4.2 Aromaticity
- Created by: holly_b
- Created on: 08-01-17 11:29
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- Aromaticity
- Structure of benzene
- Benzene is a cyclic hydrocarbon containing 6 carbon atoms and 6 hydrogen atoms.
- Kekule's Model
- A ring of 6 carbon atoms containing alternate single and double bonds called cyclohex-1,3,5-triene.
- Problems
- The C-C and C=C bond lengths are different so it wouldn't be a perfect hexagon.
- If there were 3 double bonds it would react with bromine water.
- If benzene has 3 double bonds you would expect the enthalpy change to triple that of 1 double bond but it does not.
- There are not two isomers of 1,2-dimethylbenzene.
- Key features
- All 6 bonded electrons are delocalised.
- All 6 C-C bonds have the same length, intermediate between single and double.
- The molecule is planar.
- Resistance to addition reactions
- Benzene cannot undergo addition reactions.
- Addition reactions would disrupt the ring of delocalised electrons so it undergoes substitution reactions instead.
- Benzene cannot undergo addition reactions.
- Electrophilic Substitution
- Nitration
- Benzene reacts with a 50:50 mixture of concentrated nitric and sulfuric acid under reflux at 50-55 degrees to form nitrobenzene.
- Step 1: Nitric acid is weaker than sulfuric acid and acts as a base as the electrophile is formed.
- Step 2: The NO2+ is the electrophile and attacks the delocalised ring, breaking it temporarily
- Step 3: The delocalised system then reforms itself by pulling in the electrons from the C-H bond. The H+ then recombines with the HSO4- to form H2SO4.
- Overall Reaction: C6H6 + HNO3 --> C6H5NO2 + H2O
- Benzene reacts with a 50:50 mixture of concentrated nitric and sulfuric acid under reflux at 50-55 degrees to form nitrobenzene.
- Halogenation
- Halogen carriers FeBr3 and AlCl3 help produce strong electrophiles and encourage benzene to react.
- Alkylation
- The elctrophile is CH3+ and the catalyst is AlCl3
- Stage 2: One of the Al-Cl bonds breaks and the electrons from it join to the hydrogen. This removes hydrogen from the ring and re-generates the catalyst.
- Stage 1: 2 electrons from the delocalised system are used to form a new bond with the CH3CO+ ion. The delocalisation is partly broken and the ring gains a positive charge.
- Acylation
- The elctrophile is CH3CO+ and the catalyst is AlCl3.
- Stage 1: 2 electrons from the delocalised system are used to form a new bond with the CH3CO+ ion. The delocalisation is partly broken and the ring gains a positive charge.
- Stage 2: One of the Al-Cl bonds breaks and the electrons from it join to the hydrogen. This removes hydrogen from the ring and re-generates the catalyst.
- Nitration
- Interaction between benzene and substituent groups
- The aromatic C-Cl bond is stronger than the aliphatic because it has a higher bond energy.
- This is because a non-bonding p electron pair on chlorine overlaps with the delocalised ring of pi elecrtons causing the bond to be stronger and shorter.
- The aromatic C-Cl bond is stronger than the aliphatic because it has a higher bond energy.
- Stability of benzene
- The delocalisation of the electrons makes the molecule more stable than expected for a molecule with 3 double bonds.
- The areas of high electron density above and below the plane make benzene rings very stable.
- Structure of benzene
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