrocarbons.
  • They are very less reactive towards various reagents . Hence, they are called paraffins (parum means little, affins means affinity) .

Methods of Preparation

From Haloalkanes

By Reduction of Haloalkanes

The replacement of halogen atom of haloalkanes with hydrogen is called the reduction and can be carried out by following reagents:

Zinc and Dilute HCl

HI in the Presence of Red Phosphorus

Catalytic Reduction

By Using Grignard՚s Reagent

A Grignard reagent is a compound of the type which is prepared by reacting a haloalkane with magnesium metal in the presence of dry ether.

The Grignard՚s reagents are used to prepare various compounds like hydrocarbons, ethers, alcohols and carboxylic acids.

By Wurtz Reaction

An alkyl halide reacts with sodium metal in the presence of dry ether and forms the higher alkanes.

From Unsaturated Hydrocarbons

The Unsaturated Hydrocarbon (i.e.. alkenes and alkynes) can be converted to alkanes by the addition of hydrogen in the presence of a catalyst like nickel, platinum or palladium.

This reaction is also called hydrogenation and used to prepare vegetable ghee from edible oils.

From Alcohols, Aldehydes and Ketones

Alcohols , aldehydes and ketones on reduction with , in presence of red phosphorus, give alkanes.

From Carboxylic Acids

Heating with Soda Lime

An alkane with one carbon less than those present in the parent carboxylic acid is obtained.

By Reduction of Carboxylic Acid

An alkane with same number of carbon atoms as in the starting carboxylic acid is obtained.

Kolbe՚s Electrolysis

Sodium or potassium salt of a carboxylic acid, on electrolysis, gives a higher alkane.

At anode

Ethane can be obtained by the electrolysis of sodium ethanoate.

Physical Properties of Alkanes

Physical State

  • The physical state of alkanes depends upon the intermolecular forces of attraction present between molecules which in turn, depend upon the surface area of the molecules.
  • As the molecular mass of the alkanes increases their surface area also increases, which in turn, increases the intermolecular forces of attraction and the physical state of alkanes changes from gaseous to liquid, and then to solid
  • The alkanes containing 1 to 4 carbon atoms are gases, those containing 5 to 17 carbon atoms are liquids, and the higher ones are solids.
  • In the isomeric alkanes, the straight chain alkanes will have maximum surface area, and hence, stronger intermolecular force of attraction. As the branching increases, surface area decreases and intermolecular force of attraction decreases.


  • The density of alkanes increases with the increase in molecular mass which increase with the increase in the number of carbon atoms.
  • All alkanes are lighter than water i.e.. their density is less than .

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