1. Aromatic Compounds

2. What is aromaticity
The Huckle Rule - compounds that contain (4n+2) pi electrons are considered to be aromatic (n is an integer)
Anti-aromatic compounds contain 4n pi electrons
Name comes from the strong aromas of these compounds

3. Benzene Aromatic hydrocarbon C6H6 6 pi electrons Fully conjugated
Resonance stabilized

4. Structure of Benzene
All of the following are accepted representations of benzene

5. Molecular Orbitals of Benzene
the benzene system has three molecular orbitals which accommodate the six electrons, two in each

6. Resonance Stabilization of Benzene
Cyclohexane – saturated
Benzene – unsaturated
Hydrogenation values
Cyclohexene 28.6 kcal/mole
Cyclohexadiene 55.4 kcal/mole
Cyclohexatriene 49.8 kcal/mole
Expected around kcal/mole
Therefore: resonance stabilization energy
~30-36 kcal/mole

7. Experimental Evidence
Halogenation of benzene produces no reaction without a catalyst
Most substitution reactions of benzene are endothermic (absorb energy)

8. Monosubstitued Benzenes
Toluene
Bromobenzene
Chlorobenzene
Phenol
Nitrobenzene
Alkylbenzenes

9. Toluene
Toluene is a methylated benzene

10. Halogenated Benzenes Cl

11. Phenol Structure

12. Nitrobenzene and Alkyl benzenes
Monosubstitued benzenes with a NO2 or alkyl group substituent

13. Benzene Activation vs. Deactivation
Activation - Once a substituent has been added to the benzene ring it can cause the molecule to be more reactive than the benzene molecule alone.
Deactivation – when a substituent on a benzene ring causes the molecule to be less reactive than benzene alone

14. Groups That Activate or Deactivate Benzene

15. Disubstituted Benzene
When a second substituent adds to a benzene ring the designations are:

16. Ortho – Para Directors Ring Activating
Alkyl groups
Halogens OH
OCH3 OR
NH2
NHR
NR2

17. Meta Directors Ring Deactivators
NO2
HSO3
COR CN
COOH
COOR

18. Reactions of Benzene
Electrophilic Aromatic Substitution - EAS is an organic reaction in which an atom, usually hydrogen, attached to an aromatic system is replaced by an electrophile. The most important reactions of this type that take place are nitration, halogenation, sulfonation, and acylation and alkylation.

19. Electrophile
an electrophile (literally electron-lover) is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile
Most electrophiles are positively charged, have an atom which carries a partial positive charge, or have an atom which does not have an octet of electrons. .

20. Reactions of Benzene

21. Halogenation of Benzene
Other catalyst AlCl3

22. Halogenation Mechanism
Step 1

23. Halogenation (cont) Step 2
The hydrogen is removed by the AlCl4- ion which was formed in the first stage. The aluminium chloride catalyst is re-generated in this second stage.

24. Nitration of Benzene

25. Sulfonation of Benzene

26. Alkylation of Benzene