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Química Orgânica Física · Reactive intermediates

Intermediários reativos: cátions, ânions, ilídeos, carbenos, nitrenos e arinos

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O que você vai aprender

  • Rank and explain carbocation stability from hyperconjugation, delocalisation, aromaticity, geometry and neighbouring-group (bridged) participation, and read the evidence for nonclassical ions.
  • Relate carbanion stability, geometry and configurational stability to hybridisation and delocalisation, and draw how phosphorus, sulfur and nitrogen ylides form and rearrange.
  • Distinguish singlet from triplet carbenes and nitrenes, predict the stereochemistry of their additions, and recognise carbenoid and concerted pathways (Simmons–Smith, Wolff, Curtius) that avoid a free intermediate.
  • Generate arynes from aryl halides or silyl aryl triflates, trap them in cycloadditions and nucleophilic additions, and predict the regiochemistry of unsymmetrical arynes.

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Carbocations: an empty p orbital and what fills it

A trivalent carbocation is planar and sp² hybridised, with its empty 2p orbital perpendicular to the plane. Anything that donates electron density into that orbital stabilises it. Adjacent C–H and C–C σ bonds overlap with it (hyperconjugation), so the gas-phase hydride ion affinity falls from about 1305 kJ/mol for CH₃⁺ to about 970 kJ/mol for (CH₃)₃C⁺: tertiary > secondary > primary > methyl. Allyl and benzyl cations spread the charge through π conjugation, and a lone pair on an adjacent O or N gives an oxocarbenium or iminium ion. The tropylium ion goes further: its empty p orbital completes a 6π aromatic ring, so cycloheptatriene gives up a hydride even to the trityl cation. Other cations are poor: the phenyl cation's empty orbital is an sp² orbital in the ring plane, orthogonal to the π sextet, so the ring cannot delocalise the charge; a vinyl cation puts the charge on an sp carbon; and a bridgehead cation such as 1-norbornyl cannot flatten at all. In superacid (SbF₅ in SO₂ClF) simple cations persist; the cationic carbon of the tert-butyl cation appears near δ 335 ppm in the ¹³C NMR spectrum.

Bridged cations and neighbouring-group participation

A σ or π bond held behind a leaving group can push it out and share the positive charge in a three-centre, two-electron bond. In 3-phenylbutan-2-yl tosylate the ipso carbon of the phenyl ring attacks the carbon losing tosylate, giving a phenonium ion (a spiro cyclopropane fused to an arenium ion, drawn here for the parent ion). Acetic acid opens it at either ring carbon with inversion, so the overall substitution occurs with retention, and an optically active threo tosylate gives racemic threo acetate because its phenonium ion has a mirror plane. In the 2-norbornyl system the C1–C6 σ bond plays the same role: exo-2-norbornyl brosylate solvolyses about 350 times faster than the endo isomer, and optically active exo ester gives only racemic exo product. Low-temperature NMR in superacid and a crystal structure show a symmetrical, σ-bridged (nonclassical) cation.

Carbanions: s character, delocalisation and shape

A carbanion holds a lone pair, and the more s character that lone pair has, the more stable it is: ethyne (pKa about 25) is far more acidic than ethene (about 44) or ethane (about 50). Delocalisation into π systems helps even more: in DMSO the pKa is 43 for toluene, 30.6 for triphenylmethane, 24.7 for acetophenone and 17.2 for nitromethane. A localised alkyl carbanion is pyramidal and inverts rapidly, like an amine; a carbanion conjugated with a π system, such as the benzyl anion, is planar. Cyclopropyl and vinyl carbanions invert slowly, because flattening would force a 120° angle on a carbon held near 60° or require sp hybridisation, so cyclopropyllithiums keep their configuration. Sulfur stabilises an adjacent carbanion without π conjugation, through its polarisability and the low-lying C–S σ* orbital: n-butyllithium removes the C2 proton of 1,3-dithiane, and the anion is an acyl-anion equivalent. Strong amide bases move alkynes along a chain through allenyl and propargyl anions until the terminal acetylide, the most stable carbanion available, forms.

Ylides: carbanions next to onium centres

An ylide has a carbanion bonded to a positively charged heteroatom: P⁺ in Wittig reagents, S⁺ in sulfonium ylides, N⁺ in ammonium ylides. The onium group makes the neighbouring C–H acidic, and an ester or ketone on the carbanion carbon (a stabilised ylide) makes it more acidic still, so carbonate or an amine is enough. Phosphorus ylides turn carbonyl groups into alkenes: stabilised ylides give mainly (E)-alkenes, and non-stabilised ylides under salt-free conditions give mainly (Z)-alkenes. Sulfur ylides turn carbonyl groups into epoxides instead, because sulfur leaves as a neutral sulfide rather than taking the oxygen. When an allyl or benzyl group sits on the onium atom, sulfur and nitrogen ylides rearrange by a [2,3]-sigmatropic shift through a five-membered transition state: the ylide carbon bonds to the far end of the allyl unit, the C=C moves, and the bond to the heteroatom breaks. With a benzyl group this is the Sommelet–Hauser rearrangement, which puts a new substituent ortho on the ring. The [1,2] (Stevens) shift instead goes through a radical pair that recombines in the solvent cage.

Carbenes, carbenoids and nitrenes

A carbene carbon has two bonds and two nonbonding electrons, six valence electrons in all. In the singlet state both electrons pair in an sp²-like σ orbital and a p orbital is empty; in the triplet they occupy the two orbitals with parallel spins. CH₂ is a ground-state triplet, about 38 kJ/mol below the singlet. π-Donor substituents such as Cl or OMe push a lone pair into the empty p orbital and make the singlet the ground state, as in dichlorocarbene, made from CHCl₃ and base by α-elimination. A singlet carbene adds to an alkene in one concerted step, so cis-but-2-ene gives only the cis cyclopropane; a triplet adds stepwise through a 1,3-diradical that must flip a spin before closing, so bond rotation scrambles the geometry. Singlet CH₂ also inserts into C–H bonds almost statistically. Many useful reactions avoid the free carbene. In the Simmons–Smith reaction (CH₂I₂ and Zn(Cu)) the reagent ICH₂ZnI, drawn here, transfers CH₂ to the alkene in a concerted, stereospecific step, and a nearby OH directs it to its own face. Nitrenes, the nitrogen analogues, are also six-electron species; photolysis of azides gives them, but the thermal Curtius, Hofmann and Lossen rearrangements move the migrating group to nitrogen as the leaving group departs, with retention of configuration and no free nitrene.

Arynes

Removing two ortho substituents from a benzene ring leaves an aryne. The aromatic π sextet is untouched; the extra bond comes from side-on overlap of two sp² orbitals in the plane of the ring. That in-plane bond is weak and its empty π* orbital is low in energy, so benzyne is a powerful, short-lived electrophile and dienophile. It forms when NaNH₂ removes the proton ortho to a halogen and the halide leaves, or under mild, neutral conditions when fluoride attacks the silicon of a 2-(trimethylsilyl)phenyl triflate and triflate departs. Furan, anthracene and other dienes trap it in Diels–Alder reactions, and nucleophiles add to either aryne carbon, so substitution can occur next to the carbon that bore the leaving group. A substituent that withdraws electrons inductively, such as OMe ortho to the aryne, directs the nucleophile to the far aryne carbon, so the new carbanion sits next to the electronegative group.

What to carry forward

For every intermediate, ask what orbital holds the charge or the unpaired electrons and what can overlap with it. Empty p orbitals want donors, aligned bonds or aromatic rings; lone pairs want s character, conjugation or an onium neighbour; carbenes and nitrenes behave by spin state; and arynes are strained in-plane π bonds that must be trapped. Many famous reactions (Simmons–Smith, Wolff, Curtius) deliver the chemistry of a carbene or nitrene without ever releasing one.

Problemas de prática (40)

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Desenhe o mecanismo 10

Carbanions and ylides

  • Alkyne zipper: hex-2-yne to hex-1-yne through allenyl and propargyl anionsDraw the deprotonation–reprotonation sequence through an allene that moves the triple bond of hex-2-yne to the chain end, and explain why the terminal acetylide traps the equilibrium.
  • 2-Lithio-1,3-dithiane: a sulfur-stabilised carbanion alkylated by benzyl bromideDraw the deprotonation of 1,3-dithiane at the carbon between the sulfur atoms and the SN2 alkylation of the resulting carbanion with benzyl bromide, and explain how sulfur stabilises the carbanion.
  • Sommelet–Hauser rearrangement of benzyltrimethylammoniumDraw formation of the ammonium N-methylide, its [2,3]-sigmatropic shift onto the ortho carbon of the ring, and the proton transfers that rearomatise the ring to give N,N-dimethyl-1-(2-methylphenyl)methanamine.
  • [2,3]-Sigmatropic rearrangement of a prenyl sulfonium ylideDraw formation of an ester-stabilised sulfonium ylide with carbonate and its [2,3]-sigmatropic shift, and explain why the allyl group ends up attached through its other terminus.

Arynes

  • 3-Bromoanisole and NaNH₂: 3-methoxybenzyne and m-anisidineDraw the deprotonation, bromide loss, regioselective amide addition and protonation that convert 3-bromoanisole into 3-methoxyaniline through 3-methoxybenzyne, and explain both selectivities.
  • Benzyne from 2-(trimethylsilyl)phenyl triflate, trapped by furanDraw the fluoride-triggered 1,2-elimination that turns a 2-(trimethylsilyl)phenyl triflate into benzyne, and the Diels–Alder reaction with furan that traps it as 1,4-dihydro-1,4-epoxynaphthalene.

Carbenes and nitrenes

  • Curtius rearrangement with retention: (S)-2-phenylpropanoyl azide to a methyl carbamateDraw the concerted Curtius rearrangement of an enantiopure acyl azide with loss of N₂, follow the configuration of the migrating carbon, and trap the isocyanate with methanol as a methyl carbamate.
  • Simmons–Smith cyclopropanation of (Z)-hex-3-eneDraw the concerted transfer of CH₂ from the zinc carbenoid ICH₂ZnI to (Z)-hex-3-ene and explain why the cyclopropanation is stereospecific even though no free carbene is formed.

Carbocations

  • Phenonium ion in the acetolysis of 3-phenylbutan-2-yl tosylateDraw the phenyl-assisted ionisation of 3-phenylbutan-2-yl tosylate to a bridged phenonium ion and its opening by acetic acid, and use the symmetry of the bridged ion to predict the stereochemistry of the acetate.
  • Tropylium ion from cycloheptatriene by hydride transfer to the trityl cationDraw the hydride transfer from cyclohepta-1,3,5-triene to the triphenylmethyl cation and explain, from cation stability, why the hydride moves in this direction.

Preveja o produto 4

Carbenes and nitrenes

  • Dibromocarbene and cis-but-2-enePredict the stereochemistry of a singlet dihalocarbene addition to a cis-alkene.
  • Photolysis of 2-diazocyclohexanone in waterPredict the ring-contracted acid from a Wolff rearrangement of a cyclic α-diazo ketone.

Carbanions and ylides

  • Wittig reaction with a stabilised ylidePredict the alkene geometry produced by an ester-stabilised phosphorus ylide.

Arynes

  • Benzyne meets anthracenePredict the Diels–Alder product of benzyne with anthracene, including which ring of anthracene reacts.

Classificar 5

Arynes

  • What makes the extra bond in benzyne?Classify the extra bond of benzyne as an in-plane π bond separate from the aromatic sextet.
  • Which aryl bromide cannot form an aryne?Classify aryl halides by whether they can form a benzyne with NaNH₂, from the presence of an ortho hydrogen.

Carbanions and ylides

  • Which carbanion is planar?Classify carbanions as pyramidal or planar from whether their lone pair is delocalised into a π system.

Carbenes and nitrenes

  • Which carbene is a ground-state singlet?Classify carbenes as ground-state singlets or triplets from their substituents.

Carbocations

  • Shape of the tert-butyl cationAssign the hybridisation and geometry of a trivalent carbocation and locate its empty orbital.

Comparar 2

Carbanions and ylides

  • How carbanions are stabilisedRank carbon acids in DMSO and connect each pKa to how the conjugate carbanion is stabilised.

Carbocations

  • Rank simple carbocations by stabilityOrder methyl, primary, secondary and tertiary carbocations by stability from hydride ion affinities and relate the trend to hyperconjugation.

Calcular 5

Carbenes and nitrenes

  • Electrons around a carbene carbonCount the valence electrons around a carbene carbon and recognise it as electron-deficient.
  • Indiscriminate C–H insertionCalculate the statistical product distribution for C–H insertion of singlet methylene into an alkane.

Arynes

  • How many toluidines from 3-chlorotoluene?Count the distinct products from an aryl halide that can form two different arynes.

Carbocations

  • Energy behind the exo/endo rate ratioConvert a solvolysis rate ratio into a difference in free energies of activation with ΔΔG‡ = RT ln(k₁/k₂).

Carbanions and ylides

  • Can dimsyl anion make the trityl anion?Calculate the equilibrium constant for deprotonating a carbon acid with a carbanion base from the two pKa values.

Encontre o erro 1

  • A proposed SN1 reaction of bromobenzeneDiagnose why an aryl cation cannot be stabilised by the ring π system.

Interprete as evidências 5

Carbenes and nitrenes

  • Insertion products only under lightInterpret C–H insertion products as evidence for a free acyl nitrene in the photochemical, but not the thermal, Curtius reaction.
  • Methylene in liquid alkene versus diluted gasInterpret loss of alkene stereochemistry in a carbene addition as evidence for a triplet carbene adding stepwise.

Carbocations

  • Racemic exo acetate from optically active exo brosylateInterpret the stereochemical outcome of 2-norbornyl brosylate acetolysis as evidence that C1 and C2 become equivalent in the intermediate.
  • A ¹³C signal at 335 ppmInterpret the extreme ¹³C chemical shift of a carbocation generated in superacid.

Carbanions and ylides

  • Evidence from the Stevens rearrangementInterpret CIDNP, cage-escape products and stereochemistry as evidence for a radical-pair [1,2] shift in an ammonium ylide.

Trabalhe de trás para frente 1

  • Where does the CCl₂ come from?Work back from a gem-dichlorocyclopropane to the reagents that generate dichlorocarbene by α-elimination.

Planeje uma síntese 2

Arynes

  • Making benzyne next to an esterChoose a benzyne precursor whose activation conditions are compatible with an ester-bearing trapping agent.

Carbenes and nitrenes

  • Cyclopropanate on the face of the OHChoose a carbenoid reagent that cyclopropanates an allylic alcohol on the same face as the hydroxyl group.

Explique por quê 5

Arynes

  • Why benzyne is such a good dienophileExplain the high electrophilicity and dienophile reactivity of benzyne from its weak, strained in-plane π bond.

Carbocations

  • Which tertiary bromide resists ionisation?Explain how the inability to become planar makes a bridgehead carbocation, such as 1-norbornyl, extremely hard to form.
  • What hyperconjugation meansIdentify the orbital overlap responsible for hyperconjugative stabilisation of a carbocation.

Carbanions and ylides

  • A carbanion that keeps its configurationExplain why cyclopropyl carbanions and cyclopropyllithiums are configurationally stable, using the geometry of carbanion inversion.

Carbenes and nitrenes

  • Why chlorine favours the singletExplain how π-donating, σ-withdrawing substituents stabilise the singlet state of a carbene.

Travou em intermediários reativos: cátions, ânions, ilídeos, carbenos, nitrenos e arinos? Escaneie o problema.

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