Aromatics

Question 1

Give 3 reasons why benzene is not an alkene

Answer 1

• Reactivity (with Br2)
• Enthalpy of hydrogenation
• Bond lengths

Question 2

When does benzene react with Bromine

Answer 2

• At high temperatures
• W a catalyst

Question 3

What is Benzenes formula

Answer 3

C6H6

Question 4

What is the bond length between adjacent C atoms

Answer 4

intermediate between C-C and C=C

Question 5

What happens to the 4th electron in the P orbital of each C atom in benzene

Answer 5

it delocalises to form a ring of electron density above and below the hexagon, forming rings of delocalised electron density above and below the hexagon

Question 6

What is the thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-triene ?

Answer 6

1) Hydrogenation of cyclohexene = -120Kjmol-1
2) cyclohexa-1,3,5-triene = -360Kjmol-1
3) Benzene hydrogenation = -208Kjmol-1
4) so benzene is 152Kjmol-1 more stable

Question 7

What is the appearance of benzene at 298k

Answer 7

colourless liquid

Question 8

Benzene has a MP = 279K
This is a lot higher than hexane why ?

Answer 8

1) benzene is a planar molecule and stacks well in solid state
2) makes VDW forces harder to break

Question 9

Name 2 features of the reactions of aromatic molecules

Answer 9

1) The ring is an area of high electron density due to delocalisation (react with electrophiles)
2) aromatic ring is very stable, energy has to be put in to break the ring system
3) means almost all reactions the ring stays intact at the end

Question 10

Why are arenes rarely burned

Answer 10

arenes are extremely useful substances in pharmaceutical indistry so burning is wasteful

Question 11

why do you not get as much energy from burning arenes as could be

Answer 11

1) they tend to undergo incomplete combustion
2) tend to burn with very smokey flame as carbon particles form

Question 12

what does nitration involve

Answer 12

-replacing one of the h atoms on the benzene ring with a NO2 group
-The electrophile in the reaction is the NO2+

Question 13

Name a use for nitrated arenes

Answer 13

They can be used to produce explosives ‘TNT’

Question 14

Describe the structure, bonding and stability of structure Benzene

Answer 14

Bonding
- Each C has three (covalent) bonds
- Spare electrons (in a p orbital) overlap (to form a π cloud)
- delocalisation

Shape
- Planar
- Hexagon/6 carbon ring/120o bond angle
- C–C bonds equal in length / C–C bond lengths between single and double bond

Stability
- Expected ΔH hydrogen of cyclohexatriene - –360 kJ mol–1
- ΔH hydrogen benzene (is less exothermic) by 152 kJ mol–1
- Benzene is more stable