Chemistry A2 - Aromatic Compounds

Question 1

What are arenes?

Answer 1

Aromatic compounds that contain a benzene ring as part of their structure

Question 2

Describe the structure of benzene

Answer 2

An aromatic six carbon molecule, each bonded to one hydrogen atom. The outer electron from the p-orbital of each carbon atom is delocalised to form a ring of delocalised electrons. The overlap of electrons results in the formation of pi-bonds

Question 3

What is the evidence for the structure of benzene?

Answer 3

• It was originally thought benzene had three double bonds in alternating positions (cyclohexatriene) but if this was the case then benzene would have triple the enthalpy change of cyclohexane (-120x3 = -360), but it has an enthalpy change of -208
• All the bonds between the carbon atoms are the same length, and if it has alternating double bonds then we would expect the length of the double bonds to be shorter than the single bonds
• Benzene is resistant to electrophilic addition reactions which other compounds with carbon double bonds readily undergo

Question 4

Why is benzene resistant to electrophilic addition reactions?

Answer 4

It would involve breaking the stable ring of delocalised electrons

Question 5

What is the name of the reaction benzene undergoes and why?

Answer 5

Electrophilic substitution
The delocalised ring is of high electron density and so is susceptible to electrophiles

Question 6

What is halogenation?

Answer 6

A type of electrophilic substitution reaction in which benzene reacts with halogens in the presence of a catalyst, such as AlCl3, which acts as the halogen carrier

Question 7

What is the nitration of benzene?

Answer 7

A type of electrophilic substitution reaction in which the electrophile is a NO2+ ion

Question 8

What are the conditions for the nitration of benzene?

Answer 8

Concentrated sulfuric acid and concentrated nitric acid

Question 9

Why is benzene resistant to bromination (test for unsaturation)?

Answer 9

Due to the delocalised electron density in the pi-system in benzene compared to the localised electron density of the pi-bond in alkenes, so bromine is stable and not reactive enough to decolorise bromine water

Question 10

What are phenols?

Answer 10

Organic compounds containing a benzene ring with an OH (alcohol) group

Question 11

How is phenol produced?

Answer 11

In an electrophilic substitution reaction with benzene

Question 12

What happens when phenol reacts with bromine water?

Answer 12

2,4,6-tribromophenol is produced which forms a white precipitate with a strong smell of antiseptic. This decolourises the bromine water

Question 13

Why is phenol more reactive than benzene?

Answer 13

The lone pair of electrons on the oxygen atom are delocalised into the benzene ring, increasing the electron density so it is less stable and more susceptible to attack from electrophiles