Química Orgânica 2 · Aromatic systems
Revisão de substituição aromática para a prova
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- Bromination of benzene with Br₂ and FeBr₃
- Azo coupling of benzenediazonium with phenoxide
- Bromination of phenol without a Lewis acid
- Chlorination of nitrobenzene
- Removing a sulfonic acid blocking group: 2-bromotoluene
- Friedel–Crafts acylation of benzene with propanoyl chloride
- Intramolecular Friedel–Crafts acylation: 1-tetralone
- Alkylation of benzene with cyclohexene and sulfuric acid
- Friedel–Crafts alkylation with 1-chloro-2-methylpropane: a hydride shift
Ver todos os 79 problemas de revisão de substituição aromática para a prova →
O que você vai aprender
- Decide whether a ring, ion or heterocycle is aromatic, antiaromatic or nonaromatic by counting its π electrons with Hückel's rule.
- Draw electrophilic aromatic substitution as electrophile formation, rate-determining attack to an arenium ion, and deprotonation that restores the ring.
- Predict where a second substituent goes, and how fast, from the resonance and inductive effects of the group already on the ring.
- Recognise the limits of Friedel–Crafts reactions: carbocation rearrangement, polyalkylation, deactivated rings and basic amines.
- Distinguish addition–elimination (SNAr) from elimination–addition through benzyne, and predict the products of each.
- Order the steps of a multistep arene synthesis so each directing effect works for the target.
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Aromatic, antiaromatic or neither
A ring is aromatic when it is cyclic, planar, fully conjugated (every ring atom has a p orbital) and holds 4n + 2 π electrons: 2, 6, 10, 14… With 4n π electrons (4, 8…) the same planar ring is antiaromatic and strongly destabilised; a ring with an sp³ CH₂ or one that puckers out of planarity is simply nonaromatic. Count charges carefully. The cyclopentadienyl anion (five-membered ring, drawn) has two π bonds plus a lone pair in a p orbital, 6 π electrons, so it is aromatic; that is why cyclopentadiene is unusually acidic (pKa ≈ 16). The tropylium cation (seven-membered ring, drawn) has three π bonds and an empty p orbital, also 6 π electrons and aromatic. In heterocycles, decide where each lone pair sits: the pyrrole N–H lone pair lies in a p orbital and is part of the sextet, while the pyridine lone pair lies in the ring plane, is not part of the sextet, and makes pyridine a base.
Two steps: attack, then lose a proton
Every electrophilic aromatic substitution follows the same two steps. First a strong electrophile is made: Br⁺ character from Br₂ and FeBr₃, NO₂⁺ from HNO₃ and H₂SO₄, SO₃ from fuming sulfuric acid, a carbocation or an acylium ion from an alkyl or acyl halide and AlCl₃. A ring π bond then attacks it. This step breaks aromaticity, is slow, and gives the arenium ion (σ complex) drawn here, whose positive charge is shared by the three carbons ortho and para to the new sp³ carbon. Second, a weak base (HSO₄⁻, FeBr₄⁻, AlCl₄⁻, water) removes the proton from the sp³ carbon and the ring becomes aromatic again. In nitration and bromination the C–H bond breaks only after the slow step, so C₆H₆ and C₆D₆ react at the same rate.
What the first substituent does
A substituent changes both the rate and the position of the next attack. Groups with a lone pair on the atom next to the ring (–OH, –OR, –NH₂, –NHCOR) donate by resonance: they activate the ring and direct ortho/para, because only ortho or para attack puts the arenium charge on the carbon they are bonded to, where their lone pair can share it. Alkyl groups activate weakly and direct ortho/para by hyperconjugation and induction. Halogens are the exception: they withdraw inductively more than they donate, so they deactivate, yet their lone pairs still direct ortho/para. Groups with a δ+ or + atom next to the ring (–NO₂, –CN, –SO₃H, –CHO, –COR, –CO₂R, –CF₃, –NR₃⁺) deactivate and direct meta, because meta attack is the only one that never places positive charge next to them. Of the three arenes drawn, phenol is about 1000 times faster than benzene in nitration, chlorobenzene about 30 times slower, and nitrobenzene millions of times slower.
Friedel–Crafts and its limits
Friedel–Crafts alkylation (R–Cl with AlCl₃, or an alkene or alcohol with strong acid) goes through a carbocation or a cation-like complex, so primary alkyl groups rearrange by hydride or methyl shifts: 1-chloro-2-methylpropane gives tert-butylbenzene. The alkyl group it adds activates the ring, so the product reacts again and polyalkylation follows unless the arene is in excess. Friedel–Crafts acylation (RCOCl with AlCl₃) goes through an acylium ion, R–C≡O⁺, which is stabilised by oxygen and never rearranges; the ketone it adds deactivates the ring, so the reaction stops after one acylation. The ketone binds AlCl₃, so a full equivalent of AlCl₃ is needed. Neither reaction works on rings carrying a meta director (nitrobenzene is even used as a Friedel–Crafts solvent) or a basic –NH₂/–NHR/–NR₂ group, which binds AlCl₃ and becomes strongly deactivating. To install a straight-chain alkyl group, acylate first and then reduce the ketone with Zn(Hg)/HCl or H₂NNH₂/KOH.
Nucleophilic substitution on rings: SNAr and benzyne
Aryl halides do not undergo SN1 or SN2. With strong electron-withdrawing groups ortho or para to the leaving group, a nucleophile adds to the carbon bearing the halogen and gives an anionic Meisenheimer complex (drawn here for methoxide and 1-fluoro-4-nitrobenzene), whose charge spreads onto the nitro oxygens; loss of the halide restores the ring. Addition is rate-determining, so fluoride, the poorest leaving group in SN2, is the best here: its electronegativity makes the ring carbon more electrophilic and steadies the anionic intermediate, and the strong C–F bond breaks only after the slow step. A meta nitro group cannot take the charge by resonance and helps little. Without activating groups, a very strong base such as NaNH₂ removes the proton ortho to the halogen and the halide leaves to give benzyne. Amide then adds to either end of the strained triple bond, so the new group can appear on the carbon that bore the halogen or on its neighbour. An aryl halide with no hydrogen ortho to the halogen cannot form benzyne at all.
Planning a multistep synthesis
Work backwards from the target and ask which group must already be on the ring to direct the next one. Two meta-related groups mean a meta director was installed first; ortho/para-related groups mean an ortho/para director was. Change a director when needed: reduce –NO₂ (meta) to –NH₂ (ortho/para) with H₂ and Pd/C or Fe/HCl; reduce an aryl ketone (meta) to an alkyl group (ortho/para); oxidise a benzylic alkyl group (ortho/para) to –CO₂H (meta) with hot KMnO₄. Sulfonation is reversible, so –SO₃H can block the para position and be removed later with hot dilute aqueous acid. Remember the Friedel–Crafts limits at every stage: no Friedel–Crafts step on a ring that already carries a meta director.
What to carry forward
Count π electrons to judge aromaticity; draw every EAS as electrophile → arenium ion → deprotonation; let the existing substituent pick the position (lone pair or alkyl: ortho/para; δ+ atom next to the ring: meta; halogen: deactivating but ortho/para); respect the Friedel–Crafts limits; use SNAr only with ortho/para electron-withdrawing groups and benzyne with NaNH₂; and order synthetic steps so each director is in place when it is needed.
Problemas de prática (79)
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Desenhe o mecanismo 39
Directing effects and activation
- Azo coupling of benzenediazonium with phenoxideDraw the attack of phenoxide's para carbon on the terminal nitrogen of benzenediazonium, the deprotonation that restores the ring, and the acid workup that gives 4-(phenyldiazenyl)phenol.
- Bromination of phenol without a Lewis acidDraw the attack of phenol's para carbon on Br₂, pushed by the hydroxyl lone pair, and the deprotonation by bromide that gives 4-bromophenol and HBr.
- Chlorination of nitrobenzeneDraw the activation of Cl₂ by FeCl₃, attack by the carbon meta to the nitro group, and deprotonation by FeCl₄⁻ to give 1-chloro-3-nitrobenzene.
- Friedel–Crafts tert-butylation of tolueneDraw the formation of the tert-butyl cation, attack by the carbon para to the methyl group of toluene, the arenium contributor that puts the positive charge on the carbon bearing CH₃, and deprotonation to 1-tert-butyl-4-methylbenzene.
- Meta nitration of benzaldehydeDraw the attack of the ring carbon meta to the formyl group on nitronium, a resonance contributor of the arenium ion showing that the charge never reaches the carbon bearing CHO, and the deprotonation that gives 3-nitrobenzaldehyde.
- Nitration of chlorobenzeneDraw nitronium formation in mixed acid, para attack by chlorobenzene with the chlorine lone pair stabilising the arenium ion, and deprotonation by hydrogen sulfate to give 1-chloro-4-nitrobenzene.
- Azo coupling of benzenediazonium with N,N-dimethylaniline: methyl yellowDraw para attack of N,N-dimethylaniline on the benzenediazonium ion, the iminium-stabilised arenium ion and deprotonation by acetate to methyl yellow.
- Bromination of acetanilide to 4-bromoacetanilideDraw para attack of acetanilide on bromine, the nitrogen-stabilised arenium ion and loss of the ring proton to give 4-bromoacetanilide.
- Nitration of anisole to 2-nitroanisoleDraw nitronium formation, ortho attack by anisole with the methoxy oxygen stabilising the arenium ion, and deprotonation to 2-nitroanisole.
- Nitration of 1,3-dimethoxybenzene to 2,4-dimethoxy-1-nitrobenzeneDraw nitronium formation from nitric acid, attack at C4 of 1,3-dimethoxybenzene with methoxy stabilisation, and loss of the ring proton.
- Nitration of naphthalene to 1-nitronaphthaleneDraw nitronium formation, attack at C1 of naphthalene with a benzenoid-preserving arenium ion, and deprotonation to 1-nitronaphthalene.
- Sulfonation of nitrobenzene to 3-nitrobenzenesulfonic acidDraw meta attack of nitrobenzene on sulfur trioxide, deprotonation by hydrogen sulfate and protonation of the sulfonate.
Electrophiles and Friedel–Crafts
- Bromination of benzene with Br₂ and FeBr₃Draw how FeBr₃ activates Br₂, how benzene attacks the activated bromine to give an arenium ion, and how FeBr₄⁻ removes the ring proton to give bromobenzene and regenerate the catalyst.
- Friedel–Crafts acylation of benzene with propanoyl chlorideDraw the formation of the propanoyl acylium ion from propanoyl chloride and AlCl₃, its capture by benzene, and the deprotonation that gives 1-phenylpropan-1-one.
- Intramolecular Friedel–Crafts acylation: 1-tetraloneDraw acylium-ion formation from 4-phenylbutanoyl chloride, ring closure onto the ortho carbon of the same benzene ring to form a six-membered ring, and deprotonation to give 1-tetralone.
- Alkylation of benzene with cyclohexene and sulfuric acidDraw the protonation of cyclohexene by sulfuric acid, the attack of benzene on the cyclohexyl cation, and the deprotonation by hydrogen sulfate that gives cyclohexylbenzene.
- Friedel–Crafts alkylation with 1-chloro-2-methylpropane: a hydride shiftDraw how the 1-chloro-2-methylpropane–AlCl₃ complex ionises with a hydride shift to the tert-butyl cation, how benzene captures that cation, and how AlCl₄⁻ restores the ring to give tert-butylbenzene.
- Sulfonation of toluene with fuming sulfuric acidDraw the attack of the carbon para to the methyl group of toluene on SO₃, the deprotonation that restores the ring, and the proton transfer that gives 4-methylbenzenesulfonic acid.
- Friedel–Crafts acetylation of chlorobenzene to 4-chloroacetophenoneDraw acylium ion formation from acetyl chloride and AlCl3, para attack by chlorobenzene and deprotonation to 4-chloroacetophenone.
- tert-Butylation of anisole to 4-tert-butylanisoleDraw the tert-butyl cation forming from tert-butanol in sulfuric acid, para attack by anisole and deprotonation to 4-tert-butylanisole.
Multistep arene synthesis
- Removing a sulfonic acid blocking group: 2-bromotolueneDraw the ipso protonation of 3-bromo-4-methylbenzenesulfonate by hydronium and the loss of SO₃ that removes the blocking group and gives 2-bromotoluene.
SNAr and benzyne
- SNAr: methoxide displaces chloride from 1-chloro-2-nitrobenzeneDraw the addition of methoxide to 1-chloro-2-nitrobenzene with the ortho nitro group holding the negative charge of the Meisenheimer complex, then the loss of chloride that gives 2-nitroanisole.
- Regioselective SNAr of 1,2-difluoro-4-nitrobenzene with morpholineDraw the addition of morpholine to the ring carbon para to the nitro group, the proton transfer to a second morpholine, and the loss of fluoride that gives 4-(2-fluoro-4-nitrophenyl)morpholine.
- Triptycene from benzyne and anthraceneDraw benzyne formation from bromobenzene and sodium amide, then its Diels–Alder addition across C9 and C10 of anthracene to give triptycene.
- SNAr of 1-fluoro-4-nitrobenzene with piperidineDraw piperidine adding to 1-fluoro-4-nitrobenzene to give the Meisenheimer complex, loss of fluoride, and deprotonation to 1-(4-nitrophenyl)piperidine.
Efeitos orientadores e ativação
- Acoplamento azo do benzenodiazônio com 2-naftol formando Sudan IDesenhe a desprotonação do 2-naftol, o ataque do C1 do naftóxido ao benzenodiazônio, a rearomatização e a protonação que formam o 1-fenilazo-2-naftol.
- Bromação do 2-metoxinaftaleno em C1Desenhe o ataque do C1 do 2-metoxinaftaleno ao bromo, o íon arênio estabilizado pela metoxila e a perda do próton de C1 que forma o 1-bromo-2-metoxinaftaleno.
- Bromação do 4-metilanisol a 2-bromo-4-metilanisolDesenhe o ataque do 4-metilanisol ao bromo em orto à metoxila, o íon arênio do tipo oxocarbênio e a perda do próton do anel que forma o 2-bromo-4-metilanisol.
- Nitração do 4-fluoroanisol: a metoxila orienta mais que o flúorDesenhe a formação do íon nitrônio a partir do ácido nítrico, o ataque do 4-fluoroanisol em orto à metoxila e a rearomatização a 4-fluoro-2-nitroanisol.
- Nitração do 4-metilbenzoato de metila ao éster 3-nitroDesenhe a formação do íon nitrônio, o ataque do 4-metilbenzoato de metila pelo carbono em orto à metila e em meta ao éster, e a rearomatização a 4-metil-3-nitrobenzoato de metila.
SNAr e benzino
- Aminação via benzino do 3-bromobenzotrifluoretoDesenhe a desprotonação do 3-bromobenzotrifluoreto pelo amideto, a perda de brometo formando o arino, a adição regiosseletiva do amideto e a protonação que formam a 3-(trifluorometil)anilina.
- Benzino de um triflato de sililarila capturado por furanoDesenhe o ataque do fluoreto ao silício, a perda de triflato formando o benzino e sua reação de Diels–Alder com furano, que dá o 1,4-di-hidro-1,4-epoxinaftaleno.
- SNAr do 1-cloro-2,4-dinitrobenzeno com anilinaDesenhe a adição da anilina ao 1-cloro-2,4-dinitrobenzeno formando o complexo de Meisenheimer, a perda de cloreto e a desprotonação que formam a 2,4-dinitrodifenilamina.
- SNAr da 4-fluorobenzonitrila com fenóxidoDesenhe a desprotonação do fenol pelo carbonato, a adição do fenóxido à 4-fluorobenzonitrila e a perda de fluoreto que formam a 4-fenoxibenzonitrila.
Síntese de arenos em várias etapas
- Bromação da 4-nitroacetanilida em C2Desenhe o ataque da 4-nitroacetanilida ao bromo em orto ao grupo acetamido, o íon arênio C=N⁺ e a perda do próton do anel que forma a 2-bromo-4-nitroacetanilida.
Eletrófilos e Friedel–Crafts
- Anisol e anidrido glutárico: um cetoácido de Friedel–CraftsDesenhe a abertura do anel do anidrido glutárico pelo AlCl3 a um íon acílio, o ataque do anisol pela posição para, a rearomatização e o tratamento ácido que libera o ácido 5-(4-metoxifenil)-5-oxopentanoico.
- Propanoilação de Friedel–Crafts do mesitilenoDesenhe a formação do íon acílio a partir do cloreto de propanoíla e AlCl3, o ataque do mesitileno e a perda do próton do anel que formam a 1-(2,4,6-trimetilfenil)propan-1-ona.
- Alquilação do benzeno com 2-cloro-3-metilbutano: deslocamento de hidretoDesenhe a ionização do 2-cloro-3-metilbutano pelo AlCl3, o deslocamento 1,2 de hidreto ao cátion terciário e o ataque do benzeno a ele, formando o (2-metilbutan-2-il)benzeno.
- Isopropilação de Friedel–Crafts do p-xilenoDesenhe a ionização do 2-cloropropano pelo AlCl3, o ataque do p-xileno ao cátion isopropila e a perda do próton do anel que formam o 2-isopropil-1,4-dimetilbenzeno.
Preveja o produto 7
SNAr and benzyne
- Amination of 2-bromoanisole with NaNH₂Predict the regiochemistry of nucleophilic addition to an unsymmetrical benzyne bearing an inductively withdrawing group.
- Amination of 4-chlorotoluene with NaNH₂Predict the cine-substitution products of an unactivated aryl halide reacting through benzyne.
- Trapping benzyne with furanPredict the Diels–Alder adduct when benzyne is generated in the presence of furan.
- Sanger's reagent with an aminePredict the SNAr product of 1-fluoro-2,4-dinitrobenzene with a primary amine.
Directing effects and activation
- Brominate 4-nitrotoluenePredict the position of substitution when two ring substituents reinforce each other's directing effects.
- Nitrating 4-chloroanisolePredict nitration regiochemistry when two ortho/para directors compete on 4-chloroanisole.
Electrophiles and Friedel–Crafts
- Friedel–Crafts with 2-chlorobutanePredict the alkylbenzene from Friedel–Crafts alkylation of benzene with 2-chlorobutane.
Classificar 6
Electrophiles and Friedel–Crafts
- Which arene will not undergo Friedel–Crafts acylation?Identify a ring too deactivated for Friedel–Crafts acylation.
Aromaticity and Hückel's rule
- Which species is aromatic?Apply Hückel's 4n + 2 rule to cyclic ions and neutral polyenes, including the effect of charge on the π count.
- Which nitrogen lone pair is in the aromatic sextet?Decide whether a heteroatom lone pair belongs to the aromatic π system in pyrrole and pyridine and connect it to basicity.
- Is azulene aromatic?Apply Hückel criteria to the fused 5–7 bicyclic hydrocarbon azulene.
SNAr and benzyne
- Which aryl chloride undergoes SNAr?Recognise the substrate requirement for nucleophilic aromatic substitution by addition–elimination.
Directing effects and activation
- Find the meta directorIdentify the meta-directing group among common benzene substituents.
Comparar 4
Aromaticity and Hückel's rule
- Rank three ring hydrocarbons by acidityRank hydrocarbon acidity from the delocalisation and aromaticity of the conjugate-base anions.
Directing effects and activation
- Rank five arenes by rate of nitrationOrder monosubstituted benzenes by EAS rate from the activating or deactivating character of the substituent.
- Rank arenes for brominationRank five arenes by rate of electrophilic bromination from substituent donor strength.
SNAr and benzyne
- Rank aryl chlorides for SNArRank aryl chlorides in SNAr reactivity by counting ortho/para nitro groups.
Calcular 3
Aromaticity and Hückel's rule
- π electrons in the cyclooctatetraene dianionCount the π electrons of a reduced annulene and use the count to explain a change in shape.
- Count π electrons in the tropylium ionCount the π electrons of a cyclic carbocation to decide whether it is aromatic.
Directing effects and activation
- Partial rate factor for para nitration of tolueneCalculate a partial rate factor from a relative rate and an isomer distribution.
Encontre o erro 1
- A Friedel–Crafts plan for an aminoketoneFind the flaw in a Friedel–Crafts acylation planned on a free arylamine.
Interprete as evidências 4
SNAr and benzyne
- Where did the ¹⁴C label go?Interpret an isotope-labelling result as evidence for a benzyne intermediate.
Directing effects and activation
- What a missing isotope effect meansInterpret the absence of a kinetic isotope effect in EAS nitration as evidence that attack, not deprotonation, is rate-determining.
Listas mistas de prova: dos espectros ao areno
- Um multipleto de 5H em 7,2 ppm: identifique o areno C9H12Use um multipleto aromático de cinco prótons para identificar um benzeno monossubstituído e atribuir sua cadeia lateral.
- Dois dubletos aromáticos de 2H: qual padrão de substituição?Reconheça o padrão aromático de dois dubletos no RMN de ¹H de um benzeno para-dissubstituído.
Trabalhe de trás para frente 1
- Which intermediate leads to 3-nitrobenzoic acid?Work backwards from a meta-disubstituted target to the intermediate that sets up the correct directing effect.
Planeje uma síntese 9
Multistep arene synthesis
- Turn a meta director into an ortho/para directorChoose a reagent that converts an aromatic nitro group into an amino group.
- Make 1-chloro-3-propylbenzene from benzenePlan a three-step arene synthesis that uses an acyl group as a temporary meta director and avoids carbocation rearrangement.
- Make 2-bromotoluene free of the para isomerUse a reversible sulfonic acid blocking group to direct a substitution ortho to an alkyl group.
- Make 1-bromo-3-nitrobenzene from benzeneChoose the order of two electrophilic substitutions from the relationship of the substituents in the target.
- Make 4-bromobenzoic acid from tolueneOrder a ring substitution and a side-chain oxidation so that the directing effect of the side chain is used before it changes.
Electrophiles and Friedel–Crafts
- Which sequence gives butylbenzene?Plan a route to butylbenzene that avoids carbocation rearrangement.
Síntese de arenos em várias etapas
- Ácido 3-aminobenzoico a partir do toluenoOrdene a oxidação da cadeia lateral, a nitração e a redução do grupo nitro para preparar o ácido 3-aminobenzoico a partir do tolueno.
- Ácido 4-clorobenzenossulfônico a partir do benzeno em duas etapasEscolha a ordem de cloração e sulfonação que coloca os dois grupos em para no benzeno.
- Ordene a rota do tolueno ao ácido 4-nitrobenzoicoOrdene a nitração, a separação dos isômeros e a oxidação da cadeia lateral para preparar o ácido 4-nitrobenzoico a partir do tolueno.
Explique por quê 5
Directing effects and activation
- Why does aniline give meta product in mixed acid?Explain how protonation of an amino group in strong acid converts an ortho/para director into a meta director.
Electrophiles and Friedel–Crafts
- Why alkylation over-reacts and acylation does notExplain the difference in over-reaction between Friedel–Crafts alkylation and acylation from the effect of the group introduced.
- Why acylation stops after oneExplain why Friedel–Crafts acylation gives monosubstitution while alkylation over-reacts.
Aromaticity and Hückel's rule
- One bromide is a salt, the other is notExplain a difference in ionisation between two cyclic allylic bromides from the aromaticity of the cations they would form.
- Why cyclopropenone is so polarExplain the large dipole of cyclopropenone with an aromatic charge-separated contributor.
Travou em revisão de substituição aromática para a prova? Escaneie o problema.
Tire uma foto da reação da sua lista ou do livro e desenhe o mecanismo com cada seta corrigida. 1 scan grátis agora, sem cadastro; mais 1 com uma conta gratuita.
Depois, Scan Standard: /mês