A-Level Chemistry OCR Notes

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Bonding in Benzene

Compounds that contain a benzene ring are aromatic.
The molecular formula C6H6 led to the belief that the structure of benzene was cyclohexa-1,3,5-triene. This is the Kekule model of benzene, which is incorrect

Benzene is a planar cyclic structure consisting of a ring of carbon atoms, each with a single hydrogen atom attached that sticks out into a flat plane
Benzene has a delocalised electron system, arising due to the overlap of one p-orbital from each carbon atom, above and below the plane of the ring.

Evidence for the delocalised structure:
- Bond length – X-ray diffraction patterns show the C-C bond lengths in benzene are all the same, being in between the length of carbon single and double bonds
- Hydrogenation – The enthalpy change of hydrogenation for benzene is less exothermic than that of theoretical cyclohexa-1,3,5-triene, showing that benzene is more stable than the Kekule model
- resistance to reaction – benzene does not decoulorise bromine water like normal alkenes do, and it does not undergo electrophilic substitution addition reactions

Electrophilic Substitution of Benzene

The benzene ring is a region of high electron density, which means it attracts electrophiles
Electrophilic substitution proceeds by the general mechanism, where the identity of E+ represents an electrophile

Nitration of benzene (E+ = NO+)
- A concentrated sulfuric catalyst is used and the reaction carried out under reflux, with concentrated nitric acid

Overall:

Generation of the electrophile:

Regeneration of sulphuric acid catalyst:

Halogenation (E+ = Cl+ or Br+)
The aromatic ring in benzene is too stable to react directly with halogens, therefore a halogen carrier (FeCl3/FeBr3, AlCl3/FeBr3 or Fe) is used

Overall:

Generation of the electrophile in situ:

Regeneration of the halogen carrier catalyst:

Friedel-Crafts acylation reactions involve adding an acyl group to benzene. A strong Lewis acid, such as AlCl3, is used as a catalyst. The reaction is carried out at 60°C under reflux in anhydrous conditions.

Generation of the electrophile:

A benzene derivative is a benzene ring that has undergone substitution. Depending on the identity of the substitute a different prefix is used

Double-substituted benzene derivatives are named by numbering the carbon atoms and stating what carbon each group is on
If one derivative has two different groups, the prefixes are used in alphabetical order, e.g. 1-bromo-3-chlorobenzene

Phenol

It reacts in an acid-base reaction with KOH. Normal alcohols do not partake in acid-base reactions
However, phenol can be distinguished from carboxylic acids as they don’t react with carbonates
The reactivity of phenol is greater than benzene because electrons are donated from the p-orbital of the oxygen to the delocalised aromatic ring. The increased electron density allows phenol to be more susceptible to electrophilic attack
- Phenol can induce a dipole in Br2, therefore it undergoes direct halogenation, unlike benzene
- Phenol decolourises bromine water, but benzene doesn’t

Electrophilic substitution of phenol

Unlike benzene, phenol undergoes nitration at room temperature with dilute nitric acid
- C6H6OH + HNO3→ C6H5(NO2)OH + H2O
The hydroxyl group activates the carbon atoms at the 2 and 4 positions on the aromatic ring by pushing electron density into the π-system. Therefore substitution reactions mainly occur on the 2 and 4 position
Nitration of phenol forms a mixture of 2-nitrophenol and 4-nitrophenol

If concentrated HNO3 is used, a triple substitution occurs, forming 2,4,5-trinitrophenol
The presence of an NH2 group on the ring makes the 2- and 4-directing effect stronger