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 :
- Saturated hydrocarbons
- Unsaturated hydrocarbons
- Aromatic 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.
Hydrocarbons containing at least one carbon-carbon multiple bond (double or triple) are termed as unsaturated hydrocarbons. Study the following important points :
- Open chained unsaturated hydrocarbons containing only double bonds (one or more) are termed as alkenes.
- Open chained unsaturated hydrocarbons containing triple bonds (one or more) are termed as alkynes.
- Closed rings of unsaturated hydrocarbons are termed as cycloalkenes or cycloalkynes depending upon whether the hydrocarbon contains double or triple bonds respectively.
- A compound is considered as a derivative of alkyne when both double and triple bonds are present.
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.
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 .
- Primary carbon (1°) : A carbon atom attached to one or no carbon atom is known as primary carbon or 1° carbon.
- Secondary carbon (2°) : A carbon atom attached to two other carbon atoms is called a secondary (2°) carbon.
- Tertiary carbon (3°) : A carbon is termed as tertiary carbon (3°) if it is attached to three other carbon atoms.
- 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 :
Nomenclature of hydrocarbons
There is a separate section on nomenclature of organic compounds. Refer to Nomenclature of Organic Compounds to learn nomenclature.