Reactions of aromatics

What type of reaction mechanism does benzene undergo?

What type of reaction mechanism does benzene undergo?

Electrophilic substitution.

Why do benzene molecules undergo electrophilic substitution?

The benzene ring is an area of high electron density, meaning it attracts electrophiles such as NO2+ or H+.

Why do benzene molecules not undergo electrophilic addition?

The benzene ring is too stable, and an addition reaction would destroy the benzene ring.

What are two types of electrophilic substitution reactions of benzene?

- Nitration
- Friedel-Crafts acylation

What conditions are required for nitration and what is the product?

- Warm benzene with concentrated nitric acid and sulfuric acid.
- Nitrobenzene is produced.

What is the catalyst in a nitration reaction and what does it do?

- H2SO4 catalyst
- It helps to make the NO2+ electrophile

What is an equation for the formation of the electrophile in nitration?

HNO3 + H2SO4 -> HSO4- + NO2+ + H2O

How should the temperature of a nitration reaction be controlled if mononitration (one substitution) is desired?

The temperature needs to be kept below 55 degrees celsius for mononitration.

Draw the mechanism for the nitration of benzene by electrophilic substitution (textbook page 450).

XXX.

What is an equation for the regeneration of the catalyst?

H+ + HSO4- -> H2SO4

What are some uses of nitration reactions?

- Nitro compounds can be reduced to form aromatic amines -> used to manufacture dyes/pharmaceuticals.
- Nitro compounds decompose violently when heated so can be used as explosives, such as TNT.

What conditions are required for Friedel-Crafts acylation and what products are formed?

- Reactants heated under reflux in a non-aqueous solvent.
- A phenylketone and HCl are produced.

How can electrophiles be made polarised/positive enough to attack the benzene?

Some electrophiles can be made into stronger electrophiles using a catalyst called a halogen carrier.

What catalyst is used in Friedel-Crafts acylation?

AlCl3

How is the electrophile formed in friedel-crafts acylation?

AlCl3 accepts a lone pair of electrons from the acyl chloride. As the lone pair is pulled away, the polarisation in the acyl chloride increases and it forms a carbocation. This makes it much stronger and enables it to react with benzene.

Draw the mechanism/write the equation for the formation of the electrophile in Friedel-Crafts acylation (textbook page 451).

RCOCl +AlCl3 -> RCO+ + AlCl4-

Draw the mechanism for the electrophilic substitution of benzene in friedel-crafts acylation (textbook page 452).

XXX.

What carbons should the horseshoe in the benzene intermediate be drawn between?

2 and 6.