Question

Electrophilic Aromatic Substitution//Nitration of Bromobenzene

Reagents used: nitric acid, sulfuric acid, bromobenzene

1) During the isolation procedure of this lab, you are instructed to wash your crystallized product (crystallized from ethanol) withice-cold ethanol. Why is it important to use ice-cold ethanol as opposed toroom temperature ethanol?

Answer 1

The nitration of bromo benzene would yield two products i.e. 1-bromo-4-nitrobenzene and 2-bromo-4-nitrobenzene. Since bromine is activating group with its lone pair of electrons, each of the above mentioned product will form in good amount. The para product, 1-bromo-4-nitrobenzene, has a melting point of 127ºC. The ortho product has a melting point of 43ºC. Both products will form and crystallize together during the purification process. Recrystallization of this mixture from ethanol will result in the formation of crystals of the less soluble para product. The more soluble ortho product remains in the ethanol. In order to separate each of the product, it is necessary to wash with cold ethanol so that the very soluble product gets washed and the insoluble product gets crystallised