OCR A 6.1.1 Aromatic Compounds

benzene

cyclic planar molecule

C₆H₆

benzene structure

each C bonded to 2 C and 1 H

final lone e^- in p orbital

adjacent p orbital e^- overlaps sideways above and below plane

overlapping p orbitals create pi bons which spread all of 6 C atoms in ring structure

e^- in the system r delocalised

benzene bond length

due to delocalised e^- structure

al C-C bonds have same bond length

benzene skeletal formula

kekule skeletal formula

why kekule model is wrong

using x-ray diffraction all of benzene’s C-C bond length are 0.139nm

enthalpy change of hydrogenation is less exothermic for benzene than model

benzene is less reactive as it requires a catalyst for chlorination to occur, but alkenes don’t

kekule has 3 pi bonds which overlap in 1 direction

but benzene has pi system where orbitals overlap in both direction with 6 electrons in pi bond

arene

aromatic compounds that contain benzene ring

benzoic acid

phenyl amine

benzaldehyde

phenol

naming benzene rules

grp that is attached first is n1

when benzene attached to alkyl chain w. func grp or with more than 6 C phenyl prefix used

reactions of arenes

undergo electrophilic substitution

-has high e^- density due to delcoalised ring of e^-: attract electrophiles

why arenes don’t undergo electrophilic addition

stable

would disrupt stable ring of electrons

instead electrophilic substitution occurs where grp on benzene ring subd for electrophile

benzene nitration conditions

catalysed by sulfuric acid

heat to 50 using water bath

benzene nitration equation

benzene nitration production of electrophile

benzene nitration mechanism

NO₂⁺ electrophile accepts pair of e^- from benzene ring

dative covalent bond formed

intermediate is unstable positively charged ring

e^- in C-H bond move to reform delocalised e^- ring

nitrobenzene and H⁺ formed

benzene nitration regeneration of catalyst

what happens if nitration of benzene happens in temp <50

further sub reactions may occur

leading to dinitrobenzene

halogenation of benzene conditons

doesn’t react unless halogen carrier (catalyst) present

eg: FeCl₃, AlBr₃

generated in situ from metal and halogen

halogenation of benzene equation

halogenation of benzene production of electrophile

halogenation of benzene mechanism

Br⁺ electrophile accepts pair of e^- from benzene ring

dative covalent bond formed

intermediate is unstable positively charged ring

e^- in C-H bond move to reform delocalised e^- ring

bromobenzene and H⁺ formed

halogenation of benzene reproduction of catalyst

halogen carrier for chlorination

halogen carrier for bromination

friedel crafts acylation

as benzene is too stable to react with

adding acyl grp makes easier to modify it further

acylation of benzene conditions

acyl chloride or acid anhydride

anhydrous - prevent reaction of AlCl₃

60 for 30 mins under reflux

acylation of benzene production of electrophile

AlCl₃ accepts pair of e^- away from acyl grp

polarisation increases and carbocation forms

strong electrophile made

acylation of benzene mechanism

delocalised e^- in benzene ring attracted to electrophile

2 e^- move to form bond

breaks ring and +ve charge forms

-ive AlCl₄^- attracted to +ve charged ring

1 of Cl atoms break away to form bond with H

acylation of benzene regeneration of catalyst

phenol

have hydroxide grp attached to benzene grp

c with OH grp is 1

phenol reactivity

more reactive than benzene

due to e^- density in ring is higher

polarises electrophile

more susceptible to electrophilic attacks

due to OH grp; e in O becomes part of pi ring

phenol property

weak acid; partially dissociate

form H+ ion and phenoxide ion

phenol + Br water

phenol + dilute nitric acid

activation

some groups can activate benzene ring by donating a lone pair of e^- into pi system of benzene (NH₂, OH)

aromatic ring reacts more readily w/ electrophile

therefor in presence of these extended groups the aromatic compound can react more readily with other electrophile and mutli-substituted product can form

why can Br react with phenyl amine w/out halogen carrier catalyst

NH₂ group has activated benzene group already

benzene can polarise the Br molecule

deactivation

some groups can deactivate benzene ring

so aromatic ring can react less readily w/ electrophile

also need halogen carrier catalyst when extended group are present with aromatic compounds

directing effect table