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Alkanes

A hydrocarbon is an organic molecule consisting only of hydrogen and carbon.

An alkane is a hydrocarbon in which all of the bonds are single covalent bonds (ie each of the two atoms contributes only one electron to the bond).

Alkanes are widely used as the main fuel for motors (e.g. in cars and planes) and gas cookers and heating.

Alkanes are used to fuel cars, planes and ships.
Alkanes are used to fuel cars, planes and ships.

Alkanes consisting of just one chain (without any branches) are a homologous series. The general formula for alkanes is: $$$\text{C}_\text{n}\text{H}_{2\text{n}+2}$$$

The name of alkanes always ends in '-ane'. An example is 'methane'. The letters before '-ane' (e.g. 'meth' in 'methane') depend on the number of carbon atoms.

The first four unbranched alkanes are shown in the table below.

Name Molecular and structural formula Full structural formula
Methane
  • $$\ce{CH4}$$
  • $$\ce{CH4}$$
Ethane
  • $$\ce{C2H6}$$
  • $$\ce{CH3CH3}$$
Propane
  • $$\ce{C3H8}$$
  • $$\ce{CH3 CH2CH3}$$
Butane
  • $$\ce{C4H10}$$
  • $$\ce{CH3CH2CH2CH3}$$

The physical properties of alkanes depend on the length of the carbon chain.

More energy is required to separate larger alkane molecules. This is because there can be more intermolecular forces between larger molecules. They have more surface area that can touch other molecules for intermolecular forces to act.

The need for greater energy to separate larger alkanes has several effects:

  • The melting and boiling points are higher for alkanes with longer carbon chains.
  • Liquid alkanes with longer carbon chains are more viscous (they flow less easily and are thicker).
  • Alkanes with longer carbon chains are less volatile (they evaporate less easily), making them less flammable.

Alkanes are not soluble in water. When water and alkanes are mixed, they separate into two separate layers.

When oil spills occur, the crude oil floats on top of the seawater because it mainly contains alkanes, which are less dense than water.
When oil spills occur, the crude oil floats on top of the seawater because it mainly contains alkanes, which are less dense than water.
Boiling points of some straight chain alkanes
Number of carbon atoms 1 2 3 4 5 10 15
Boiling point ($$^{\circ}\text{C}$$) $$-164$$ $$-89$$ $$-42$$ $$-1$$ $$36$$ $$176$$ $$269$$

Complete combustion occurs when there is sufficient or excess oxygen present.

There must be enough oxygen atoms to convert all of the alkanes into the products of the reaction: carbon dioxide $$(\ce{CO2})$$ and water $$(\ce{H2O})$$.

The complete combustion of methane has the equation: $$$\ce{CH4 + 2O2 -> CO2 + 2H2O}$$$

Incomplete combustion occurs when there is insufficient oxygen. Such a combustion produces soot (carbon particles $$\ce{C}$$) and carbon monoxide $$(\ce{CO})$$ (in addition to water) instead of carbon dioxide.

The incomplete combustion of methane can occur via two different reactions: $$$\ce{2CH4 + 3O2 -> 2CO + 4H2O}$$$ $$$\ce{CH4 + O2 -> C + 2H2O}$$$

Carbon monoxide $$\ce{CO}$$ is a very poisonous gas.

In a substitution reaction, an atom in a compound is replaced by another atom.

An example is the reaction of an alkane in which at least one hydrogen atom of the alkane is replaced by (substituted with) a halogen atom (e.g. fluorine, chlorine or bromine).

The C-C bonds inside the alkane remain intact.

The substitution reaction between alkanes and halogens is exothermic (it generates heat). Ultraviolet (UV) light (included in sun light) is usually required to start this reaction.

The reaction of ethane with chlorine is an example:

Depending on the relative quantities of the alkane and chlorine, the products of the reaction may include different numbers of chlorine atoms.

Possible products from reacting chlorine and methane include $$\ce{CH3Cl}$$, $$\ce{CH2Cl2}$$, $$\ce{CHCl3}$$ or $$\ce{CCl4}$$. Hydrogen chloride $$\ce{HCl}$$ is always a second product of the reaction.

Trichloromethane $$(\ce{CHCl3})$$, commonly known as chloroform, was used as an anaesthetic in the 19th century.
Trichloromethane $$(\ce{CHCl3})$$, commonly known as chloroform, was used as an anaesthetic in the 19th century.

Alkanes are relatively unreactive. because the C-H and C-C bonds are very stable.

There are two main types of reactions of alkanes:

  • In a combustion reaction, a fuel reacts with oxygen and heat is released. The reaction of alkanes with oxygen is an example of combustion.

    An example is the complete combustion reaction of methane with oxygen: $$$\ce{CH4 + 2O2 -> CO2 + 2H2O}$$$

  • In substitution reactions, alkanes react with halogens (typically chlorine or bromine). The halogen replaces one or several hydrogen atoms on the alkane while the alkane's carbon-carbon bonds remain intact.

    An example is the reaction between methane and chlorine: $$$\ce{CH4 + Cl2 -> CH3Cl + HCl}$$$

Both combustion and substitution reactions involving alkanes require an initial input of energy.

  • Combustion reactions usually get the energy from heat.
  • Substitution reactions usually get the energy from ultraviolet (UV) light or sunlight.

Once under way, combustion and substitution reactions with alkanes are exothermic (they generate heat).

Burning of gasoline. Gasoline contains alkanes as well as other hydrocarbons. When it burns, a combustion reaction takes place.
Burning of gasoline. Gasoline contains alkanes as well as other hydrocarbons. When it burns, a combustion reaction takes place.

Branched alkanes contain side chains of carbon that branch off the main chain.

The fact that alkanes can be branched means that there can be many alkanes with the same molecular formula but a different structure. Such molecules with a different structure but the same molecular formula are called isomers.

Alkanes with four or more carbon atoms have multiple isomers that each have the same number of carbon and hydrogen atoms.

Hexane (left) and 2-ethylbutane (right) are both isomers with the molecular formula $$\ce{C6H14}$$
Hexane (left) and 2-ethylbutane (right) are both isomers with the molecular formula $$\ce{C6H14}$$

Methane, ethane and propane do not have isomers as any form of arrangement will still give the same molecular structure, with only a single chain of carbon atoms.

All branched and unbranched alkanes have the general formula $$\text{C}_\text{n}\text{H}_{2\text{n}+2}$$.