Before spectroscopy was invented, Körner's absolute method was used to determine whether a disubstituted benzene derivative was the ortho, meta, or para isomer. Körner's method involves adding a third group (often a nitro group) and determining how many isomers are formed. For example, when o-xylene is nitrated, two isomers are formed.

(a) How many isomers are formed by nitration of m-xylene?
(b) How many isomers are formed by nitration of p-xylene?
(c) A turn-of-the-century chemist isolated an aromatic compound of molecular formula C6H4Br2. He carefully nitrated this compound and purified three isomers of formula C6H3Br2NO2 Propose structures for the original compound and the three nitrated derivatives.

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NO CH CH 3HNO H,SO CH CH CH3 CH3