AROMATIC HYDROCARBONS (ARENES)

  • Most of the aromatic compounds have pleasant smell (aroma means pleasant smelling) and most of them contain benzene ring.
  • Aromatic compounds containing benzene ring are called benzenoid compounds and those which do not contain benzene ring are called non-benzenoid compounds.

Structure of Benzene

  • The molecular formula of benzene is C6H6, which indicates a high degree of unsaturation.
  • But benzene was found to be a stable molecule and form a triozonide which indicates the presence of three double bonds.
  • Also it produces only one monosubstituted derivative which indicates that all the six carbon and six hydrogen atoms of benzene are identical.

On the basis of these observations, August Kekulé proposed the following structure for benzene having cyclic arrangement of six carbon atoms with alternate single and double bonds.

The Kekule structure indicates the possibility of two isomeric1,2-disubstituted derivatives. But actually benzene forms only one 1,2-disubstituted derivative.

In order to overcome this problem, Kekule suggested the concept of oscillating nature of double bonds in benzene.

Kekule structure could not explain the stability of benzene and the preference of benzene to substitution reaction rather than addition reaction.

Resonance concept of Benzene

According to this concept, benzene is a hybrid of the following two resonace structures.

The actual structure of benzene is not A or B . it is in between thes two resonating structures. So benzene is denoted by a hexagon with a dotted circle, which represents the delocalised π-electrons.

Orbital Overlap Concept of Benzene

  • In Benzene all the six carbon atoms are sp2 hybridized. Two sp2 hybrid orbitals of each carbon atom overlap with sp2 hybrid orbitals of adjacent carbon atoms to form six C—C sigma bonds which are in the hexagonal plane.
  • The remaining one sp2 hybrid orbital of each carbon atom overlaps with 1s orbital of a hydrogen atom to form six C—H sigma bonds.

Now each carbon atom contains one unhybridised p orbital perpendicular to the plane of the ring. They overlap laterally to form three π-bonds. There are two possible overlapping.

These give two Kekule structures with localized π electrons. But in benzene all the C-C bonds are identical and the bond length is 139 pm. To explain this, it is suggested that the p-orbitals of all the C atoms overlap each other. Thus in benzene, there is an electron cloud in the form two rings one above and one below the hexagonal ring as follows:

So the six π electrons are delocalised and can move freely about the six carbon nuclei. Presence of delocalised π electrons in benzene makes it more stable. The delocalised π electrons can be denoted by a circle inside a hexagonal ring. So benzene is best represented as:

Aromaticity

  • Aromaticity is defined by a rule called ‘Huckel rule’. According to this rule, “cyclic, planar, conjugated systems containing (4n+2) π electrons are aromatic”. Where n is the number of rings. n may be 1,2,3,….
  • For benzene n = 1, so it should contain 6 delocalised π electrons. If n = 2, the no. of delocalised π electrons =10 and so on. Example for some aromatic compounds are:

Preparation of Benzene

1. Cyclic polymerisation of ethyne (acetylene):

3 C2H2 Red hot iron tube & 873K C6H6

2.Decarboxylation of aromatic acids: Sodium salt of benzoic acid on heating with sodalime gives benzene.

3.Reduction of phenol: Phenol is reduced to benzene by passing its vapours over heated zinc dust.

Chemical Properties

Aromatic compounds generally undergo electrophilic substitution reactions. Under special conditions, they can also undergo addition and oxidation reactions.

I) Electrophilic Substitution Reactions

These are reactions in which a weak electrophile is replaced by a strong electrophile. The important electrophilic substitution reactions are Nitration, Sulphonation, Halogenation and Friedel-Crafts alkylation and acylation.

1. Nitration: It is the introduction of nitro (-NO2) group to a benzene ring. For this benzene is heated with a mixture of conc. HNO3 and conc. H2SO4 (nitrating mixture).

2. Halogenation: It is the introduction of halo (-X) group to a benzene ring. For this benzene is treated with a halogen (Cl2 or Br2) in presence of Lewis acids like anhydrous FeCl3, FeBr3 or AlCl3.

3. Sulphonation: It is the introduction of sulphonic acid (-SO3H) group to a benzene ring. It is carried out by heating benzene with fuming sulphuric acid (H2S2O7 or oleum).

4. Friedel-Craft’s reaction: It is the introduction of alkyl (-R) group or acyl (-CO-R) group to a benzene ring. It is of two types:

a) Friedel-Craft’s Alkylation reaction: It is the introduction of alkyl (-R) group to a benzene ring. Here the reagents used are alkyl halide in presence of anhydrous AlCl3.

b) Friedel-Craft’s Acylation reaction: It is the introduction of acyl (-CO-R) group to a benzene ring. Here the reagents used are acyl halide in presence of anhydrous AlCl3.

5. Benzene on treatment with excess of chlorine in the presence of anhydrous AlCl3 in dark to form hexachlorobenzene (C6Cl6).

II) Addition Reactions

1. Addition of H2: Benzene add hydrogen in presence of nickel catalyst at high temperature and pressure to form cyclohexane.

2. Addition of halogen: Benzene adds chlorine in presence of uv light to form benzene hexachloride (BHC). It is also known as Gammexane or Lindane or 666

Directive influence of a functional group in mono-substituted benzene

  • When monosubstituted benzene undergoes further substitution, two types of products are formed – either ortho and para products or meta product.
  • This behaviour depends on the nature of the substituent already present in the benzene ring. This is known as directive influence of substituents.

There are two types of substituents – ortho and para directing groups and meta directing groups.

1. Ortho and para directing groups:

  • The groups which direct the incoming group to ortho and para positions are called ortho and para directing groups.
  • Example for such groups are –OH, –NH2, –NHR, -NHCOCH3, –OCH3, –CH3, –C2H5 etc.
  • Generally, orho-para directing groups are activating groups, since they increases the electron density on benzene ring. Or, they activate the benzene ring for the attack by an electrophile.

Halogens are deactivating eventhough they are ortho-para directing. This is because of their strong –I effect.

2. Meta direcing groups:

  • The groups which direct the incoming group to meta position are called meta directing groups.
  • They are generally deactivating groups, since they reduces the electron density on benzene ring.
  • Examples are –NO2, –CN, –CHO, –COR, –COOH, –COOR, –SO3H, etc