Chapter 25 Aromatics

• kerkule suggested a cyclic structure for benzene with 3 alternating carbon-carbon double bonds • kerkule proposed benzenes low reactivity was due to rapid equilibrium between 2 isomers

• the lack of reactivity - if benzene had C-C double bonds present then it should decolorise bromine in an electrophilic addition reaction, this didn’t happen. • isomers - compounds such as 1,2-dichlorobenzene should form isomers, but only 1 form exists • bond length - C-C and C-C double bonds have different bond lengths so benzene would be an irregular hexagon, however imaging shows benzene is a regular hexagon with the same bond lengths • hydration enthalpy - cyclohexene has a hydration enthalpy of -120kj/mol, this would imply that benzene had a hydration enthalpy of -360kj/mol, but in reality benzene had a hydration enthalpy of -208kj/mol

• each carbon creates 3 covalent sigma bonds - 2 with neighbouring carbons and 1 with hydrogen • the remaining electron is found in the p-orbital at a right angle above and below the carbon atom • the p-orbitals overlap sideways in both directions, this evenly creates a ring of delocalised pi electrons above and below the structure, creating a ring of electron density

• empirical formulae of CH

• electrophile - electron pair acceptor • unlike alkenes benzene cannot undergo electrophilic addition, since it would disrupt the delocalised pi-electron ring and the product would be less energetically stable

• replaces hydrogen for another group, retaining the delocalised ring • C6H6 + X+ -> C6H5X + H+

• the electrophile is attracted to the ring due to the high electron density caused by delocalised electrons • a pair of electrons from the ring are donated to the the electrophile to form a covalent bond • an unstable intermediate forms with an incomplete ring and a positive charge • the bond with H undergoes heterolytic fission to donate electrons to the ring structure • the stable ring structure returns leaving substituted product and a hydrogen ion

• The nitration of benzene substitute an N02 to the ring structure • reagent: conc. nitric acid (HNO3) • Catalyst: conc. sulfuric acid (H2SO4) • conditions: 50-55 celcius

• H2SO4 + HNO3 -> HSO4- + H2NO3+ • H2NO3+ -> H2O + NO2+ • this leaves a water molecule in the overall equation

• friedel-craft reactions use halogen carriers (AlX3, FeX3 or Fe) as catalysts to substitute R-Cl groups to rings via: • halogenation - sub a halogen • Alkylation - sub an alkyl chain using haloalkanes • Acylation - sub a ketone chain using acyl chlorides, acyl chloride are same as carboxylic acids but have Cl instead of OH all reactions are performed under reflux in anhydrous condition

• electrophile formation: R-X + AlX3 -> R+ + AlX4- • electrophile attack: the electrophile attack the Benzene ring structure to substitute the R group for a hydrogen • Catalyst reformation: the H+ ion formed by substitution reacts with the chlorine donated to the halogen carrier to form HX

• benzene is too stable to react with non-polar bromine • Br2 + FeBr3 -> FeBr4- + Br+ , where Br + is the electrophile

• H atom in the ring is substituted by an alkyl group • react benzene with haloalkane in presence of of AlX3

• benzene reacting with an acyl chloride with AlCl3 catalyst an aromatic ketone is formed

• Phenols are aromatic compounds that have an alcohol’s hydroxyl group attached to the ring • the hydroxyl takes position 1, with 1 OH group attached to benzene

• oxygen atom in hydroxyl has 2 lone pairs of p-orbital electrons • as the ring is electron deficient one of the p-orbitals overlap into the ring structure to become delocalised • this increases the ring’s electron density, this allows electrophilic substitution reactions to occur more easily

• the p-orbital overlap into the ring weakens the O-H bond so phenol can donate a H+ • forms phenoxide ions (C6H5O) when dissolved in water this means phenol solutions are acidic: • more acidic than alcohols and less then CA • phenols and CA react with strong bases • only CA can react with weak bases

• phenol + strong bases -> salt + water • phenol + metal -> salt + hydrogen

• phenols can react with bromine water by electrophilic substitution at RTP without catalyst

• the addition of p-orbital electrons to the ring structure from the oxygen atom of a phenol increases the electron density • the increase in electron density is enough to induce a dipole in a bromine molecule allowing substitution without a catalyst

• in aromatic compounds benzene is considered the parent chain - e.g 1-chlorobenzene • UNLESS it is attached to an alkyl chain of length 7 or more or if it contains a functional group (not halogens or nitro groups) then it is considered a constituent group • prefix = phenyl

• the presence of substituted groups on the benzene ring will have an effect on the substitution of further groups, the directing effect • releasing electrons - increases electron density • withdrawing electrons - decreases electron density

• side chains that increase electron density allow electrophiles to react quicker with benzene, activating groups • electron releasing groups direct further substitution to position 2 and 4

• side chains which decrease the electron density will slow reactions with an electrophile compared to benzene, so are deactivating groups • the group directs further substitution to position 3