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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
Do you remember the content of A-level Chemistry regarding Cyclohexene? 👨🏫
Cyclohexene will:
- Decolourise 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!
Drafted by Cherry (Chemistry)