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Reactions of Benzene & Its Derivatives

organic lecture Series1 Reactions of Reactions of Benzene & Benzene & Its DerivativesIts DerivativesChapter 22 organic lecture Series2 Reactions of BenzeneReactions of BenzeneThe most characteristic reaction of aromatic compounds is substitution at a ring carbon:++ChlorobenzeneHalogenation:HClCl 2 FeCl3 HCl++NitrobenzeneNitration:HNO2 HNO3H2SO4H2 OOrganic lecture Series3+Benzenesulfonic acidSulfonation:HSO3 HSO3H2SO4++An alkylbenzeneAlkylation:RRXAlX3HX++Acylat ion:An acylbenzeneHRCXAlX3 HXOCROHR eactions of BenzeneReactions of BenzeneOrganic lecture Series4 RArenesAlkylbenzenesAlCl3 RClCarbon-Carbon Bond Formations: organic lecture Series5 Electrophilic Aromatic SubstitutionElectrophilic Aromatic Substitution Electrophilic aromatic substitution:Electrophilic aromatic substitution: a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile In this section: several common types of electrophiles how each is generated the mechanism by which each replaces hydrogen++HEE+H+ organic lecture Series6 EAS: General MechanismEAS: General Mechanism A general mechanism

Organic Lecture Series 23 • The electrophile is an acylium ion R-C Cl O Cl Cl Al-Cl O R-C Cl Al Cl Cl Cl O R-C+ AlCl 4-Aluminum chloride An acyl chloride A molecular complex with a positive charge charge on chlorine A n ion pair containing an acylium ion + - •• •• •• + (1) (2) •• •• Friedel-Crafts Acylation Organic Lecture ...

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Transcription of Reactions of Benzene & Its Derivatives

1 organic lecture Series1 Reactions of Reactions of Benzene & Benzene & Its DerivativesIts DerivativesChapter 22 organic lecture Series2 Reactions of BenzeneReactions of BenzeneThe most characteristic reaction of aromatic compounds is substitution at a ring carbon:++ChlorobenzeneHalogenation:HClCl 2 FeCl3 HCl++NitrobenzeneNitration:HNO2 HNO3H2SO4H2 OOrganic lecture Series3+Benzenesulfonic acidSulfonation:HSO3 HSO3H2SO4++An alkylbenzeneAlkylation:RRXAlX3HX++Acylat ion:An acylbenzeneHRCXAlX3 HXOCROHR eactions of BenzeneReactions of BenzeneOrganic lecture Series4 RArenesAlkylbenzenesAlCl3 RClCarbon-Carbon Bond Formations: organic lecture Series5 Electrophilic Aromatic SubstitutionElectrophilic Aromatic Substitution Electrophilic aromatic substitution:Electrophilic aromatic substitution: a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile In this section: several common types of electrophiles how each is generated the mechanism by which each replaces hydrogen++HEE+H+ organic lecture Series6 EAS: General MechanismEAS: General Mechanism A general mechanism Key question: What is the electrophile and how is it generated?

2 +E+HEH+s low , ratedeterminingStep 1:Step 2:EH+fast+H+EEl e ct ro - phileResonance-stabilized cation intermed iateOrganic lecture Series7++ organic lecture Series8 ChlorinationChlorinationStep 1: formation of a chloronium ionStep 2: attack of the chloronium ion on the ring+++Resonance-stabilized cation intermediate; the positivecharge is delocalized onto three atoms of the ring+slow , ratedeterminingClHHClHClClClClClClClFeCl ClClFeClClClFe Cl4-+-A molecular complex w ith a po sitiv e ch arg e o n ch l ori neFe rri c chl o ri de(a Lew is acid )Chlorin e(a Lew isbas e)++An ion p air contain ing a ch l oro ni um i o nOrganic lecture Series9 Step 3: proton transfer regenerates the aromatic character of the ringChlorinationChlorinationClHCl-F eCl3 ClHClFeCl3 ChlorobenzenefastCationintermediate+++-+ organic lecture Series10++BromobenzeneHBrBr2 FeBr3 HBrBrominationBrominationThis is the general method for Substitution of halogen onto a Benzene ring(CANNOT be halogenated by Free Radical Mechanism) organic lecture Series11 BrominationBromination--Why not Why not addnaddnof Brof Br22?

3 ?Regains Aromatic EnergyOrganic lecture Series12 Generation of the nitronium ion, NO2+ Step 1: proton transfer to nitric acid Step 2: loss of H2O gives the nitronium ion, a very strong electrophileHSO3 OHHONOOHSO4 ONOOHHCo n ju gate ac i dofnitric acid++SulfuricacidNitricacidThe nitro niumionONOOHHOHH+ONON itrationNitrationpKa= -3 organic lecture Series13 Step 1: attack of the nitronium ion (an electrophile) on the aromatic ring (a nucleophile)Step 2: proton transfer regenerates the aromatic ringONOHNO2NO2 HHNO2++++Resonance-stabilized cation intermediate+OHHOHHHHNO2NO2 OHHHOHHH++++++NitrationNitrationOrganic lecture Series14 A particular value of nitration is that the nitro group can be reduced to a 1 amino groupCOOHNO23H2 NiCOOHNH22H2O4-Aminobenzoic acid4-Nitrobenzoic acid+(3 atm )+NitrationNitrationOrganic lecture Series15 SulfonationSulfonation Carried out using concentrated sulfuric acid containing dissolved sulfur trioxideB enzenesulfonic acidBenzene+SO3 HSO3H2SO4(SO3in H2SO4is sometimes called fuming sulfuric acid.)

4 organic lecture Series16 FriedelFriedel--Crafts AlkylationCrafts Alkylation Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl groupClAlCl3 HCl+Benzene2-Chloropropane(Iso prop yl chlo rid e)Cumene(Isopropylbenzene)+The electrophilic partner is a carbocation; it will arrange to the most stable ion: allylic>3o>2o>1oOrganic lecture Series17 Step 1: formation of an alkyl cation as an ion pairStep 2: attack of the alkyl cation on the aromatic ringStep 3: proton transfer regenerates the aromatic ring+R+RHRHRHA resonance-stabilized cation+++HRCl A lCl3 RAlCl3 HCl+++RClClAlClClRClClClAl ClR+ AlCl4-A n ion pair contain ing a carbo catio n+-+A molecular co mp l e xFriedelFriedel--Crafts AlkylationCrafts AlkylationOrganic lecture Series18 There are two major limitations on Friedel-Crafts alkylations:1.

5 Carbocation rearrangements are common:+Isobutylchloridetert-Butylbenzen eBenzeneAlCl3+HClClCH3 CHCH2-ClCH3 AlCl3CH3C- C H2-Cl-AlCl3CH3 HCH3C+AlCl4-CH3CH3I sobutyl chl ori de+-+amolecularcompl exan i on pai rFriedelFriedel--Crafts AlkylationCrafts AlkylationOrganic lecture Series192. F-C alkylation fails on Benzene rings bearing one or more of these stronglyelectron-withdrawing groupsYRXAlCl3SO3 HNO2NR3+CF3 CCl3 CNCHOCROCO HOCOROCNH2O+ N o reacti o nWhen Y Equals Any of These Groups, the BenzeneRing Does Not Undergo Friedel-Crafts AlkylationFriedelFriedel--Crafts AlkylationCrafts AlkylationOrganic lecture Series20 organic lecture Series21 The The DeDe--activationactivation of of Aromatic SystemsAromatic SystemsNote: deactivation refers to the rateof EASO rganic lecture Series22 Friedel-Crafts acylationacylationforms a new C-C bond between a Benzene ring and an acylgroup.

6 OClCH3 CClOAlCl3 AlCl3 OOHClHCl+BenzeneAcetophenoneAcetylch l ori de4-Phenylbutanoylchl o ri d e -Tetralon e++FriedelFriedel--Crafts Crafts AcylationAcylationOrganic lecture Series23 The electrophile is an acyliumacyliumionionR-C ClOClClAl-ClOR-CClAl ClClClOR-C+AlCl4-AluminumchlorideAn acylchlorideA molecular complexwith a positive charge charge on chlorineA n ion pair containing an acylium ion+- +(1)(2) FriedelFriedel--Crafts Crafts AcylationAcylationOrganic lecture Series24 an acylium ion is a resonance hybrid of two major contributing structures F-C acylations are free of a major limitation of F-C alkylations; acyliumions do not rearrange.:++complete valence shellsThe more importantcontributing structureOOR-CR-C::FriedelFriedel--Craft s Crafts AcylationAcylationOrganic lecture Series25A special value of F-C acylations is preparation of unrearrangedalkylbenzenes:+AlCl3N2H4, KOHdiethylene glycolIsobutylbenzene2-Methyl-1-phenyl-1 -propanone2-Methylpropanoyl chlorideClOOFriedelFriedel--Crafts Crafts AcylationAcylationOrganic lecture Series26 DiDi--and and PolysubstitutionPolysubstitutionOnly a traceOrganic lecture Series27 DiDi--and and PolysubstitutionPolysubstitutionOrientat ion on nitration of monosubstitutedbenzenes:OCH3 ClBrCOOHCNNO2orthometaparaortho + bstitu entCH358438964 organic lecture Series28 Orientation.

7 Certain substituents direct preferentially to ortho & parapositions; others to meta positions substituents are classified as either orthoortho--paraparadirectingdirecting or meta meta directing directing toward further substitutionDiDi--and and PolysubstitutionPolysubstitutionOrganic lecture Series29 Rate certain substituents cause the rate of a second substitution to be greater than that for Benzene itself; others cause the rate to be lower substituents are classified as activatingactivatingor deactivatingdeactivatingtoward further substitutionDiDi--and and PolysubstitutionPolysubstitutionOrganic lecture Series30 organic lecture Series31 -OCH3is ortho-para directing: -CO2H is meta directingOCH3HN O3CH3 COOHOCH3NO2 OCH3NO2H2Op-N itro anis ole (55%)o-Nitroanisole (44%)An isole+++COOHHN O3H2SO4NO2CO O HCO O HNO2NO2 COOH100 Cm-Nitro-ben zoicacid(80%)Ben zoicacid+++o-Nitro-ben zoic acid(18%)p-Nitro-benzoicacid(2%)DiDi--an d and PolysubstitutionPolysubstitutionOrganic lecture Series32 DiDi--and and PolysubstitutionPolysubstitutionWeakly activatingOrtho-para DirectingWeakly deactivatingModerately activatingStrongly activatingNH2 NHRNR2 OHN HCRN HCArOROCArOCRRFClBrI:::::::::::::::::::: :::::Strongly deactivatingModerately deactivatingCHOOCRCOHSO3 HCOROCNH2NO2NH3+CF3 CCl3 Meta DirectingCNOOOOOOO rganic lecture Series33the order of steps is important.

8 CH3K2Cr2O7H2SO4HN O3H2SO4CH3NO2 COOHH2SO4 HNO3K2Cr2O7H2SO4 COOHNO2 COOHNO2m-N itro ben zoicacidp-N itro ben zoic acidDiDi--and and PolysubstitutionPolysubstitutionOrganic lecture Series34 Theory of Directing EffectsTheory of Directing Effects The rate of EAS is limited by the slowest step in the reaction For almost every EAS, the rate-determining step is attack of E+on the aromatic ring to give a resonance-stabilized cation intermediate The more stable this cationintermediate, the faster the rate-determining stepand the faster the overall reactionOrganic lecture Series35 For ortho-para directors, ortho-paraattack forms a more stable cationthan meta attack ortho-para products are formed faster than meta products For meta directors, meta attack forms a more stable cation than ortho-paraattack meta products are formed faster than ortho-para productsTheory of Directing EffectsTheory of Directing EffectsOrganic lecture Series36-OCH3.

9 Examine the meta attack:OCH3NO2+OCH3NO2 HOCH3NO2 HOCH3NO2 Hslowfast-H++OCH3NO2+++(a)(b)(c)Theory of Directing EffectsTheory of Directing EffectsNitration of anisoleNitration of anisoleOrganic lecture Series37-OCH3: examine the ortho-para attack:OCH3NO2+fast+(d)(e)(f)OCH3 HNO2 OCH3 HNO2 OCH3 HNO2 OCH3 HNO2 OCH3NO2-H++slow+++(g):::::::This resonance structure accounts for the selectivityNitration of anisoleNitration of anisoleOrganic lecture Series38-NO2; examine the meta attack:CO O HNO2+CO O HHNO2CO O HHNO2CO O HHNO2-H+CO O HNO2+slowfast(a)(b)(c)Theory of Directing EffectsTheory of Directing EffectsNitration of benzoic acidNitration of benzoic acidOrganic lecture Series39-NO2: assume ortho-para attack:COOHNO2+CO O HHNO2CO O HHNO2 COOHHNO2-H+CO O HNO2+slowfast(d)(e)(f )

10 The most disfavoredcontribu ting structu reThis resonance structure accounts for the selectivityNitration of benzoic acidNitration of benzoic acidOrganic lecture Series40 ActivatingActivating--DeactivatingDeacti vating Any resonance effectAny resonance effect, such as that of -NH2, -OH, and -OR, that delocalizes the positive charge on the cation intermediate lowers the activation energy for its formation, and has an activatingeffect toward further EAS Any resonance effectAny resonance effect, such as that of -NO2, -CN, -CO, and -SO3H, that decreases electron density on the ring deactivatesthe ring toward further EAS organic lecture Series41 Any inductive effectAny inductive effect, such as that of -CH3or other alkyl group, that releases electron density toward the ringactivatesthe ring toward further EAS Any inductive effectAny inductive effect, such as that of halogen, -NR3+, -CCl3, or -CF3, that decreases electron density on the ring deactivatesthe ring toward further EASA ctivatingActivating--DeactivatingDeactiv atingOrganic lecture Series42 Generalizations: alkyl, phenyl, and all other substituentsin which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing; all other substituents are meta directing all ortho-para directing groups except the halogens are activating toward further substitution.


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