Aromatic Compounds

- Benzene is a cyclic planar hydrocarbon, in a hexagonal ring. Each carbon atom is bonded to 2 other Carbon atoms and a Hydrogen atom

- The shape around each carbon atom is trigonal planar, 120

- Each Carbon atom has 4 outer shell electrons, 2 bonded to Carbon atoms and one to the hydrogen atom. Leaving a 4th electron in a P orbital above and below the plane of the carbon atoms. 

- Adjacent p-orbital electrons overlap sideways in both directions above and below the plane of the carbon atoms.

- This overlap produces a system of Pi bonds that spreads over all 6 carbon atoms. The pi electrons are no longer held between 2 carbon atoms, but are spread over the whole ring and said to be delocalised.

- Carbon-carbon bond lengths are all the same in benzene, an intermediate between the length of C-C and C=C/

- Enthalpy change of hydrogenation of benzene was less exothermic than expected (-208, was expected to be -360)

- Benzene doesnt decolourise bromine water. It only reacts with bromine under high temperatures with halogen carrier catalysts.

1. The electrophile accepts a pair of pi electrons from the benzene ring to form a covalent bond

2. An intermediate forms containing the electrophile and the Hydrogen atom being replaced. The delocalised pi electron could is disrupted, and the intermediate is unstable.

3. The unstable intermediate rapidly loses a Hydrogen atom. The delocalised ring of electrons is reformed and stability is reformed.

SUBSTITUTION = an atom/group of atoms is replaced by another atom/group of atoms

ELECTROPHILE = electron pair acceptor

Theres insufficient electron density in benzene to polarise the halogen molecule. But benzene undergoes electrophillic substitution reactions.

Benzene

• Pi electrons are delocalised, has a lower electron density than alkenes.
• Polarises bromine molecules less, and so attracts bromine molecules less than alkenes.
• Benzene induces a weaker dipole in bromine molecules

Alkenes

• Pi electrons are localised, has a higher electron density than benzene.
• Polarises bromine molecules more, and so attracts bromine molecules more than benzene.
• Alkenes induce a stronger dipole in bromine molecules

• Conditions = 50°C, reflux, concentrated H2SO4 catalyst
• Reagents = Concentrated HNO3

If the temperature increases above 90°C further substitution occurs, leading to the production of 1,3-dinitrobenzene. So this reaction should be refluxed in a thermostatically controlled water bath to maintain a constant temperature.

Uses of nitrobenzene:

1. Dyes

2. Pharmaceuticals

3. Pesticides

Reacts in the presence of a halogen carrier catalyst:

• FeCl3 - Iron III chloride
• FeBr3 - Iron III bromide
• AlCl3 - Aluminium chloride
• AlBr3 - Aluminium bromide

Reagents = Bromine liquid

Conditions = heat under reflux, halogen carrier catalyst

To generate the electrophile (Br+) you need to react the halogen carrier and bromine together:

Br2 + FeBr3 ---> FeBr4- + Br+

Phenol is a benzene ring with a hydroxyl (-OH) group is attatched directly to it. (If the -OH group is on a side chain it is then referred to as an aromatic alcohol, not a phenol)

Used for plastics, antiseptics, disinfectants and resins for paints,

Solid at room temperature, but when dissolved in water it forms a slightly acidic solution that reacts with sodium and bases.

Phenol's arent strong enough to react with sodium carbonate unlike carboxylic acids, so no fizzing will be produced - allowing you to distinguish between a phenol and a carboxylic acid.

Phenol reacts with both sodium hydroxide and sodium metal to produce a salt (sodium phenoxide).

• The lone pair of electrons in the p orbital of the oxygen atom in the phenol group is delocalised into the ring

• This increases electron density, and so phenol polarises bromine molecules more, that are then attracted more strongly towards the ring structure.

• Benzene has a lower electron density than phenol and polarises bromine molecules less.

The nitro group deactivates the aromatic ring. The ring now reacts less readily with electrophiles.

NO2 is said to be electron withdrawing, and is 3 directing. It is an example of a deactivating group.

The amine group activates the benzene ring, and so it can then react more readily with electrophiles.

NH2 and OH are electron donating, and are both 2,4 directing. They are both examples of activating groups.