Chemistry (A-Level)/Rings, acids and amines

An arene is any aromatic hydrocarbon that contains at least one benzene ring.

Benzene

Structure of benzene

A benzene ring is a cyclic hydrocarbon with the molecular formula C₆H₆. It's structure isn't what would be expected from generic organic compounds.
The six carbons form a planar hexagonal ring with σ C-C bonds. Attached to each carbon is a hydrogen, bonded with a σ C-H bond. At each carbon, there are 3 σ bonds (two C-C and one C-H) in a trigonal planar shape with bond angles of 120°. Parallel to the carbon ring on either side is a ring of overlapping π orbitals where the electrons are delocalised. This gives a benzene ring a high electron density above and below the plane of the molecule.

Discovery

In 1825, Michael Faraday managed to isolate benzene and determined that it had an empirical formula of CH. Later in 1834, Eilhard Mitscherlich found that it had a relative molecular mass of 78 and thus a molecular formula of C₆H₆. Despite this knowledge, the structure of benzene remained unknown. Several structures were theorized but none matched benzene's chemical properties. It was in 1865 when Friedrich August Kekulé suggested that benzene had a cyclic structure with alternating double and single bonds. Kekulé claims that he thought of this structure while daydreaming about a snake biting it's own tail. However Kekulé's structure did not explain several chemical properties of benzene. This was proof that his structure needed to be expanded upon.
Some of the proofs that Kekulé's structure was still flawed include:

• Benzene's low reactivity

If benzene contained 3 double bonds as Kekulé suggested, it would be very reactive with bromine (Br₂). Bromine will decolourise in the presence of C=C bonds, however when mixed with benzene, it is not decolourised.

• The bond lengths of benzene

C-C and C=C bonds have different bonds lengths of 0.153nm and 0.134nm respectively. X-ray studies had shown that all of the carbon to carbon bond lengths in benzene are the same at 0.139nm.

• Hydrogenation of benzene

The predicted enthalpy change for Kekulé's model is different from the actual change in the hydrogenation of benzene.

For the hydrogenation of carbon-carbon double bonds:

C=C + H₂ → CH-CH ΔH= -120kJmol^-1

Therefore for Kekulé's model, ΔH= -360kJmol^-1, however from experiment, ΔH= -208kJmol^-1

Nitration of benzene (NO₂⁺)

To form nitronium ions (NO₂⁺), the following two reactions take place.

H₂SO₄ + HNO₃ → H₂NO₃⁺ + HSO₄^-

H₂NO₃⁺ → NO₂⁺ + H₂O

For the nitration of benzene, the following reaction takes place:

C₆H₆ + NO₂⁺ → C₆H₅NO₂ + H⁺

The reagents for this reaction are benzene(C₆H₆), concentrated sulphuric acid(H₂SO₄) and concentrated nitric acid(HNO₃). The reaction must take place up to 50°C. If it takes place at a higher temperature, more than one nitro group(-NO₂) will substitute with a hydrogen.
This type of reaction is called electrophilic substitution. The nitronium ion acts as the electrophile. In A-level Chemistry, you will need to know the mechanism for this reaction.

Reaction mechanism (Arrows represent movement of electron pairs)

Halogenation of benzene

This is the reaction between benzene and a positive halogen ion. This is another example of electrophilic substitution. In the reaction between benzene and chlorine, the Cl⁺ ion acts as the electrophile. To get the positive ion needed for the reaction, a halogen carrier is required.
Some some possible halogen carriers are:

• Fe
• FeBr₃/FeAl₃
• AlBr₃/AlCl₃

The following reaction can take place to form the necessary ion:

Cl₂ + FeCl₃ → FeCl₄^- + Cl⁺

The reaction for the chlorination of benzene is as follows:

C₆H₆ + Cl⁺ → C₆H₅Cl + H⁺

The mechanism for this reaction is very similar to the nitration of benzene

Only difference from nitration is the electrophile

After these reactions, the products FeCl₄^- and H⁺ react.

FeCl₄^- + H⁺ → FeCl₃ + HCl

This means that the halogen carrier acts as a catalyst in this set of reactions. These reactions all work the same with bromine(Br) in the place of chlorine(Cl).

Carbonyl Compounds

Able to be reduced to their respective alcohols

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Further reading on:
Organic chemistry
Benzene
Phenol