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.

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:

• De-colourise Bromine water
• React with strong acids
• React with halogens 

Nitrification of Benzene:

Reagents required:

• Concentrated HNO3
• Concentrated H2SO4

Conditions Required:

• 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.

Why does Bromine water react more readily with Cyclohexene compared

to 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) 

Compared to:

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)

References:

https://regenesis.com/en/glossary/benzene/

https://commons.wikimedia.org/wiki/File:Cyclohexene_structures.svg

This is the end of the topic!

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