Aromatic Hydrocarbons

Aromatic Hydrocarbons

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)

Nomenclature and Isomerism

• 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.

Aromaticity

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

Structure of benzene:

1. Molecular formula:v

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.

2. Straight chain structure not possible:

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.

3. Evidence of cyclic structure:

i) substitution of benzene:

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.

ii) addition of 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.

4. Kekule’s structure of benzene:

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.

5. Resonance description of benzene:

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.

6. Spectrosscopic measurments:

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°.

7. Molecular orbital structre:

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.

8. Representation of benzene:

Hence, there are three ways in which benzene can be represented.

Benzene and its homologous series

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.

Sources of aromatic compound:

Benzene and other aromatic compound are obtained from coal tar and petroleum

It can also be prepared in laboratory using some simple aliphatic compounds

1. Preparation of benzene

(i) industrial preparation of benzene from coal tar :

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.)

(ii) from acetylene.

Acetylene on passing through a red –hot tube trimerises to give benzene. We have already studied this concept in polymerization of alkynes.

(iii) Laboratory Methods Of Preparing Benzene And Toluene

(a) Decarboxylaation Of Aromatic Acid.

When sodium benzoate in heated with sodalime, benzene vapours distil over.

(b) Preparation Of Benzene From Phenol

When phenol vapours are passed over zinc dust, then it is reduced to benzene.

(c) Wurtz – Fittig Reaction:

When a solution of bromo benzene and iodo methane in dry ether is treated with metallic sodium, toluene is formed.

(d) Friedel Craft’s Reaction:

When benzene is treated with methyl chloride in the presence of anhydrous aluminium chloride, toluene is formed.