ARENES

ArenesArenes are aromatic hydrocarbons which contains

benzene ring.General formula : CnH2n-6 , n 6E.g :

Structure of benzene :Kekule structure, planar, symmetrical and non-

polar molecule.

methylbenzenebenzene

ethylbenzene 1,4-dimethylbenzene

Structure of benzeneEach carbon atom in benzene has 3 σ-bond and 1

-bond has sp2 hybridised orbital. Bond angle = 120Planar molecule.

All the 6 carbon are joined in a ring, the unhybridised 2p orbital (each has 1 electron) will overlap sideways forming a ring of electron density above and below the carbon atoms.

These 6 electrons are free to move throughout the ring, thus the -electrons are delocalised around the ring.

Bond length for C – C (0.154nm) and C=C (0.134) are different, but in benzene all the C –C and C=C bond length are the same (o.134nm).

This is due to the delocalisation of -electronsThe delocalisation of -electrons makes benzene

electron rich and very stable hence cannot undergo addition and oxidation reaction (which will destroy the ring structure).

It mainly undergoes electrophilic substitution reaction.

Naming of Aromatic Compounds

For benzene ring that carries these functional group, the systematic name will be written as ‘functional group-benzene’ (except for carboxylic acid)

Substituent group

Systematic name

Carboxylic acid Benzoic acid

Methyl, -CH3 Methylbenzene

Chloro, -Cl Chlorobenzene

Nitro, -NO2 Nitrobenzene

The carbons in benzene ring are numbered according to alphabetical arrangement of substituent. Lowest number goes to the first alphabet.

Example:

The prefix ‘phenyl’ (C6H5- group) and are used when carbon in benzene ring is attached to functional groups such as –OH group and –NH2 group. The carbon attached to the functional group is labeled carbon 1.

E.g:

NH2

phenylamine

OH

phenylmethanol

ExcersiceName the following compunds.

Write the structural formula of :a) 2-chloromethylbenzeneb) 2-phenylpropanec) 2,4,6-trinitromethylbenzene

Reactions of benzene1) Halogenation (Electrophilic Substitution) Reagent : Cl2 or Br2 in CCl4 Condition : Room temperature, catalyst (‘halogen

carriers’), e.g FeCl3, FeBr3, AlCl3, AlBr3. Product : Chlorobenzene or bromobenzene.

Yellowish-green colour of Cl2 decolourised and white fumes of HCl detected.

Reddish-brown colour of Br2 fades and white fumes of HBr detected.

Reaction mechanism :Step 1 : Formation of Br+

Step 2 : Electrophilic attack on benzene ring.

Step 3 : Loss of proton.

2) Nitration of benzeneReagent : mixture of concentrated HNO3 and

H2SO4

Condition : < 60CProduct : nitrobenzene

If temperature is above 60C, nitrobenzene reacts with more HNO3 producing dinitrobenzene.

Reaction mechanism for nitration of benzene:

Step 1 : Production of nitronium ion, NO2+

Step 2 : Electrophilic addition.

Step 3 : Loss of proton.

Electrophilic substitution reactions in methylbenzene is quicker than benzene because of the electron-donating methyl group attached to it which activates the benzene nucleus.

Substitution takes place at the 2- and 4- position of the benzene ring.

Whether reaction occurs in the side chain or the benzene ring depends on the type of reagent and condition of reagent.

The position of the second substituent is determined by the substituent X, already present in the ring.

Exception : halogen atoms (-F, -Cl, -Br, -I) are electron withdrawing (ring deactivating) but are 2- and 4-directing.

Reaction of benzene ring in methyl benzene Electrophilic substitution.

Reaction mechanism is similar to benzene.

Write the reaction mechanism of nitration of methylbenzene.

Reaction of methyl group (side chain)

1) Halogenation – free radical substitution. Condition : u.v light

1) Oxidation Reagent : Acidified KMnO4

Condition : Reflux for several hours

Any carbon group on benzene ring will be oxidized to -COOH