Take a moment and think of sub-stances that have a strong fragrance. What kind of things come to your mind?
Perfume, Vanila or cinnamon? They smell differently, they have something in common. These substances are made of aromatic compounds [Greek: Aroma-Pleasant smelling]. However, some compounds are chemically aromatic but do not have distinct smell. The aromatic hydrocarbons are classified depending upon number of rings present in it.
(i) Monocyclic aromatic hydrocarbon (MAH)
(Ex) Benzene (C6H6) and Toluene (C7H8)
(ii) Polycyclic aromatic hydrocarbon(PAH)
(Ex) Naphthalene (C10H8) and Anthra-cene (C14H10)
• We have already discussed about nomenclature of aromatic hydrocarbons in Unit:11. The first member of aromatic hydrocarbon is benzene (C6H6) represented by a regular hexagon with a circle inscribed in it.
• Since, all the six hydrogen atom in benzene are equivalent, it can give only one mono-substituted compound (Ex) methyl benzene (C6H5-CH3) which named as toluene.
• When di substitution occurs either by a similar monovalent atom or two different atoms or groups in benzene, then three different position isomers are possible. Their relative positions are indicated as ortho (1,2), meta (1,3) and para (1,4). For example, consider dimethyl benzene which is named as xylene.
Huckel proposed that aromaticity is a function of electronic structure. A compound may be aromatic, if it obeys the following rules
i. The molecule must be co-planar
ii. Complete delocalization of π electron in the ring
iii. Presence of (4n+2) π electrons in the ring where n is an integer (n=0,1,2….)
This is known as Huckel’s rule.
Some of the examples for Huckel rule
Elemental Analysis and molecular weight determination have proved that the molecular formula of benzene is C6H6. This indicates that benzene is a highly unsaturated compound.
Benzene could be constructed as a straight chain or ring compound but it not feasible since it does not show the properties of alkenes or alkynes.for example, it did not decolourise bromine in carbon tetrachloride or acidified KMnO4. It did not react with water in the presence of acid.
Benzene reacts with bromine in the presence of AlCl3 to form mono bromobenzene.
Formation of only one monobromo compound indicates that all the six hydrogen atoms in benzene were identical. This is possible only if it has a cyclic structure of six carbons each containing one hydrogen.
Benzene can add on to three moles of hydrogen in the presence of nickel catalyst to give cyclohexane.
This confirms cyclic structure of ben-zene and the presence of three carbon-car-bon double bond.
In 1865, August Kekule suggest-ed that benzene consists of a cyclic planar structure of six carbon with alternate sin-gle and double bonds.
There were two objections:
i) Benzene forms only one orthodisub-stituted products whereas the Kekule’s structure predicts two o-di substituted products as shown below.
ii) Kekule’s structure failed to explain why benzene with three double bonds did not give addition reactions like other alkenes.To overcome this objection, Kekule suggested that benzene was mixture of two forms (1 and 2)which are in rapid equilibrium.
The phenomenon in which two or more structures can be written for a substance which has identical position of atoms is called resonance. The actual structure of the molecule is said to be resonance hybrid of various possible alternative structures. In benzene, Kekule’s structures I & II represented the resonance structure, and structure III is the resonance hybrid of structure I &II
The structures 1 and 2 exist only in theory. The actual structure of benzene is the hybrid of two hypothetical resonance structures.
Spectroscopic measurements show that benzene is planar and all of its car-bon-carbon bonds are of equal length 1.40A°. This value lies between carbon-car-bon single bond length 1.54A° and car-bon-carbon double bond length 1.34A°.
The structure of benzene is best de-scribed in terms of the molecular orbital theory. All the six carbon atoms of benzene are sp2 hybridized. Six sp2 hybrid orbitals of carbon leanerly overlap with six one is or-bitals of hydrogen atoms to form six C - H sigma bonds. Overlap between the remain-ing sp2 hybrid orbitals of carbon forms six C-C sigma bonds.
All the σ bonds in benzene lie in one plane with bond angle 120°. Each carbon atom in benzene possess an un hybridized p-orbital containing one electron. The lateral overlap of their p-orbital produces 3 π- bond The six electrons of the p-orbitals cover all the six carbon atoms and are said to be delocalised. Due to delocalization, strong π-bond is formed which makes the molecule stable. Hence unlike alkenes and alkynes benzene undergoes substitution reactions rather addition reactions under normal conditions.
Hence, there are three ways in which benzene can be represented.
Benzene and its homologous series are colorless liquids with pleasant odour .They are lighter than water and insoluble in it. Their vapours are highly flammable, and volatile and toxic in nature.
Benzene and other aromatic compound are obtained from coal tar and petroleum
It can also be prepared in laboratory using some simple aliphatic compounds
Coal tar is a viscous liquid obtained by the pyrolysis of coal. During fractional distillation, coal tar is heated and distills away its volatile compounds namely benzene, toluene, xylene in the temperature range of 350 to 443 K. These vapours are collected at the upper part of the fractionating column (Table 13.5.)
Acetylene on passing through a red –hot tube trimerises to give benzene. We have already studied this concept in polymerization of alkynes.
When sodium benzoate in heated with sodalime, benzene vapours distil over.
When phenol vapours are passed over zinc dust, then it is reduced to benzene.
When a solution of bromo benzene and iodo methane in dry ether is treated with metallic sodium, toluene is formed.
When benzene is treated with methyl chloride in the presence of anhydrous aluminium chloride, toluene is formed.