6.1 Aromatic Chemistry

Benzene (C6H6). Consists of a hexagonal ring of 6 carbon atoms with each carbon atom joined to 2 other carbon atoms and one hydrogen.

Suggested that the structure of benzene was a hexagonal ring joined by alternate double bonds.

1 - Benzene did not decolourise bromine water so cannot have any double bonds. 2- All bonds were same length, less than single but more than a double. 3- Should have enthalpy value of -360 for braking 3 double bonds (3x 120). Actual value was -208, so is more stable than Kekule model.

Planar, cyclic hexagonal hydrocarbon. Each carbon atom has 1 delocalised electron in p-orbital that overlap sideways with each other to create a system of pi bonds.

Electrophilic substitution.

Benzene reaction with nitric acid to form nitrobenzene. The reaction is catalysed by concentrated sulfuric acid and heated to 50 degrees. Produces water.

Benzene reacts with bromine during electrophilic addition in the presence of a halogen carrier (FeBr3). Produces HBr.

Reacts in same way of bromine. Used AlCl3. Produces Chlorobenzene and HCl.

Reacting benzene with haloalkane in presence of AlCl3, that acts as a halogen carrier.

Benzene reacts with acyl chloride in presence of AlCl 3 to form an aromatic ketone.

This is because benzene has delocalised pi electrons spread above and below the plane. Electron density around 2 carbon atoms is less than that in C=C double bond. When bromine approaches ring, there is insufficient electron density to polarise bromine molecule.

Type of organic chemical with hydroxyl functional group. C6H5OH.

Less soluble in water than alcohols due to non-polar benzene ring. It partially dissociates forming phenoxide ion and proton. More acidic than alcohols but less acidic than carboxylic acids.

React to form salt, sodium phenoxide, and water.

Halogen carrier is not required and reaction decolourises bromine water. Bromine is attached at the 2,4 position.

Phenol reacts with dilute nitic acid, where NO2, joins in the 2.4 position.

There is a lone pair of electrons from oxygen in OH group being donated into the pi system. The electron density of benzene ring in phenol is increased, so attracts electrophiles more easily.

Group that attaches and activates the ring, as the ring reacts more readily with electrophiles.

Group that deactivated the aromatic ring as the ring reacts less readily with electrophiles.

Different groups can have different directing effects where groups attaching can be directed to attach at different points.

They have 2,4 directing, except halogen which are deactivating but still direct in the 2.4 position. Eg. NH2, OH, R

They are 3 directing, except halogens which are deactivating but direct in the 2,4 position. Eg. NO2, COOH, COOR, CHO.