Lesson Introduction

Organic compounds have been classified on the basis of their structure. Open-chain structures containing carbon–carbon single (C–C), double (C=C) and triple bonds are said to be aliphatic compounds. The term ‘aliphatic’ is derived from the Greek word for fat or oil. In alicyclic molecules, the carbon atoms form a cyclic structure. The generic name aromatic was given originally to a structurally diverse collection of compounds, which had one common property, a fragrant odour. The discovery of benzene in 1825 by Faraday (isolated from whale oil) and 1845 by Hofmann (from coal tar) may be considered to institute the constitutional study of aromatic compounds. The term ‘aromatic’ acknowledges the fact that many fragrant compounds contain benzene rings. Aromatic compounds are cyclic molecules having delocalized π-electrons. Aromatic compounds may be carbocyclic (ring containing only carbon atoms) or heterocyclic (ring containing one or more heteroatoms). Heterocyclic compounds may be aromatic or alicyclic. For a compound to be classified as aromatic, it must fulfill both of the following criteria.

1. It must have an uninterrupted ring of p orbital-bearing atoms so that it has an uninterrupted cloud of delocalized π-electrons above and below the plane of the molecule, meaning the molecule must be cyclic and planar.
2. The π cloud must contain an odd number of pairs of π electrons.