Hydrocarbons are the parent of all organic compounds. Hydrocarbons consist of only carbon and hydrogen atoms.
Hydrocarbons : Organic compounds containing only carbon and hydrogen atoms are called hydrocarbons.
Examples of hydrocarbons
CH4, CH3−CH3, CH2=CH2 etc.
All other organic compounds are derived from hydrocarbons by replacing one or more of their hydrogen atoms. The groups that replace hydrogen from hydrocarbons are known as Functional Groups.
Examples of functional groups
CH3−OH, CH3−CHO, CH3−COOH etc.
Functional groups determine the chemical properties of organic compounds. For example CH3−OH and CH3CH2CH2−OH have similar chemical properties due to the presence of the same functional group (−OH) despite being attached to the chains containing different number of carbon atoms.
A functional group may be defined as an atom or a group of atoms present in a molecule which largely determines its chemical properties.
All molecules other than hydrocarbons (which only contains carbon-hydrogen framework) consist of two parts
Carbon-hydrogen frameworks (usually denoted by R) mainly affect the physical properties of the compound such as melting point, boiling point, density, solubility, refractive index etc.
Functional groups as said above determine chemical properties.
Note : A molecule may show exceptions if it is very large or contains many functional groups.
Some of the most important functional groups are given below :