aromatic compounds Flashcards
(10 cards)
Describe the Kekulé model of benzene with the subsequent delocalised model for benzene in terms of p-orbital overlap.
Kekulé proposed a six-membered ring with alternating single and double carbon to carbon bonds. This would have alternating localised pi bonds around the ring. In the delocalised model, each carbon atom is bonded to two other C atoms and hydrogen with sigma bonds. Each carbon atom has an electron left over in a p orbital. The p orbitals overlap sideways around the ring to create a pi system above and below the plane of the molecule. The electrons are delocalised around the ring.
Describe the 3 pieces of evidence for a delocalised, rather than Kekulé, model for benzene.
Benzene doesn’t readily undergo addition reactions, the enthalpy change of hydrogenation is less exothermic than expected, the C-C bond lengths are all the same and are in between a typical C-C single bond and a C=C double bond
Explain the relative resistance to bromination of benzene, compared with alkenes, in terms of electron density and π-bonding
In alkenes, the pi bond is localised between the carbon atoms. There is high electron density which is able to polarise the bromine and so alkenes react readily. In benzene, the electrons are delocalised around the ring, the electron density is lower and benzene is unable to polarise the bromine. Benzene needs a halogen carrier to react with bromine.
Describe the acidity of phenols by referring to its reactivity with NaOH and with carbonates. Compare this to a carboxylic acid like ethanoic acid
Phenol is a very weak acid. It reacts with NaOH, a strong alkali but not with carbonates, weak bases. A carboxylic acid reacts with both NaOH and also with a carbonate. This property can be used as a characteristic test for a phenol – it shows that it is an acid when tested with indicator paper, but does not fizz when a carbonate is added.
What reagent is needed to turn phenol into 2,4,6-tribromophenol and what type of reaction is this
Bromine water can be used as the reagent. This is a substitution /halogenation reaction.
What reagent is needed to turn phenol into 2-nitrophenol and what type of reaction is this? write a balanced equation
Dilute nitric acid can be used as the reagent. This is a substitution / nitration reaction. C6H5OH + HNO3 > C6H5NO2 + H2O
Why does phenol more readily undergo electrophilic substitution compared with benzene?
A lone pair on the O in the OH group is partially delocalised into the ring. This increases the electron density and the ring is more susceptible to electrophilic attack. The ring is more easily able to polarise an electrophile.
Considering the substituents OH, NH2 and NO2, which groups are 2- and 4-directing and which are 3-directing in the electrophilic substitution of aromatic compounds? Explain this in term of electron donating or electron withdrawing effects
The OH and NH2 groups are 2-, 4-directing because a lone pair on the O or N can be donated into the delocalised pi system in the benzene ring. This activates the ring and other groups are substituted in positions 2,4 and 6 around the ring. The NO2 group is 3-directing. This group is electron withdrawing
why is phenol a weak acid?
partially dissociates when dissolved in water to produce protons
evidence which cast doubt on kekule model
- Carbon to carbon bond lengths all the same
-enthalpy change of hydrogenation less exothermic than expected
-resistant to reaction/needs a catalyst- substsitoons not addition
