Depto. Química Orgánica

AROMATIC HALOGENATION

Image Link UAM Excelencia UAM Facultad Ciencias UAM
15/07/2017
page counter
Where I am Problems and exercises Molecular panel Home
Image Link
The price to producing a SEAr is the temporary loss of aromaticity.
That means an energy barrier that has to be surmounted by increasing the electrophilicity of the attacking species.
Image
In the case of the bromination, it is necessary to activate the Br2 molecule with FeBr3 as follows:
Image
The thermodynamic balance of the reaction depends on the used halogen:
Image
For one, the energy diagram of the chlorination is the following:
 Image
deltaHº= (deltaHº)Cl-Cl + (deltaHº)H-C6H5 -[(deltaHº)H-Cl + (deltaHº)ClC6H5] = = 58 + 111 - (103 +96) = -30 kcal/mol

The SEAr reactions are usually thermodynamically favorable.

The use of catalysts is mandatory in order to surmount the activation energy and get a reasonable reeaction rate.