Classifcation of organic compounds

Organic compounds can be classified into aliphatic and aromatic compound depending on their molecular  structure which is the way atoms are arranged in the molecules.
 
Aliphatic compounds

compounds whose molecules are composed of chains of carbon atoms are known as aliphatic compounds.
There is no limit to the number of carbon atoms in a given chain. Often, a given carbon chain may even have one or more branches. Some examples of aliphatic compounds are

1. peptane(a straight chain compound), and 
2. 2-methylbutane(a branched chain compound).

Such straight and branched chain aliphatic compounds are called acyclic compounds
In the actual molecular structure, the carbon chains are not in truly straight lines as is conveniently represented on paper. Instead, the chains are zig-zag  lines because of the tetrahedral nature of the carbon bonds

Sometimes, the end carbon atoms of an open aliphatic chain can also join together to form a closed system or rings as in cyclopropane and cyclohexane.Such  compounds  are know as cyclic compounds



Aromatic Compounds
 
Aromatic compounds are a special class of cyclic compounds based on benzene, C6H6, a 6-carbon ring compound. All other aromactic compounds are derivatives of benzene, .e.g. pheylamine(aniline) and phenol. Some derivatives may also contain straight carbon chains as side chains. The following are some examples of aromatic compounds.