REACTIVITY OF ALKENES AND BENZENE

Question 1

Explain why even though Br2 is a non polar molecule its still able to react with the double bond in alkenes

Answer 1

• The π bond contain localised electrons above and below the plane of the two carbon atoms in the bond.
• This creates a region of high electron density
• When bromine approaches an alkene,the elctrons in the π bond repel the elctrons in the Br-Br bond
• This induces a dipole in the Br2 molecule with one end being slighly negative and the other slightly positive.
• The Br2 molecule is polarised
• the π electron pair of the double bond is attracted to the slightly positive bromine atom,causing the double bond to break
• A new bond forms between one of the carbon atoms and a bromine atom,forming a positively charded carbocation
• The Br-Br bond breaks by heterolytic fisiion forming a bromide ion
• The bromide ion is attracted to the carbocation forming a covalent bond

Question 2

what type of reaction is this between bromine water and cylohexene

Answer 2

electrophilic addition

Question 3

explain why when bromine is addded to benzene no reaction takes place.

Answer 3

• Benzene has delocalised electrons spread over all 6 carbon atoms in the ring structure
• Alkenes have localised electrons above and below the plane of two carbon atoms in the double bond
• Benzene has a lower π electron density and alkenes have a higher π electron density.
• Therefore when a non polar molecule like Br2 approaches the benzene ring there is insufficient π elctron density above and below the plane of two carbon atoms to cause the necessary polarisation of the bromine molecule
• hence this makes Benzene resistance to reactions with non polar molecules
• For Br2 to react with benzene a halogen carrier is required to generate a poweful electrophile Br+
• The greater charger on the Br+ is able to attract the π electrons from the benzene so that a reaction can occur

Question 4

what does PCB stand for

Answer 4

Polychlorinated biphenyls

Question 5

How many benzene rings make up PVBs

Answer 5

2

Question 6

name some of the uses of PCBs

Answer 6

1. coolants
2. insulating fluids for transformers
3. plasticers in paint
4. cement

Question 7

what is a particular problem with PCBs

Answer 7

PCBs are fat soluble therefore can be stored in the body