19. Reactions of Aromatics: EAS and Beyond

Propose a mechanism for nitration of pyridine at the 4-position,

and show why this orientation is not observed.

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 (by a method shown in Chapter 17), two isomers are formed.

(a) How many isomers are formed by nitration of m-xylene?

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 (by a method shown in Chapter 17), two isomers are formed.

(b) How many isomers are formed by nitration of p-xylene?

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 (by a method shown in Chapter 17), two isomers are formed.

(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