AROMATIC HYDROCARBONS
Benzene and it derivatives are known as aromatic hydrocarbons. Aromatic compounds form one of the major divisions of organic compounds. They contain a closed ring, which is usually of carbon atoms, but possess a system of bonding that gives them quite different properties to aliphatic or alicyclic compounds. The most common aromatic structure is the six-membered carbon ring, commonly known as the benzene ring because the compound benzene is the simplest molecule to contain this structure. Some aromatic compounds may also contain one or more atoms of an element other than carbon – usually nitrogen – in the ring structure.

Bonding in Aromatic Rings

Aromatic compounds show very different properties from aliphatic compounds and alicyclic compounds of the same molecular formula. This is because some of the electrons from the atoms forming the aromatic ring are delocalized – that is, they are not restricted to forming a conventional bond with just one other atom, but are able to circulate around the aromatic structure in one large molecular orbital along with electrons from other ring atoms. This delocalization gives such structures much greater stability than aliphatic compounds with the same molecular formula, since the lack of any weak point in the structure makes it difficult for other molecules to attack. Aromatic compounds easily undergo substitution reactions (which allow the aromatic structure to be preserved), whereas aliphatic and alicyclic compounds containing double bonds do not.

Nomenclature of Aromatic Hydrocarbons

C₆H₆ -->benzene    C₆H₅ --> phenyl   NO₂ --> nitro  NH₂ -->amino  CH₂=CH- --> vinyl 

 

Bromo benzene

amino benzene (aniniline)

nitrobenzene

vinyl benzene (styrene)

methyl benzene (toluene)

 hydroxy benzene (phenol)

    
                                                                        
    

Ortho      meta               para

 

Preparation of Benzene

 

The structure of benzene was for many years a problem for chemists. Its molecular formula, C6H6, was established in 1834, but no sufficient model of the structure was provided until many years later.

The Nature of Aromaticity
Examination of the behaviour of benzene led Friedrich Kekulé in 1865 to the conclusion that the benzene molecule is symmetrical, and that each carbon atom is directly united to one, and only one, atom of hydrogen. As carbon is assumed to be tetravalent (forming a total of four bonds with neighbouring atoms in a molecule), any graphic representation of the molecule should show four bonds between the carbon atom and the atoms directly bonded to it. This had proved to be a problem for many years, with various linear structures being proposed. It was not until Kekulé produced his resonance structures that a useful model became available.

Kekulé’s Resonance Model
Kekulé initially proposed a six-membered ring structure for benzene in which the carbon-to-carbon bonds were alternately single and double bonds. However, this formula, like others produced around that time, was not wholly satisfactory. He then attempted to find an explanation for the fact that when another atom was substituted for one of the hydrogen atoms, a single compound with a unique structure resulted, but when two atoms were substituted, three isomers were formed. To explain this, he suggested, in 1872, that the double bonds were shared by all the carbon atoms, so that each carbon-to-carbon bond is effectively a single bond half the time and a double bond for half the time:

This phenomenon is known as resonance, and according to the model the molecule oscillates, or resonates, between the two structures. This remained the most satisfactory explanation of the bonding in benzene well into the twentieth century.

• C₆H₁₄ --> C₆H₆ + 4H₂  (at 500 °C)
• C₆H₅-COONa + NaOH + heat --> C₆H₆ + Na₂CO₃

Reactions of Aromatic Hydrocarbons

A characteristic subsitution reaction of aromatic compounds is that with nitric acid (HNO₃), producing nitro compounds (compounds containing the -NO₂ group).
Aromatic compounds react similarly with sulphuric acid (H₂SO₄), to give sulphonic acids (containing the -SO₃H group). When nitro compounds are reduced, they are converted into aromatic amino compounds. Further treatment with nitrous acid (HNO₂) at low temperatures produces diazo compounds (compounds containing a (single bond)N(double bond)N(single bond) linkage); if the solution is warmed, phenols are obtained. Aromatic alcohols are prepared by methods analogous to those employed in producing aliphatic alcohols – the corresponding halogen derivatives are heated with water, weak alkalis, or silver hydroxide.

• Halogenation

C₆H₆ + Cl₂ --> C₆H₅-Cl + HCl (catalayst: FeCl₃)

• Nitration

C₆H₆ + HNO₃ --> C₆H₅-NO₂ + H₂O  (catalyst: H₂SO₄ , 50-60 °C)

• Alkylation(Friedel-Crafts Reaction)

C₆H₆ + R-X --> C₆H₅-R + HX

• Sulfonation

C₆H₆ + H₂SO₄ + --> C₆H₅-SO₃H + H₂O

• Hydrogenation

C₆H₆ + 4H₂ --> C₆H₆  (heat, pressure, Ni catalyst)

Polyhydric Aromatic Hydrocarbons

                 

Naphtalene            anthracene                phenantherene