arenes

benzene

-cyclic, planar to achieve maximum overlap

-C6H6

-C bonded to 2 other Cs and one H by sigma bonds, leaving a lone e- in p-orbital (sits above and below planar ring)

-p-orbitals overlap sideways to form pi system

-delocalised e- ring

-high e- density above and below ring

-very stable

electrophilic substitution

-generation of electrophile (requires catalyst)

-electrophile attached to benzene to form intermediate

-H+ lost from intermediate, reforming delocalised ring

NO2 replaces H

-regeneration of catalayst

activating and deactivating groups

-OH and NH2 (and CH3) are activating, they stabilize substitutions at 2,4 positions, electron donating groups

-NO2 and Cl deactivate ring by attracting e- out of benzene ring, electron withdrawing, so less likely for multiple substitutions, direct at 3,5 positions

thermochemical evidence for benzene’s structure

-thermochemical: enthalpy change of hydrogenation was predicted to be -360kJ, 3x enthalpy change of cyclohexene

-it was actually -208kJ (less negative/exothermic than expected) so more stable than cyclohexatriene

x-ray diffraction to prove benzene model

-every benzene C-C bond length in intermediate length between double and single bond, so can’t be cyclohexatriene

reaction with bromine to prove benzene structure

-doesn’t decolourise without catalsyt, so no double bonds

halogenation with bromine and chlorine

-Br2 + FeBr3 (catalyst/halogen carrier) => Br+ + FeBr4-

-Cl2 + AlCl3 => Cl+ + AlCl4-

-FeBr3 + HBr OR AlCl3 + HCl are regenerated as hydrogen lost from intermediate

nitration

-conditions: 50 degrees C with conc. HNO3 and H2SO4

-HNO3 + H2SO4 => H2NO3+ + HSO4- (H+ transferred)

-H2NO3+ => H2O + NO2+ (electrophile)

-regeneration of H2SO4: H+ + HSO4-

alkylation

-conditions: room temp., AlCl3 catalyst, haloalkane

-CH3Cl + AlCl3 = AlCl4- + CH3+ (alkyl group)

-multiple substitutions

acylation

-conditions: 60 degrees C, reflux, AlCl3 catalyst, anhydrous

-monosubstitution

-RCOCl (acyl chloride) + AlCl3 (halogen carrier) = AlCl4- + RCO+ (electrophile)

-regeneration of AlCl3: H+ + AlCl4-

why is benzene more resistant to bromination compared to alkenes

In benzene pi e- are delocalised (but localised in C=C),

-it has a lower e- density so polarises br2 less

it’s more stable compared to localised e- density of pi bond in alkenes

why are multiple substitutions for alkylation possible…

phenols

-not alcohols

-white crystalline solids at room temp.

-C6H5OH

-weak acid (so only react with strong bases e.g. NaOH, not NaCO3)

-kills bacteria (disinfectant) but corrosive to skin

-burns a smoky flame due to high ration of C atoms and incomplete combustion

-sparingly soluble, non-polar benzene ring disrupts formation of hydrogen bonds

-can lose H+ as phenoxide ion is stabilised, -ve charge on oxygen is delocalised around ring, overlaps with delocalised e- ring

Bromination of phenol

-triple substitution with bromine water and room temp.

-forms white ppt (test for phenols), 2,4,6- tribromophenol (C6H2Br3OH)

And Hbr

Why is phenol more reactive than benzene

(OH activates e- ring)

-lone e- pair from oxygen’s p-orbital is delocalised/donated into pi ring

-increasing e- density in ring

-so polarises br-br, generating electrophile br+, more susceptible to electrophilic attack

- (halogen carrier isn’t needed)

Phenol + dilute nitric acid

2 or 4 nitrophenol

doesn’t require conc. H2SO4 catalyst or HNO3 to be conc. as phenol more reactive tha benzene

phenols in alkalis/strong bases

-soluble in sodium hydroxide as phenoxide ions attract water molecules, making ionic-dipole interactions, stronger than hydrogen bonds