ARENES

Arenes Arenes are aromatic hydrocarbons which contains benzene ring. General formula : CnH2n-6 , n  6 E.g : Structure of benzene : Kekule structure, planar, symmetrical and non-polar molecule.

Structure of benzene Each carbon atom in benzene has 3 σ-bond and 1 -bond has sp2 hybridised orbital. Bond angle = 120 Planar 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 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 -electrons The 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:

Excersice Name the following compunds.

Write the structural formula of : a) 2-chloromethylbenzene b) 2-phenylpropane c) 2,4,6-trinitromethylbenzene

Reactions of benzene 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 benzene Reagent : mixture of concentrated HNO3 and H2SO4 Condition : < 60C Product : nitrobenzene If temperature is above 60C, 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.

Reactions of methylbenzene Reactions of methyl group Substitution – halogenation Oxidation Reactions of benzene ring Electrophilic substitution 1. Halogenation 2. Nitration 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.

Electron-donating group Electron-withdrawing group 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. Substituent, X Electron-donating group  2- and 4- directing group.  Ring activating. increase rate of reaction. -NH2 -C6H5 -OH -CH3 -NHCOCH3 -CH2CH3 Electron-withdrawing group  3-directing group.  Ring deactivating. decrease rate of reaction. -NO2 -SO3H -CN -CHO -COOH -COR

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) Halogenation – free radical substitution. Condition : u.v light Oxidation Reagent : Acidified KMnO4 Condition : Reflux for several hours Any carbon group on benzene ring will be oxidized to -COOH