Benzene (The Arenes)

These compounds were formally called aromatic due to their strong aromas.

The arenes differ from aliphatic compounds such as alkanes and alkenes, in possessing one or more rings of carbon atoms in which the bonding electrons are delocalised. The best known of these compounds is benzene.

1. A normal sigma bond is formed between each pair of carbons and each contains two electrons.

   

2. The remaining six electrons (one from each carbon) occupy p-orbitals. These p-orbitals overlap to form a bond. However, in the case of benzene, this bond extends over all six carbon atoms.

   

Hence, there are two rings of delocalised electrons above and below the benzene ring (containing six electrons). This has led to the following symbol being used for benzene:

The delocalisation of some of the bonding electrons of benzene has some important consequences:

1. Benzene is much more stable than expected.
2. The extra stability means that benzene will less readily undergo addition reactions.
3. The more loosely held electrons are open to attack by electrophiles. Hence, the characteristic reaction of benzene is electrophilic substitution.

1. Resistance to addition:

   Addition across a double bond usually leads to a more stable structure, saturated compound since single bonds are stronger than double bonds. However, benzene is extra stable due to delocalisation. Addition would lead to a loss of delocalisation and a less stable product.

   

2. Electrophilic substitution: during substitution reactions the delocalisation and extra stability is retained.

   

Naming of Aromatic Compounds

Combustion

Benzene burns in oxygen with a smoky flame due to its high carbon content. This is a general test between aromatic and aliphatic compounds.

2C₆H_6(l) + 15O_2(g) 12CO_2(g) + 6H₂O_(g)

Electrophilic substitution

We shall consider two types:

1. Nitration
2. Halogenation

Both of these reactions involve:

1. Initial attack by an electrophile (often a cation), to form an intermediate addition compound.
2. Loss of hydrogen ion, H+, leaving a substitution product.

Nitration

A mixture of conc. Sulphuric acid and Nitric acid below 50°C, react exothermically with benzene to form mononitrobenzene. At temperatures above 50°C, dinitrobenzene will be formed:

Mechanism:

1. Formation of electrophile:

   

   The nitronium ion NO₂⁺ is thought to be the electrophile.

2. Electrophilic attack on benzene:
   

   Apair of electrons are taken to make the bond, leaving four electrons delocalised.

3. Loss of hydrogen ion.

   The electrons in the C-H bond are drawn towards the carbon thus leaving the hydrogen open to nucleophilic attack by HSO₄^-

   

   Note: The NO₂ group withdraws electrons from the ring so deactivating it towards further attack by NO²⁺. Higher temps. are therefore needed to make dinitrobenzene. The NO₂ group also directs electrophilic attack to the 3 position resulting in the 1,3 isomer.

Halogenation

Chlorine or bromine react with benzene at room temperature in the dark. However, a suitable catalyst (a halogen carrier) such as anhydrous AlCl₃, anhydrous AlBr₃, iron filings or iodine crystals must be present.

The reaction is exothermic, forming mono-chloro or bromobenzene. Further substitution occurs if the reaction time is long, and products up to C₆Cl₆ and C₆H₃Br₃ can be obtained. This is not a catalyst free reaction.:

The catalyst is to produce Cl⁺ and the HCl obtained proves that this is a substitution reaction and not addition.

1. Formation of electrophile

   

2. Electrophilic attack on benzene

   

3. The removal of hydrogen ion.