Aromatic Compounds

Question 1

Is benzene stable?

Answer 1

Delocalised ring of electrons makes it very stable

Question 2

Structure of benzene?

Answer 2

Each carbon atom has 3 covalent bonds - one to a hydrogen atom and the other two to carbon atoms.

All the carbon carbon bonds are the same length

The fourth electron of each carbon atom is found in a p orbital which there are 6 of.

The electrons in the p orbital are delocalised.

Question 3

Problems with Kekulé benzene structure?

Answer 3

C-C bond length
All the bonds are the same size but in kekulé you would expect 3 shorter C=C and 3 longer C-C

Addition Reactions
Benzene does not undergo addition reactions - doesn’t declolourise bromine water.

Enthalpy of hydrogenation
Enthalpy change is less exothermic than expected meaning its more stable than the expected triene molecule.
Extra stability from the delocalised electrons.

Question 4

Why do arenes undergo electrophilic substitution reactions?

Answer 4

High region of electron density so attracts electrophiles.

Doesn’t undergo addition but substitution occurs.

Friedel crafts acylation and nitration

Question 5

What is nitration used for?

Mechanism etc…

Answer 5

Nitro compounds can be reduced to form aromatic amines which are used to manufacture dyes and pharmaceuticals

Can be used as explosives. - TNT

Question 6

What is Friedel Crafts Acylation used for?
What’s the point?

Mechanism:

Answer 6

Benzene is used widely in pharmaceuticals and dyes but it is very stable.
Adding acyl groups (RCO-) can make the molecule more reactive as the benzene structure is weaker.

Heat reactants under reflux in a non aqueous solvent

Question 7

How does AlCl3 make acyl chlorides stronger electrophiles?

Answer 7

AlCl3 accepts a lone pair of electrons from the acyl chloride.
The acyl chloride becomes more polarised and it forms a carbocation.

Now has a strong enough charge to react with the benzene ring.