Unit 3: Section 6 - Aromatic Compounds and Amines

Question 1

How does the delocalised electron ring system arise in benzene?

Answer 1

1. Benzene is a planar cyclic structure witht the formula C6H6
2. Each carbon atom forms single covalent bonds to the carbons on either side of it and to one hydrogen atom
3. The final unpaired electron on each carbon atom is loacated in a p-orbital that sticks out above and below the plane of the ring
4. The p-orbitals on each carbon atom combine to form a ring of delocalised electrons
5. All the carbon bonds in the ring are the same, so they’re the same length - 140pm which lies between the C-C and C=C

Question 2

How do we know benzene is more stable than cyclohexa-1,3,5-triene?

Answer 2

1. Cyclohexene has one double bond - when it’s hydrogenated, the enthalpy change is -120kJmole^-1, if benzene had 3 double bonds, you’d expect it to have an enthalpy of hydrogenation of -360kJmol^-1
2. The experimental enthalpy of hydrogenation of benzene is -208kJmol^-1 - far less exothermic than expected
3. More energy must have been put in to break the bonds in benzene than would be needed to break the bonds in a theoretical cyclohexa-1,3,5-triene molecule
4. This difference indicates benzene is more stable - which is thought to be due to the delocalised electron ring system

Question 3

What is an arene?
(aromatic compound)

Answer 3

Compounds containing a benzene ring

Question 4

What mechanism do arenes undergo?

Answer 4

Electrophilic substitution
* As the benzene ring is a region of high electron density, it attracts electrophiles
* As the benzene ring’s so stable, it doesn’t undergo electrophilic addition reactions, which would destroy the delocalised ring of electrons
* Instead it undergoes electrophilic substitution reactions where one of the hydrogen atoms (of other functional group) is substituted for the electron

Question 5

Why is Friedel-Crafts acylation used?

Answer 5

1. Useful chamicals such as dyes and pharmaceuticals contain benzene rings but because benzene is so stable, it’s fairly unreactive - it can be difficult to make chemicals that contain benzene
2. Friedel-Crafts acylation rections are used to add an acyl group (RCO-) to the benzene ring
3. Once an acyl group has been added, the side chains can be modified using further reactions to make useful products

Question 6

What electrophile is used for Friedel-Crafts acylation?

Answer 6

An electrophile has to have a strong positive charge to be able to attack the stable benxene ring - most aren’t polarised enough but some can be made into stronger electrophiles using a catalyst called a halogen carrier
An acyl chloride is used as a nucleophile and the halogen carrier

Question 7

How is an acyl chloride substituted into the benzene ring?

Answer 7

1. Electrons in the benzene ring are attracted to the positively charged carbocation.
2. 2 electrons from the benzene bond with the carbocation, which paratially breaks the delocalised ring and gives it a positive charge
3. The negatively charged AlCl4- ion is attracted to the positively charged ring
4. One chloride ion breaks away from the aluminium chloride ion and bonds with the hydrogen ion - this removes the hydrogen from the ring forming HCl - allows the catalyst to reform
5. A phenylketone is formed

Question 8

What conditions are needed for Friedel-Crafts acylation to occur?

Answer 8

The reactants need to be heated under refluc in a non-aqueous solvent (dry ether) for the reaction to occur

Question 9

How do you form the electrophile for a nitration reaction?

Answer 9

HNO3 + H2SO4 ->