6.1 AROMATIC COMPOUNDS,CARBONYLS & ACIDS

• Straight or branched organic substance

1) Both have sideways overlap of p-orbitals 2) Both have π bonding above and below the carbon atoms

1) Kekule has three alternating π bonds with localised π electrons 2) Delocalised has a π ring system where all p-orbitals overlap

1) All six carbons have the same bond length 2) Benzene is resistant to addition reactions 3) ΔH hydrogenation is less exothermic that expected

• The carbon bond lengths in delocalised benzene are a value between the C=C double bonds and C-C single bonds • So, all bond lengths are equal

• Benzene doesnt readily undergo addition reactions like alkenes as benzene is less reactive than alkenes

• The ΔH hydrogenation of benzene is less exothermic than expected so is more stable than predicted

• The H in the OH group can only partially dissociate to release H+ in aqueous solution

• Because phenols are weak acids so cannot react with a weak base like a carbonate • Only reacts with hydroxides

1) Bromination of phenols to form 2,4,6-tribromophenol 2) Phenols with NaOH to form sodium phenoxide 3) Nitration of phenols to form a mixture of 2-nitrophenol and 4-nitrophenol (depending on HNO₃ concentration)

• OH • NH2 • (electron donating groups)

• NO2 • (electron withdrawing groups)

• The phenols p-orbital electrons are donated to the π system of the aromatic ring • The π system of the aromatic ring now is more nucleophilic/electron dense • The ring becomes more susceptible to electrophilic attack as it can induce dipoles in nonpolar molecules better

1) Oxidation of aldehydes to form carboxylic acids 2) NaBH₄ reduction of carbonyl compounds to form primary/secondary alcohols 3) HCN addition to carbonyl compounds to form hydroxynitriles