What are the properties of alkanes?

Structural features

Alkanes are the simplest of the hydrocarbons, comprising carbon and hydrogen chains with only single bonds. The alkanes have no functional groups and only single bonds. This means that they are relatively unreactive, due to the strength of the carbon-carbon and the carbon-hydrogen bonds.

Average bond enthalpy terms

  • carbon-carbon bond = 348 kJ mol-1
  • carbon-hydrogen bond = 412 kJ mol-1

Another factor that contributes to the low reactivity of the alkanes is their lack of polarity. The electronegativity of carbon and hydrogen is similiar and the electrons are shared evenly between them in single bonds. No dipoles are formed, meaning that there are no regions of partial positive, or negative charge on the hydrocarbon chain that can invite attack by nucleophiles or electrophiles.


Combustion

Like most organic compounds, alkanes burn in air, or oxygen, giving carbon dioxide and water:

CH4 + 2O2 CO2 + 2H2O

When the supply of oxygen is insufficient, carbon monoxide and microparticulates of carbon may also be formed. This is called incomplete combustion.

Carbon monoxide is an odourless, colourless, toxic gas that poisons by replacing the oxygen molecules in the haemoglobin units of the red blood corpuscles, preventing oxygen reaching the brain. Symptoms are drowsiness, coma and death.

For this reason, gas appliances must be regularly checked to ensure good air flow.

Combustion reactions proceed via a free radical mechanism, obviating the high activation energy needed to break the C-C and O-H bonds.

Halogenation

In the presence of ultraviolet light, the alkanes react with halogens via a free radical mechanism. Free radical processes are chain reactions that proceed in three stages:

  1. 1 Initiation
  2. 2 Propagation
  3. 3 Termination

In the initiation stage, chlorine molecules are broken into free radicals by the UV light.

Cl2 2Cl

In the propagation stage the chlorine free radicals react with the alkane, making alkyl free radicals and hydrogen chloride.

Cl + CH4   CH3 + HCl

The alkyl free radical can then react with another chlorine molecule, making a haloalkane and propagating the reaction further:

CH3 + Cl2 Cl + CH3Cl

The chloride free radical can then react with another alkane molecule and continue the process.

The final stage in a free radical reaction is termination, when two free radicals collide to end the chain reaction:

Cl +  CH3  CH3Cl

Although this reaction proceeds easily, it is of limited use in synthesis, as a mixture of products is formed. Free radicals react with the first molecule that they encounter, meaning that several by-products are also formed. To try to limit the number of by-products, the gases are mixed in an appropriate ratio, according to the stoichiometry of the equation. In the case of the reaction between methane and chlorine in the presence of ultraviolet light, the equation is:

CH4 + Cl2 CH3Cl + HCl

The stoichiometry of the reaction suggests that the ideal molar ratio of chlorine to methane is 1:1