Benzene and aromatic compounds

Define an arene

Aromatic hydrocarbons containing one or more benzene rings

Give some characteristics of benzene

• A colourless, sweet smelling, highly flammable liquid

• Classified as a carcinogen

• Burns with smoky flame

What is benzene naturally found in?

• Crude oil

• Petrol

• Cigarette smoke

• Volcanoes and forest fires

What are some things benzene is used to produce?

• Detergents

• Explosives

• Pharmaceuticals

• Dyes

What was Kekulé’s proposed model of benzene?

Briefly state the 3 pieces of evidence which disproved Kekulé’s model

1. Benzene’s lack of reactivity

2. The lengths of the carbon-carbon bonds in benzene

3. Hydrogenation enthalpies

Explain how the benzene’s low reactivity disproved Kekulé’s model

Benzene cannot have any C=C bonds in its structure because:

• Benzene does not decolourise bromine water

• Benzene doesn’t take part in electrophilic addition reactions expected from the C=C

Explain how X-ray crystallography disproved Kekulé’s model

Using X-ray crystallography, scientists found that

• All the C-C bonds were the same length

• All C-C-C bonds had bond angles 120

Explain how hydrogenation enthalpies disproved Kekulé’s model

The enthalpy of hydrogenation was less exothermic than expected

• If Kekulé’s model was correct, benzene would have an enthalpy change of hydrogenation 3 times that of cyclohexene

  • The enthalpy change was less exothermic, so benzene must therefore be more stable than Kekulé’s model

Show the delocalised model of benzene and give the main features

• Planar (flat), cyclic, hexagonal hydrocarbon

• Each carbon uses three of its four electrons to bond to 2 carbons and one hydrogen (3 σ bonds per each C)

  • Each carbon has one electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms

• Bond angle 120

• Overlapping of p-orbitals creates delocalised system of π-bonds above and below plane of carbon atoms

What types of reactions does benzene take part in?

Only substitution reactions

How is a benzene derivative formed?

When one of the hydrogen atoms on the benzene is replaced with an atom or group

• Benzene therefore forms the root of the name

How do we name monosubstituted aromatic compounds?

• The benzene ring is the parent-chain

• Alkyl groups, halogens and nitro groups are all considered prefixes to benzene

When do we use the term phenyl?

• When a benzene ring is attached to:

  • An alkyl chain with a functional group

  • An alkyl chain with seven or more carbon atoms

• Benzene is considered a substituent

Ethylbenzene

Chlorobenzene

Nitrobenzene

Phenylethanone

2-phenyloctane

Benzoic acid

Phenylamine

Benzaldehyde / Benzanal

1,4-benzoic acid

Phenyl ethanoate

2,4,6-trinitrotoulene (TNT)

phenylethanol

methylbenzene (toulene)

4-methylchlorobenzene

2-hydroxybenzoic acid

phenylethene

2-phenylbutane

phenylethanone

methyl benzenecarboxylate (methyl benzoate)

N-phenylethanamide

phenol

2,4,6-trichlorophenol (TCP)

benzene-1,2-diol

2-nitrophenol