c25 - benzene and phenol

general properties of benzene

colourless, sweet smelling, highly flammable, carcinogen

kekulé’s model of benzene

alternate single and double bonds on 6 carbon ring

evidence to disprove kekulé’s model of benzene

lack of reactivity (no electrophilic addition, no decolourising bromine), lengths of c-c bonds (x-ray diffraction showed all bonds were same length), hydrogenation enthalpy (not 3x hydrogenation enthalpy of cyclohexene, lower enthalpy so more stable irl)

delocalised model of benzene

planar, cyclic, hydrocarbon with 6C and 6H, each C only uses 3 of 4 electrons in bonding, one p orbital electron delocalised per C, electrons form pi bond/ overlap sideways above and below C plane

nitration of benzene to form nitrobenzene

reagents = conc. HNO₃ + Conc. H₂SO₄ (catalyst), conditions = 50–60°C (temperature control prevents multiple nitration), mechanism = electrophilic substitution, electrophile = NO₂⁺ (nitronium ion)

halogenation of benzene to form halobenzene

reagents = Cl₂ or Br₂, conditions = halogen carrier catalyst (AlCl₃ or FeBr₃), heat under reflux, mechanism = electrophilic substitution, electrophile = Cl⁺ or Br⁺ (generated by catalyst)

friedel–crafts alkylation to form alkylbenzene

reagents = haloalkane (e.g. CH₃Cl), conditions = AlCl₃ catalyst, heat under reflux, mechanism = electrophilic substitution

friedel-crafts acylation to form aromatic ketone

reagents = acyl chloride (e.g. CH₃COCl), conditions = AlCl₃ catalyst, heat under reflux, mechanism = electrophilic substitution, electrophile = RCO⁺ (acylium ion)

why does benzene require a halogen carrier catalyst

benzene has delocalised pi electrons above and below carbon ring so lower e density around any 2 carbon atoms compared to alkene so cannot induce dipole

phenol as a weak acid

partially ionises in water, pos of equ lies well to left, more acidic than alc, less acidic than carboxylic acid

phenol reacting with naoh

neutralisation, forms sodium phenoxide and water

phenol reaction with sodium (same for all metals)

sodium phenoxide and hydrogen

phenol reaction with bromine

no catalyst, rtp, decolourises bromine water, forms white ppt of 2,4,6 tribromophenol

why phenol reacts more readily than benzene with br

lone pair of electrons from oxygen atom donated to delocalised electron cloud increasing electron density so can induce dipole in br

reaction of phenol with dilute nitric acid

rtp, forms mixture of 2-nitrophenol and 4-nitrophenol

uses of phenols

antiseptics, detergents, pharmaceuticals, dyes

amine directing group

open up at position 2,4,6

nitrites directing group

open up at position 3 and 5

activates

substituents which increase reactivity of ring

deactivates

substituents which decrease reactivity of ring

electron donating groups

OH, NH2, CH3 - increases e density, easier subsitution at 2,4,6

electron withdrawing groups

NO2, COOH, C=O-R - decreases e density, harder substitution at 3 and 5

ortho and para (2 and 4) directing groups

NH2 or NHR

meta (3) directing groups

NO2