Aromatics

Question 1

Benzene structure

Answer 1

C6H6

6 delocalised e-
One hydrogen and one carbon at each point

Question 2

Naming aromatic compounds

Answer 2

Question 3

Kekule benzene

Answer 3

ring of 6 carbons
alternating single an double bonds

Question 4

Cylcohexa-1,3,5-triene vs Benzene- Reactivity

Answer 4

Cylcohexa-1,3,5-triene:
Electrophilic addition
Bromine water would turn from orange to colourless

Benzene:
Benzene actually undergoes substitution reactions, not addition reactions.
Benzene only reacts with bromine at high temperature and with a catalyst.

Question 5

Cylcohexa-1,3,5-triene vs Benzene- Enthalpy of Hydrogenation

Answer 5

Benzene is more stable less reactive
takes more energy to break bonds

If we do hydrogenate benzene only about -208 KJmol-1 is released, about 150 Kjmol-1 less exothermic than expected.

Benzene is about 150Kjmol-1 more stable than expected – this is what we call the delocalisation stabilisation of benzene.

Question 6

Cylcohexa-1,3,5-triene vs Benzene- Bond lengths

Answer 6

Single (sigma) bonds are longer than double (pi) bonds.
X-ray Crystallography shows benzene has bond lengths that are all the same.
These bond lengths a length in-between a single and a double bond.

Question 7

The evidence that led scientists to doubt the model of Benzene proposed by Kekule

Answer 7

• Bond lengths are an intermediate between (short) C=C and (long) C–C.
• ΔH hydrogenation less exothermic than expected (when compared to ΔH hydrogenation for cyclohexene) because benzene is more stable.
• Benzene is less reactive than an alkene. It doesn’t undergo an addition reaction to decolourise bromine.
• Benzene undergoes substitution reactions in order to maintain the ring of delocalised electrons.

Question 8

Delocalised model of benzene

Answer 8

bonds are formed from the overlap of atomic orbitals.
All these bonds are sigma (σ) bonds.

uses three out of the four electrons on each carbon.
The fourth electron is in a p orbital; there are six “unused” electrons in six p orbitals

When two p orbitals overlap side by side, a pi (π) bond is formed.
In benzene, all the p orbitals can overlap to form an extended pi electron system.

benzene is a PLANAR molecule. This is a key difference from the similar cyclohexane molecule.

Question 9

Key Facts: Bonding, Shape and Stability

Answer 9

Bonding
1a) Each C has three (covalent) bonds
1b) Spare electrons (in a p orbital) overlap (to form a π cloud)
1c) delocalisation

Shape
2a) Planar
2b) Hexagon/6 carbon ring/120o bond angle
2c) C–C bonds equal in length

Stability
3a) Expected ΔH hydrogen of cyclohexatriene = –360 kJ mol–1
3b) ΔH hydrogen benzene (is less exothermic) by 152 kJ mol–1
3c) Benzene is more stable

Question 10

Physical properties of Arenes

Answer 10

• Benzene is a colourless liquid at room temperature.
• Benzene has a BP = 353K. This is very similar to that of hexane (354K)
• Benzene has a MP = 279K. This is a lot higher than hexane because benzene being a planar molecule can stack together very well in the solid state. This makes the Van der Waals intermolecular forces harder to break.

Question 11

The reactivity of aromatic compounds

Answer 11

(1) The ring is an area of high electron density due to the delocalisation. This means arenes react with electrophiles.

(2) The aromatic ring is very stable. Energy has to be put in to break the ring system. This means in almost all reactions the ring remains intact at the end of the reaction.

Question 12

Combustion of Arenes

Answer 12

Arenes are rarely burned because:
- Arenes are extremely useful substances in the pharmaceutical industry and so burning them is wasteful.

• Arenes, as they are unsaturated, tend to undergo incomplete combustion. This means you don’t get as much energy out from burning the molecule as could be, and they tend to burn with a very smoky flame as solid carbon particles form.

Question 13

Electrophilic Substitution - Nitration

Answer 13

Reagents: Conc H2SO4 and Conc HNO3
Electrophile Reaction: H2SO4 + HNO3 → HSO4- + H2O + NO2+

Question 14

Uses of Nitrated Arenes

Answer 14

-They can be used to produce explosives.

Toluene is the old name for methyl benzene.

If this is nitrated 3 times it produces trinitrotoluene ‘TNT’ used in explosives.

• the nitro group on an arene can then be converted into an amine (-NH2) group. This can then be used in a range of different reactions.

Question 15

Electrophilic Substitution- Friedel-Crafts Acylation

Answer 15

Reagents: An Acyl Chloride e.g. RCOCl and catalyst AlCl3

Reaction: CH3COCl + AlCl3 → CH3CO+ + AlCl4-