Resonance energy: The amount of energy by which a molecule is more stable than expected. Bezne has a resonance enrgey of about 150kjmol-1.
- Benzenes enthalpy of hydrogenation is -209kjmol-1, but was expected to be -372kjmol-1. It is a lot more stable than first predicted.
- Doesn't undergo addition reactions like ethene, but subsitituion reactions.
- It's bonds are all the same length, and if it had double and single bonds, it be would irregular.
SP HYBRID ORBITALS
Benzene is linked by sp hybrid orbitals.
- SP hybrid orbitals are created when a s1 electron is promoted to a p orbital, making the s and p oribitals of the same energy.
- The three sp2 orbitals arrange themselves in trigonal planar,
- but there is one p orbital which isn't hybridised.
- These overlap between each carbon, creating a ring of delocalised electrons or a pi system.
BENZENE SHAPE AND BONDING
- Benzene is a planar regular hexagon,
- with bond angles of 120degrees.
- 6 carbon atoms joined in a ring
- with a delocalised pi ring above and below the chain
The pi system makes the molecule more stable, because the electron density is less.
Addition reactions would mean Benzene would have to break the pi system, and make its self more unstable.
Chlorination of Benzene
You need: DRAW THIS PLEASE
- AlCl3 (catalyst)
- room temperature
Step 1: The creation of positive chloride ion.
AlCl3 +Cl2 > AlCl4- and Cl+
Step 2: Subsitutiing hydrogen for chlorine
Delocalised pi ring donates a electron to the chlorine ion, and is then bonded to both a hydrogen and a chlorine, with an unstable positive pi system. The hydrogen donates its electron to the pi system, and becomes a H+ ion.
Step 3: Regenerating the catalyst
AlCl4- + H+ >>> AlCl3 + HCl
Nitration of Benzene
You need: DRAW THIS PLEASE
- Concentrated HNO3
- Concentrated H2SO4 (catalyst)
- Temperature below 50 celcius.
Step 1: Generating the nitronium particle.
H2SO4 + concentrated HNO3 >>> HSO4- +H2O + NO2+
Step 2: Subsitituing hydrogen for nitronium
Delocalised pi ring donates electron to the nitronium ion, and benzene becomes bonded to both hydrogen and nitronium. This makes the pi system unstable, and the hydrogen donates an electron to make it stable. This leaves a H+ ion.
Step 3: regenerating the catalyst
HSO4- +H+ >> H2SO4
OH group adds a lone pair to the delocalised pi system, make it increase in electron density, and induce dipoles and react more easily.
It has a weak acidity in water e.g. C6H5OH <> C6H50- + H+
Reacts with NaOH e.g. C6H5OH + NaOH >>> C6H5O-Na+ + H2O
Reacts with Sodium e.g. 2C6H5OH + 2Na >>> 2C6H5O-Na+ +H2
Reacts with Bromine water by electrophilic subsitution e.g.
increased electron density induces dipole in bromine, causing it to react.
C6H5OH +3Br >>> C6H2Br3OH + 3HBr
Subsitution always takes place at carbons 2, 4 and 6.
Benzene v phenol v cyclohexene
IN EASE OF BROMINATION
More difficult to brominate as low electron density in delocalised pi system. This means it struggles to induce a dipole in a halogen.
It's double bond is of high electron density and induces a dipole easily, allowing addition reactions.
Easy, because the OH group donates an lone pair to the pi system,which then increases in electron density.
Uses of Phenol
- Epoxy resins in paints