1. Aromatics
AH Chemistry, Unit 3(b)

2. Benzene was first isolated in 1825 by Michael Faraday, by heating crude oil and distilling coal.
In 1865, German chemist August Kekule proposed that benzene was a mixture of two compounds in rapid equilibrium (see picture).
However, the reactivity did not conform to the proposal and in 1931, Linus Pauling proposed the resonance structure, which we wtill use today.

3. Bonding in benzene
What information about bonding do these structure suggest to you?
sp2 hybridisation
Sigma and pi bonding
Electron delocalisation

4. Reactivity of benzene How reactive would you say benzene is? Not very.
Why?
The delocalisation of 6  electrons over the ring stabalise the structure.
What does it react with?
(Strong) electrophiles
What is it’s characteristic reaction type?
Electrophilic substitution, producing substituted benzene rings.
What are the products used for?
Medicines, dyes, insecticides, pesticides and resins.

5. Stability of benzene
Hydrogenation of different molecules (exothermic).
Hydrogenation is exothermic because the two C-H bonds formed are stronger (and therefore RELEASE more energy in being formed) than the C=C bond which is broken to give C-C and the H-H which also breaks (and both REQUIRE energy to be broken).

6. Characteristic reactions
Halogenation (chloronation and bromination) to produce chlorobenzene and bromobenzene
Alkylation to produce alkylbenzenes
Nitration to produce nitrobenzene
Sulphonation to produce benzene sulphonic acid

7. General mechanism 1

8. General mechanism 2

9. General mechanism 3

10. Bromination
Benzene reacts with bromine or chlorine in the presence of a catalyst – FeBr3, FeCl3 or AlCl3
The first step of the mechanism involves the bromine or chlorine reacting with the catalyst to form a powerful electrophile

11. Chlorination

12. Alkylation (Friedel-Crafts alkylation)

13. Nitration (concentrated) (concentrated)
In step 1, the suphuric acid donates a proton to the nitric acid

14. Sulphonation
The reaction is with fuming sulphuric acid (oleum) – a mixture of sulphuric acid and sulphur trioxide. These react to form the electrophile.

16. Acidity of phenol Phenol is a weak acid
Phenoxide ion is stabalised due to the negative charge of the oxygen atom is delocalised over the ring.
Phenol is therefore more acidic than aliphatic alcohols.

17. Basic nature of aminobenzene (analine)
+ H+
NH3 +
Phenylamine is basic
There is delocalisation of the lone pair of electrons on the nitrogen atom in aminobenzene, into the aromatic system
It therefore accepts a proton less readily than ammonia of aliphatic amines and is less basic