Properties Of Alkanes

What are Alkanes?

Alkanes are referred to as saturated hydrocarbons that are, 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, \(C_1\) to \(C_4\) are gases, \(C_5\) to \(C_{17}\) are liquids and those containing 18 carbon atoms or more are solids at 298 K. They are colorless and odorless.

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Why Alkanes are Saturated Hydrocarbons?

Saturated & Unsaturated Compounds

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 \(sp^3\) hybridized that is, every carbon atom forms four sigma bonds with carbon or hydrogen atoms. General configuration of alkane is \(C_nH_{2n+2}\). They exhibit tetrahedral geometry with a bond angle of 109.47° between them.

The methane molecule has a symmetrical tetrahedral structure.

Structures of Alkanes

Structures of Alkanes – Methane, Ethane & Propane

2. Solubility of Alkanes

  • Due to very little difference of electronegativity between carbon and hydrogen and 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 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.

Physical-Properties-Of-Alkanes-And-Their-Variations

It is generally observed that even-numbered alkanes have 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-organized 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 the products.

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, 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 occurs often 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

Also Read: Pyrolysis of Hydrocarbons: Alkanes

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