Hydrocarbons Introduction


Organic compounds containing only carbon and hydrogen atoms are called hydrocarbons.

Classification of hydrocarbons

Depending upon the type of carbon-carbon bonds present, they can be classified into the following three categories :

  1. Saturated hydrocarbons
  2. Unsaturated hydrocarbons
  3. Aromatic hydrocarbons

Saturated hydrocarbons

Saturated hydrocarbons contain only carbon-carbon single bonds. In other words, there is no double or triple bonds present in the compound.

If different carbon atoms of saturated hydrocarbons are linked together to form open chain, they are termed as alkanes. On the other hand, if carbon atoms form a closed ring, they are termed as cycloalkanes.

Saturated hydrocarbons


Saturated hydrocarbons example : alkane


Alkane example


Alkane example : Ethane


Closed ring example : Cyclohexane

Unsaturated hydrocarbons

Hydrocarbons containing at least one carbon-carbon multiple bond (double or triple) are termed as unsaturated hydrocarbons. Study the following important points :

Unsaturated hydrocarbons


Unsaturated hydrocarbons example : Ethene


Alkyne example


Closed ring example : Cyclohexene


alicyclic ring example : Cyclohexyne


cyclic ring example : Cyclohexen-3-yne


Alkyne example : Buten-3-yne

Aromatic hydrocarbons

Aromatic hydrocarbons are a special type of cyclic compounds. Most of aromatic compounds contain benzene ring and are known as benzenoids. However, there are some examples of aromatic hydrocarbons that do not contain benzene but instead contain other highly unsaturated ring. Such compounds are termed as non-benzenoids.

Aromatic hydrocarbons


Aromatic hydrocarbons example : Benzene


Aromatic hydrocarbons example : Toluene

Primary, secondary, tertiary and quaternary carbons

Based upon the number of carbon atoms attached to a carbon, the carbon atom is termed as primary (1°), secondary (2°), tertiary (3°) and quaternary (4°) carbon .

  1. Primary carbon (1°) : A carbon atom attached to one or no carbon atom is known as primary carbon or 1° carbon.
  2. Secondary carbon (2°) : A carbon atom attached to two other carbon atoms is called a secondary (2°) carbon.
  3. Tertiary carbon (3°) : A carbon is termed as tertiary carbon (3°) if it is attached to three other carbon atoms.
  4. Quaternary carbon (4°) : A carbon is termed as quaternary (4°) carbon if it is attached to four other carbon atoms.

The hydrogen atoms attached to primary, secondary and tertiary carbon atoms are termed as primary, secondary and tertiary hydrogen atoms. Since a quaternary carbon does not contain any hydrogens, there is nothing like quaternary hydrogen atoms. Take a look at the following example :

primary secondary tertiary and quaternary carbon atoms

Nomenclature of hydrocarbons

There is a separate section on nomenclature of organic compounds. Refer to Nomenclature of Organic Compounds to learn nomenclature.