Naturalsources like resins, balsams, aromatic oils etc., contain aliphatic compounds and also a group of compounds with pleasant odorour. These odorous compounds were arbitrarily classified as aromatic. Most of the simple aromatic hydrocarbon compounds were found to contain six carbon atoms. Further when aromatic compounds were subjected to various methodsof treatment, they often produced benzene or a derivative of benzene. Hence these aromatic compounds are called benzenoid compounds.
Physical properties of aromatics:
Theorganic chemistry the word aromatic be use to explain aromatic substances such like benzaldehyde, toluene etc. Aromatic represent to benzene and structural homologues.
Manyhydrocarbon compound inaccessible from natural source are aromatic to benzene toluene and benzaldehyde, complex compounds such as the female steroidal harmone estrone and the well known analgesis, morphine have aromatic rings. Many synthetic drugs used medicinally are also aromatic in part.
Benzene is the first in the aromatic series of compounds. Physical properties of the whole series of compounds which contain one or more benzene rings in their molecules are called aromatic hydrocarbon compoundas. Certain heterocyclic compounds like pyridine which do not have benzene rings also come under this classification.
Compoundswhich contain benzene rings or a condenses system of benzene rings havethe following Physical properties which are not shown by the analogous aliphatic and alicyclic compounds.
With respect to benzene the enthalpy of hydrogenation, the enthalpy much smaller than unsaturated compound is hydrogenated.
Liquids or solids with characteristic. They are insoluble in water but are miscible in all proportions with organic solvents such as ethanol, ether etc. they are inflammable and burn with sooty flame. Toxic also carcinogenic in character. The boiling points increase with increase in molecular weight, but their melting points do not exhibit regular gradation melting point to depend on molecular symmetry then on molecular weight.