BENZENE

Three Flaws in Kekules structure of Benzene:

• Low Reactivity (3 Double bonds should of made Benzene reactive/unstable however it is a stable molecule)

• Lower Enthalpy (Expected Enthalpy = -360kjmol-1 -- Actual Enthalpy - 208kjmol-3)

• Carbon-Carbon bond lengths are not symmetrical as stated (x-ray diffraction proved this)

Reactions Under normal Conditions:

Benzene will not BUT cyclohexene will

•De-colourise Bromine water

• React with strong acids

• React with halogens

Features of Benzene:

Benzene is a cyclic hydrocarbon with a structural formula of C6H6, the shape around each carbon is trigonal planar which has a bond angle of 120 degrees.

The electrons in the P-orbitals overlap, this results in a ring of electron density above and below the plane of Carbon atoms. This produces a 'pi-cloud' where one carbon atom contributes one electron forming a delocalised system of six electrons.To preserve stability, Benzene takes part in Electrophilic substitution reactions (Electrophile substitutes an atom)

Electrophile: is an atom that is attracted to an electron rich centre which accepts a pair of electrons forming a dative covelant bond.

Delocalised: Electrons are shared between more than two atoms.

Nitrification of Benzene:

Reagents/ Conditions required:

• Concentrated HNO3, Concentrated H2SO4, 50 degrees Celsius, Reflux

Bromination of Benzene:

• Benzene is too stable to react with Br2, therefore a Halogen carrier is required.

• The halogen carrier (FeBr3) is required to generate the Bromonium ion (Br+) this is a stronger electrophile allowing it to react with Benzene.

Cyclohexene:The pi bond contains localised electrons above and below the plane of carbon atoms, this produces an area of high electron density, this allows the alkene to polarise the Br2 molecule, this forms the electrophile Br+ which is attracted to the electron rich centre accepting a pair of electrons to form a dative covelant bond. (Cyclohexene decolourises bromine water - Orange to Colourless) 

Benzene: Benzene has delocalised electrons over all six carbon atoms (1 electron per carbon - carbon bond) this results in a lower electron density, the low electron density is therefore insufficient to polarise the Br2 molecule, therefore a halogen carrier is required to generate a bromonium ion (Br+) which is a stronger electrophile allowing the reaction to take place. (No reaction takes place with out a halogen carrier -- however with a halogen carrier - Bromine is decolourised and white fumes of hydrogen bromide are given out)

Features of Phenol:    • Solubility in water - Phenol is sparingly soluble in water as the OH group is able to form hydrogen bonds, however the benzene ring limits the solubility allowing it to only form weak Van der waals force of attraction.• Acidic Properties - Phenol is a weak acid as it partially dissociates in water.

Uses of Phenol: • Antiseptics • Detergents • Synthesis of aspirin and dyes • Explosives • Resin

State why Phenol is more reactive than Benzene:

A lone pair of electrons occupying the P-orbital on the oxygen is delocalised into the Benzene ring, this creates a high electron density and the ring is said to be activated, this increased electron density allows polarisation of molecules therefore increasing the attraction of the atom to the electron rich centre.