Arenes

Question 1

compare the Kekulé model for benzene in terms of p-orbital overlap forming
bonds;

Answer 1

alternate single and double bonds

Question 2

compare the delocalised mode for benzene in terms of p orbital overlap forming bonds

Answer 2

• 6 carbons and 6 hydrogens
• planar hexagonal ring
• shape around each carbon atom is trigonal planar, 120 degrees
• each carbon atoms has 4 outer shell electrons, 3 bond to 2 carbon atoms and 1 hydrogen atom. there is 1 outer shell electronin the 2p orbital above and below plane.
• electron in p orbital overlaps to form pi bonds which spread over all 6 carbon atoms

Question 3

what evidence is there to support the delocalised model of benzene?

Answer 3

• low reactivity: the c=c would be expected to react with Br water, decolourising it. However, this does not happen.
• bond length: kekule structure is presented as symmetrical BUT c=c and c-c have different bond lengths.
• enthalpy change of hydrogenation: 152kJ less than expected enthalpy change; more stable than expected

Question 4

Equation for the nitration of benzene (write and draw it)

Answer 4

C6H6 + HNO3 -> C6H5NO2 + H20

H2S04 IS USED AS CATALYST, 50 DEGREES
refer to notes for drawing

Question 5

Describe the mechanism for nitration of benzene

Answer 5

1) formation of NO2+ electrophile:
HNO3 + H2SO4 –> NO2 + HS04- +H20

2) NO2+ electrophile then reacts with benzene
3) H+ + HS04- —> H2S04

Question 6

Draw the overall equation for the halogenation of benzene

Answer 6

Benzene + BR2 –> Bromobenzene + HBr

NEED halogen carrier (Eg FeBr3 or AlBr3)

Question 7

Describe the mechanism for the halogenation of benzene

Answer 7

1) formation of Br+:
Br2 + FeBr3 –> Br+ + FeBr4-

2) little mechanism of Br electrophile reacting with benzene
3) H+ + FeBr4- –> FeBr3 + HBr

Question 8

Describe the electrophillic addition of arenes

Answer 8

1) electron dense ring in benze attracts the electrophile.
2) electrphile accepts a pair of pi electrons from the ring and forms a covalent bond
3) an intermediate forms, the structure has been disrupted and so is less stable than benzene
4) the unstable intermidate loses the hydrogen atom as an ion. delocalised ring is reformed.

Question 9

Describe the mechanism for the reaction between Br2 and an alkene.

Answer 9

• Br2 is non polar. Pi bond has localised electrons above and below plane = high electron density
• Br2 approaches alkene, electrons in pi bond repel electrons in Br-Br bond
• Induces a dipole in Br2 molecule = polar
• pi elecrons from double bond are attracted to slightly positive Br, causing the double bond to break.
• new bond forms between a carbon and a bromine, forming a positive carbocation
• bond between 2 br atoms breaks by heterolytic fission, forming Br-
• The Br- ion is attracted towards intermediate, forming a covalent bond

Question 10

Why is benzene unable to react with bromine without a halogen carrier?

Answer 10

Benze has delocalised electrons over all 6 carbons whereas alkenes have localised pi electrons above and below the double bond.
Benzene has insufficient electron density to react with non polar halogens.
Halogen carrier generates more powerful electrophile.