Classification of organic compounds

Acyclic or open chained compounds:

• Also called aliphatic compounds.
• Consist of straight or branched compounds.

Alicyclic or closed chained compounds:

• Also called aliphatic cyclic.
• Carbon atoms are bonded in the form of a ring.
• Homocyclic - consisting only of carbon atoms joined in the ring.
  
• Heterocyclic - consist of atoms other than carbon joined in the ring.
  

Aromatic compounds:

• Benzenoid: These compounds consist of at least one benzene ring, i.e., a six-membered carbocyclic ring having alternate single and double bonds. Generally, these compounds have some fragrant odour and hence, named as aromatic (Greek word aroma meaning sweet smell).
  
• Non-benzenoid aromatics: There are aromatic compounds, which have structural units different from benzenoid type and are known as Non-benzenoid aromatics
  Ex: Tropolone, azulene etc.
  
  

Functional group:

• It is an atom or group of atoms responsible for the characteristic chemical properties of the compounds.Ex: (-OH) group - alcohol      (-CHO) group - aldehyde

Homologous series:

• Compounds containing a common functional group.
• The successive members differ by a –CH2 unit.Ex: alkanes - methane, ethane, propane
        Alkanols - methanol, ethanol, propanol