Alkanes

Hydrocarbons which contain only single bonds are called alkanes. They are called saturated hydrocarbons because there is a hydrogen in every possible location. This gives them a general formula C_nH_2n+2.

The first four alkanes are methane, ethane, propane, and butane with the Lewis symbols shown below.

Past this number of carbons, the -ane suffix is retained and the number prefixes penta-, hexa-, hept-, oct-, non-, dec-, etc are used. Alkyl groups are used as substituents, and alkane derivatives have many applications.

The alkanes are highly combustible and are valuable as clean fuels, burning to form water and carbon dioxide. Methane, ethane, propane and butane are gases and used directly as fuels. Alkanes from pentane up to around C₁₇H₃₆ are liquids. Gasoline is a mixture of alkanes from pentane up to about decane. Kerosene contains alkanes from about n=10 to n=16. Above n=17 they are solids at room temperature. Alkanes with higher values of n are found in diesel fuel, fuel oil, petroleum jelly, paraffin wax, motor oils, and for the highest values of n, asphalt.

Alkane derivatives are used in hundreds of products such as plastics, paints, drugs, cosmetics, detergents, insedticides, etc., so the fossil fuel resource from which we obtain the alkanes is much too valuable to burn it all as a motor fuel.

Alkyl Groups

If a hydrogen is removed from an alkane, it can be used as a substituent and is called an alkyl group. Alkyl groups are named by dropping the -ane suffix of the alkanes and adding the suffix -yl. Methane becomes a methyl group, ethane an ethyl group, etc.

Alkane derivatives can be formed by substituting an alkyl group for one of the hydrogens.

Alkane Derivatives

Almost an unlimited number of derivatives can be made from the alkanes since any hydrogen can be substituted by an alkyl group, a halide, etc.

If the substituent is an alkyl group, then the derivative will have the same empirical formula as a larger alkane, so the empirical formula for an organic compound is insufficient to identify it. For example, if a methyl group is substituted for for one the hydrogens on the center carbon of a propane molecule, the result is called methylpropane. It has the same molecular formula as butane. These two molecules are said to be structural isomers.

Because of multiple substitution possibilities, a naming convention has been developed to name such derivatives.

Hydrocarbon Naming Convention

Because of the multitude of ways organic molecules may be formed, a set of rules for naming alkane derivatives is a part of the IUPAC system for organic compounds.

1. Find the longest chain of carbon atoms (the backbone) and name the atom as a derivative of the alkane with that number of carbons.
2. Use the positions and names of the substituents that replace the hydrogen as prefixes. When more than one substituent is present, either on the same carbon atom or on different carbon atoms, list them alphabetically. If there is more than one of the same substituent, use prefixes di-, tri-, tetra-, etc to indicate the number of them.
3. Number the carbon atoms on the backbone by counting from the end nearest the substituents. The position of attachment of each substituent is identified by the number of the carbon atom to which it is attached. Commas are used to separate the numbers for the same substituent and a dash separated the numbers from the name.