Benzene and its properties
- Starting material for all arenes.
- Benzene is carcinogenic
- Colourless liquid
- Sweet odour
- Highly Flammable
- six membered carbon ring with alternating single and double bonds
- Fits with molecular formula C6H6
Problems with Kekule's structure:
- Benzene does not decolourise bromine water, if double bonds were present it would.
- Does not take part in other electrophilic addition reactions
- Benzene is more stable than most alkenes, less reactive
- The bonds were all the same length in benzene, C-C bonds and C=C bonds are known to have different lengths
- The enthalpy change in benzene is less than the predicted enthalpy change for kekule's structure of benzene
Delocalised model of Benzene
- Cyclic hydrocarbon, 6 carbon atoms and 6 hydrogen atoms
- Planar hexagonal ring
- Each carbon is bonded to two other carbons and one hydrogen
- 4 electrons in outer shell of each carbon, three are involved in sigma bonds and one electron is in orbital above and below the plane of atoms.
- P-orbital for each carbon overlap
- Delocolised ring of pi bonds shared over all six carbon atoms, above and below the plane of carbon atoms.
- does not decolourise bromine water
- does not react with strong acids (HCl)
- does not react with halogens (Cl, Br, I)
This is because addition reactions would disrupt the delocalisation of the ring structure.
Instead, Benzene undergoes electrophillic substitution reactions.
Nitration of Benzene
- H2SO4 General Equation
- 50 degrees C6H6 + HNO3 ---> C6H5NO2 + H2O
Generation of the electrophile
HNO3 + H2SO4 --> NO2+ + HSO4- + H2O
Forming the intermediate
(print and draw)
Regenration of the catalyst
H+ + HSO4- --> H2SO4
Halogenation of Benzene
Conditions: general eqn
- FeCl3 or AlCl3 (halogen carrier) C6H6 + Br2 --> C6H5Br + HBr
Generating the electrophile
Br2 + FeBr3 --> Br+ + FeBr4-
Regenerating the catalyst
H+ + FeBr4- --> FeBr3 + HBr
Benzene, Cyclohexene and Bromine
Why benzene doesn't react with bromine:
- As benzene's electrons ae spread out over all 6 carbon atoms, there is a lower electron density in the ring.
- There is insufficient electron density to polarise the Br-Br bond
- Halogen carrier is needed is polarise bond and attract pi-electrons from ring
Why cyclohexene does react with bromine:
- the pi-bond (double bond) in cyclohexane contains localised electrons, shared between only two carbon atoms. This produces a region of high electron density.
- The electron density is sufficient to polarise the Br-Br bond and create an induced dipole.
- The pi-electron pair are attracted to the slightly polar bromine, causing the double bond to break.
- Bromide ions are covalently bonded to the two carbon atoms
- Benzene ring with -OH group attatched.
Phenols in water
C6H5OH (dissolved in water) --> C6H5O- + H+
Phenols, when dissolved in water, lose a H+ ion fom the OH group.
Phenol to salt
C6H5OH + NaOH(aq) --> C6H5O-Na+ + H2O (neutralisation in aqueous NaOH)
2C6H5OH + 2Na --> 2C6H5O-Na+ + H2 (phenol + sodium = salt + hydrogen gas)
Bromination of phenols
- electrophillic substitution
- room temperature and no halogen carrier
C6H5OH + 3Br2 --> C6H3OBR3 + 3HBr
Ease of bromination of phenol
- lone pair of electrons on oxygen atom is drawn to ring
- creating higher electron density, ring is activated
- increased electron density polarises bromine molecules, which are then attracted to ring.