6.1.1 aromatic compounds

straight or branched chain organic substance

includes one or more ring of six carbon atoms with delocalised bonding

simplest arene C6H6 6 delocalised electrons over 6 bonds above & below plane of carbon atoms has a lower electron density so it cannot induce a dipole/polarise

3 double bond alternating benzene

lacks reactivity - if benzene contained the C=C bond, it should decolourise Br2 in electrophilic addition - but, doesn’t undergo electrophilic addiction & doesn’t decolourise Br2 so no C=C all bonds were found to be equal length, inbetween the value of the single and double bond length enthalpy change of hydrogenation expected to be 3x of cyclohexenes (double bonds), -360 but only -208 so less exothermic - more stable than thought

instead of benzene C6H5- e.g. phenylamine, phenylethene

react easily at room temperature localised pi electrons above and below plane of 2 carbon atoms so has a higher electron density can polarise/induce dipole in Br2

carcinogen methylbenzene is less toxic and reacts more readily than benzene as methyl side group releases electrons into delocalised system making it more attractive to electrophiles

benzene > nitrobenzene conc nitric acid in presence of conc H2SO4 electrophilic substitution electrophile: NO2+ 50/60C as doing at higher temperatures results in further substitution

HNO3 + 2H2SO4 > (NO2+)+ (2HSO4-) + H3O+

benzene > bromobenzene or chlorobenzene reagents: bromine or chlorine conditions: FeBr3 or AlCl3 catalyst electrophilic substitution

AlCl3 + Cl2 > AlCl4- + Cl+ FeBr3 + Br2 > FeBr4- + Br+ to reform AlCl3 catalyst H+ + AlCl4- > AlCl3 + HCl

benzene > alkylbenzene reagents: chloroalkane in presence of anhydrous aluminium chloride catalyst conditions: heat under reflux mechanism: electrophilic substitution

AlCl3 + Ch3CH2Cl > CH3CH2+ AlCl4- reform AlCl3 catalyst: H+ + AlCl4- > AlCl3 + HCl

benzene > phenyl ketone reagents: acyl chloride in presence of anhydrous aluminium chloride catalyst conditions: heat under reflux (50C) mechanism: electrophilic substitution

AlCl3 + CH3COCl > CH3CO+ AlCl4- reform catalst: H+ + AlCl4- > AlCl3 + HCl

OH group directly attached to benzene ring lone pair of electrons on oxygen p-orbital is donated into pi system - higher electron density so can polarise weakly acidic - weaker than carboxylic acid

H from alcohol replaced by Na+ (sodium phenoxide) + 0.5H2

H from alcohol replaced by Na+ (sodium phenoxide) + H2O sodium phenoxide more soluble than phenol so solid phenol dissolves on addition of NaOH

Br2 reagent room temperature doesn’t need a FeBr3 catalyst like benzene and undergoes multiple substitutions whereas benzene will only add one Br white solid formed (2,4,6-tribromophenol) and HBr

production of plastics antiseptics disinfectants

4M HNO3 reagent - single substitution room temp forms 2-nitrophenol or 4-nitrophenol

position on ring of substitution reactions electron-donating groups (OH, NH2) will force further substitutions to occur on 2nd and 4th positions of ring electron-withdrawing (NO2) will have a 3-directing effect of in electrophilic substitution of aromatic compounds

if a -OH group, Cl atom or NH2 group is directly attached to benzene ring, delocalisation in benzene ring will extend to include lone pairs on N, O and Cl > changes properties and reactions of side group