A2.8 Aromatics Flashcards
Benzene bonding
Six carbon atoms in a ring
Each carbon has 3 valence electrons in sigma bonds
6 delocalised electrons in delocalised pisystem above and below the plane of the ring
Kekules model
Cyclohexa-1,3,5-triene
Kekules limitations
1) benzene carbon atoms in ring are equivalent bond distance
- average of c-c and c=c bond distances
2) benzene is resistant to addition reactions
3) benzenes hydrogenation was lower than expected
- difference must be due to bonds broken so bonds broken value must be higher in benzene because they are stronger - delocalised pi system is more stable
Benzene on a chain
X- phenylalkane
Benzene and aldehyde
Benzaldehyde
Benzene and carboxylic acid
Benzoic acid
Benzene and amine
Phenylamine
Benzene reactions
Does not readily react like alkenes
Density of electrons around any two carbons in delocalised pi system is lower than between two carbons in alkene localised pi bond
Therefore requires halogen carrier
Why does benzene only undergo electrophillic substitution
Intermediate is not stable and breaks down to restore aromaticity
Phenol solubility
Struggles to form hydrogen bonds with water due to large non polar aromatic ring so less soluble than most alcohols
Phenol as an acid
Weak acid
Able to react with strong bases
Unable to react with carbonates
Phenol reactivity compared to benzene
Far more reactive
Lone pair from oxygen atom is donated to the delocalised pi system
Raises electron density
Causing it to attract electrophiles
Phenol halogenation
Doesn’t require halogen carrier
Phenol nitration
Doesn’t require catalyst
Doesn’t need to be concentrated HNO3
Electron donating
OH and NH2
Direct to 2 and 4
