Organic Chemistry 201 - Electrophilic Aromatic Substitution

Question

Explain the mechanism of nitration in benzene through electrophilic aromatic substitution. Describe each step of the process and discuss the role of the nitrating agent in facilitating the reaction.

Answer 1

In general this is an electrohilic aromatic substitution by an electrophile.

What does that mean?

A molecule or ion searching out electrons is attracted to a benzene ring because of its ring of π electrons. This is that ring of alternating double bonds we draw on a benzene ring. Essentially the electrophile will replace one hydrogen on the benzene ring. There is an intermediate step with the creation of a carbocation intermediate.

So specifically for a nitration

the electrophile is created by treating HNO₃ with an acid , H₂SO_4, to protonate it_. During protonation the nitric acid will accept a positive hydrogen ion (proton) from the acid. Then lose a water molecule to become ⁺NO₂. ⁺NO₂ is now our electrophile. It will react the electrons on the benzene ring. Initially 1 carbon on the ring will not have a double bond, instead it will have 2 constituents the ⁺NO₂ and its original hydrogen. The hydrogen will be removed as a proton by a base.