Saturated Hydrocarbons: Alkanes

Methane (CH4) is the simplest alkane in which four hydrogen atoms are linked to the carbon atom in a tetrahedral fashion.

If instead of a hydrogen atom, the carbon atom is further linked to another carbon atom, you get another alkane, namely ethane (C2H6).

Similarly, more carbon atoms can link with each other and the carbon chain can further extend to give a variety of hydrocarbons. The general formula of alkanes is CnH2n+2 where n is the number of carbon atoms in the alkane molecule.

Alkanes are colourless and odorless compounds. They have very low reactivity. Many of these compounds are gases or liquids.

Each compound differs from the previous one by a -CH2 unit. Such a series of compounds is known as homologous series. Each homologous series has a general formula.

Isomerism in Alkanes

Isomers are compounds which have the same molecular formula but have different structures. The first three hydrocarbons have only one isomer because there is only one way in which one, two or three carbon atoms can link to each other.

But when there are four carbon atoms, they can join in two different ways. Corresponding to the two carbon skeletons, there are two hydrocarbons - butane and isobutane. They are the isomers of butane as they have the same molecular formula (C4H10) but have different structures.

  • Butane: straight chain arrangement
  • Isobutane: branched chain arrangement

The number of possible structures in which different carbon atoms can link to each other increases with the increase in number of carbon atoms in the alkane molecules.

IUPAC Nomenclature of Alkanes

Earlier organic compounds were known by their popular or common names which mostly originated from the sources of these compounds. But as the number of these compounds increased, it became difficult to correlate the structure and name of the compound. This let to the need of a systematic nomenclature of compounds.

In 1892, International Union of Chemists met in Geneva, Switzerland and framed the rules for nomenclature. Later, this organization was named as International Union of Pure and Applied Chemistry (IUPAC) and the names approved by it are called IUPAC names of compounds.

For IUPAC naming, you must have idea about word root of carbon skeletons.

  1. meth
  2. eth
  3. prop
  4. but
  5. pent
  6. hex
  7. hept
  8. oct

1. For straight chain alkanes, the suffix -ane is added to the Greek root for the number of carbon atoms. For example,

C3H8, word root + ane → Prop + ane → Propane

2. For branch chain alkanes, the longest continuous chain of carbon atoms is selected as the main chain which gives the root name of the hydrocarbon. 

3. Then, the substituent alkyl groups are identified and named. The alkyl groups are named by replacing -ane suffix of the alkane with -yl suffix. For example, methyl (-CH3), ethyl (-C2H5), propyl (-C3H7), and so on.

4. The location of the substituent alkyl group on the main chain is specified by counting its number from that end of the carbon chain which gives it the lowest possible number. The number and alphabets are separated by a hyphen (-) and there is no space between the substituent and the root name.

Example 1: 3-ethylhexane

5. When more than one substituent is present on the carbon chain, then they are listed in alphabetical order.

Example 2: 3-ethyl-3-methylhexane

6. The identical substituents are indicated by the prefixes di (two), tri (three), tetra (four) and so on. The prefixes not considered while arranging the substituent's in the alphabetical order.

Example 3: 2,2-dimethylbutane