Physical & Chemical Properties of Alkanes with Examples & Videos

What are Alkanes?

Alkanes are referred to as saturated hydrocarbons, that is, hydrocarbons having all carbon atoms bonded to other carbon atoms or hydrogen atoms with sigma bonds only.

As the alkanes posses weak Van Der Waals forces, the first four members, C1 to C4 are gases, C5 to C17are liquids and those containing 18 carbon atoms or more are solids at 298 K. They are colourless and odourless.

Some salient physical and chemical properties of alkanes are:

Physical Properties of Alkanes

1. Structures of Alkanes

All the carbon atoms present in an alkane are sp3 hybridised that is, every carbon atom forms four sigma bonds with carbon or hydrogen atoms. General configuration of alkane is CnH2n+2. They exhibit tetrahedral geometry with a bond angle of 109.47° between them.

The methane molecule has a symmetrical tetrahedral structure.

Alkanes are referred to as saturated hydrocarbons, that is, hydrocarbons having all carbon atoms bonded to other carbon atoms or hydrogen atoms with sigma bonds only.

2. Solubility of Alkanes

  • Due to very little difference in electronegativity between carbon and hydrogen and the covalent nature of C-C bond or C-H bond, alkanes are generally non-polar molecules.
  • As we generally observe, polar molecules are soluble in polar solvents whereas non-polar molecules are soluble in non-polar solvents. Hence, alkanes are hydrophobic in nature, that is, alkanes are insoluble in water.
  • However, they are soluble in organic solvents as the energy required to overcome the existing Van Der Waals forces and the energy required to generate new Van Der Waals forces is quite comparable.

3. Boiling Point of Alkanes

As the intermolecular Van Der Waals forces increase with the increase of the molecular size or the surface area of the molecule we observe,

  • The boiling point of alkanes increases with increasing molecular weight.
  • The straight-chain alkanes are observed to have a higher boiling point in comparison to their structural isomers.

4. Melting Point of Alkanes

  • The melting point of alkanes follows the same trend as their boiling point, that is, it increases with an increase in molecular weight.
  • This is attributed to the fact that higher alkanes are solids and it’s difficult to overcome intermolecular forces of attraction between them.

It is generally observed that even-numbered alkanes have a higher trend in melting point in comparison to odd-numbered alkanes as the even-numbered alkanes pack well in the solid phase, forming a well-organised structure which is difficult to break.

Chemical Properties of Alkanes

Alkanes are the least reactive type of organic compound. Alkanes are not absolutely unreactive. Two important reactions that they undergo are combustion, which is the reaction with oxygen and halogenation, which is the reaction with halogens.

1. Combustion

A combustion reaction is a chemical reaction between a substance and oxygen that proceeds with the evolution of heat and light. Alkanes readily undergo combustion reactions when ignited. When sufficient oxygen is present to support total combustion then carbon dioxide and water are formed.

CH4 + 2O2 → CO2 + 2H2O + energy
2C6H14 + 19O2 → 12CO2 + 14H2O + energy

The exothermic nature of alkane combustion reactions explains the extensive use of alkanes as fuels. Natural gas which is used in home heating is predominantly methane.

2. Halogenation

Halogenation of an alkane produces a hydrocarbon derivative in which one or more halogen atoms have been substituted for hydrogen atoms. An example of an alkane halogenation reaction is

CH3-CH3 + Br2 → CH3-CH2-Br + HBr

Alkane halogenation is an example of a substitution reaction, a type of reaction that often occurs in organic chemistry.

A general equation for the substitution of a single halogen atom for one of the hydrogen atoms of an alkane is

R-H + X2 → R-X + H-X

Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations. In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS". He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.