Question #7ea91

1 Answer
Jan 19, 2017

If you are not given the conditions, you should assume "ordinary" conditions.

Ordinary conditions

I would consider ordinary conditions to be temperatures less than 100 °C and only "ordinary" catalysts.

Thus, the bromination of benzene at 65 °C to 70 °C gives bromobenzene as the major product.


The problem is that #"Br"# is a deactivating group, and the addition of more #"Br"# atoms to bromobenzene requires progressively more vigorous conditions.

You can, however, get a 93 % yield of hexabromobenzene if you heat a solution of benzene in a solvent such as tetrachloroethane (b.p. 146 °C) with 7 molar equivalents of bromine and an #"Fe/I"_2# catalyst at 80 °C for 10 h and then gradually raise the temperature to 143 °C over the next 5 h.


The methyl group is activating, so the reaction conditions don’t have to be so vigorous to get 2,4,6-tribromotoluene.

Heating excess bromine with a solution of toluene in acetic acid and a #"Zn/I"_2# catalyst on a steam bath for 5 h gives a 65 % yield of 2,4,6-tribromotoluene.