Reaction of arenes

Question 1

How are arenes combusted?

Answer 1

arenes burn in air with flames that are noticeably smoky

this is because they have high carbon : hydrogen ratio compared with alkanes

there is usually unburnt carbon remain when they burn in the air and this produced soot

a smoky flame suggests an aromatic compound

Question 2

How does the reactiiobn of a benzene differ from an alkene?

Answer 2

the most typical reaction is an electrophillic substituion that leaves the aromatic system unchanged

rather than addition which would require the input of the delocalisation energy to destroy the aromatic system

Question 3

Why is it unusual that benzene reacted in this way by electrophillic sub?

Answer 3

benzene is unsaturated

Question 4

How the electrophillic substituion take place?

Answer 4

1. the electrophile, El+, is attracted to the delocalised electrons
2. a bond forms between one of the carbon atoms and the electrophile
3. to do this, the carbon must use electrons form the delocalised system
4. this destroys the aromatic system

Question 5

How is the stability of the aromatic system retunred?

Answer 5

the carbon oses an H+ ion with the electron in the C-H bond, returning to the delocalised system

Question 6

What is the sum of these reactions?

Answer 6

the substitutuon of H+ by El+

Question 7

Draw the mechanism for the electrophillic substituion of benzene

Answer 7

Question 8

Why will CN react with a halogenoalkane but not benzene?

Answer 8

• halogenoalkane
  
  • electron deficit carbon in the halogneoalkane attacked by nucleophile
• beneze
  
  • electron rich benzene repels CN the nucleophile