Chapter 25 (6.1.1) Aromatic compounds

properties of benzene

colourless

sweet smelling

highly flammable

liquid

found naturally in crude oil and cigarette smoke

carcinogen

evidence to disprove Kekule’s model

1) lack of reactivity of benzene. does not undergo electrophilic addition reactions, does not decolourise bromine under normal conditions

2) the lengths of the c-c bonds. the lengths of the c-c bonds in benzene were between the lengths of a single and a double bond

3) hydrogenation enthalpies. was expected to have an enthalpy change of hydrogen 3x that of cyclohexene but had a lower enthalpy change instead. actual structure of benzene is more stable than the Kekule model

main features of delocalised model of benzene

planar, cyclic, hexagonal hydrocarbon

each C atom has one electron in a p-orbital at right angles to the plane of the bonded C and H atoms

adjacent p-orbital electrons overlap sideways to form a ring of electron density = system of pi-bonds

6 delocalised electrons

nitration of benzene

benzene + HNO₃ = nitrobenzene + H₂O

H₂SO₄ catalyst

50°C

electrophilic addition

diagram of nitration of benzene

uses of nitrobenzene

preparation of dyes, pharmaceuticals, pesticides

mechanism for nitration of benzene

1) HNO₃ + H₂SO₄ = NO₂⁺ + HSO₄^- + H₂O

2) benzene = unstable intermediate = nitrobenzene + H⁺

3) H⁺ + HSO₄^- = H₂SO₄

diagram of mechanism for nitration of benzene

examples of halogen carriers

AlCl₃, FeCl₃, AlBr₃, FeBr₃

diagram of bromination of benzene

mechanism for bromination of benzene

1) Br₂ + FeBr₃ = FeBr₄^- + Br⁺

2) Br⁺+ benzene = unstable intermediate = bromobenzene + H⁺

3) H⁺ + FeBr₄^- = FeBr₃ + HBr

diagram of mechanism for bromination of benzene

diagram of alkylation of benzene

diagram of acylation of benzene

why does benzene not react with bromine unless a halogen carrier catalyst is present

1) benzene has delocalised pi-electrons spread above and below the plane of C atoms in the ring structure

2) electron density around any two C atoms in benzene ring is less than that in a C=C in an alkene

3) there is insufficient pi-electron density around any C atoms in polarise the Br

why are phenols classified as weak acids

phenol is less soluble in water due to the presence of the non-polar benzene ring

when dissolved in water phenol partially dissociates a phenoxide ion and a proton

diagram of partial dissociation of phenol

comparing acid dissociation of alcohols with phenols and carboxylic acids

ethanol does not react with NaOH (strong base) or Na₂CO₃ (weak base)

phenols and carboxylic acids react with solution of strong bases

only carboxylic acids are strong enough acids to react with Na₂CO₃

how to distinguish between a phenol and a carboxylic acid

Na₂CO₃ + carboxylic acid produces CO₂(g)

diagram of reaction of phenol with NaOH (aq)

bromination of phenol

phenol + 3Br₂ = 2,4,6-tribromophenol + 3HBr

white precipitate formed

decolourises bromine water (orange = colourless)

RTP

nitration of phenol

phenol + HNO₃ = 2-nitrophenol + 4-nitrophenol

RTP

why is phenol more reactive than benzene

lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the pi-system of phenol

the electron density of the benzene ring in phenol is increased

the increased electron density attracts electrophiles more strongly than with benzene

activation with benzene

-NH₂ in nitrobenzene activates the benzene ring as the ring reacts more readily with electrophiles

positions 2 or 4

deactivation with benzene

-NO₂ deactivates the benzene ring as the ring reacts less readily with electrophile

position 3

ortho

position 2

meta

position 3

para

position 4

examples of 2- and 4- directing groups

-NH₂ or -NHR, -OH, -OR, -R or C₆H_5, -X

examples of 3- directing groups

RCOR, -COOR, -SO₃H, -CHO, -COOH, -CN, -NO_2, -NR₃⁺