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Química Inorgânica · Main group and Lewis acids

Ácidos e bases duros e moles, catálise por ácidos de Lewis e superácidos

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O que você vai aprender

  • Classify ions and molecules as hard, soft or borderline Lewis acids and bases and use the hard–soft rule to predict binding, exchange, solubility and site selectivity.
  • Rank Lewis acids by strength and explain the trends (BF₃ < BCl₃ < BBr₃, the Al₂Cl₆ dimer, acceptor numbers, Drago E and C values).
  • Draw Lewis-acid-promoted mechanisms with BF₃·OEt₂, AlCl₃, TiCl₄, ZnCl₂, Me₃SiI and Ag⁺, showing where the acid binds and how it is released.
  • Explain what makes a superacid, read the Hammett acidity scale and describe how superacids give persistent carbocations.

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Lewis acids accept an electron pair

A Lewis acid accepts an electron pair and a Lewis base donates one. Boron in BF₃ and aluminium in AlCl₃ have only six valence electrons and an empty p orbital, so they accept a lone pair from an ether, an amine, a halide or a carbonyl oxygen. In the adduct the acceptor becomes four-coordinate and tetrahedral, and the formal charges show the donation: in BF₃·OEt₂ (drawn) oxygen is +1 and boron −1. Metal cations (Ag⁺, Zn²⁺, Al³⁺, Ti⁴⁺ in TiCl₄) and SiMe₃⁺ sources such as Me₃SiI act the same way. BF₃ itself is a gas, so chemists use the liquid etherate BF₃·OEt₂, from which the weakly bound ether is displaced by the substrate.

Hard and soft

Pearson's rule: hard acids bind hard bases best and soft acids bind soft bases best. Hard acids are small, highly charged and hard to polarise: H⁺, Li⁺, Na⁺, Mg²⁺, Al³⁺, Ti⁴⁺, Fe³⁺, BF₃, R₃Si⁺. Soft acids are large, low-charged and polarisable: Cu⁺, Ag⁺, Au⁺, Hg²⁺, Pd²⁺, Pt²⁺, I₂, CH₃ in an SN2 transition state. Hard bases have small, electronegative donor atoms (F⁻, OH⁻, H₂O, ROH, R₂O, NH₃, carboxylates); soft bases have large, polarisable donors (I⁻, RS⁻, R₂S, R₃P, CO, CN⁻, alkenes). Borderline acids include Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺; borderline bases include Br⁻, pyridine and N₃⁻. The same element is softer in a lower oxidation state: Cu⁺ is soft, Cu²⁺ borderline. Absolute hardness puts a number on this: η = (I − A)/2, half the gap between ionisation energy and electron affinity. Fluorine (η ≈ 7.0 eV) is harder than iodine (η ≈ 3.7 eV).

Predictions from the hard–soft rule

Exchanges run toward hard–hard plus soft–soft pairs: LiI + CsF → LiF + CsI. Soft Hg²⁺ binds halides in the order F⁻ ≪ Cl⁻ < Br⁻ < I⁻, and silver halides become less soluble from AgF (very soluble) to AgI, while hard Li⁺ gives the opposite solubility trend (LiF poorly soluble, LiI very soluble). Thiocyanate binds soft Hg²⁺ through sulfur but hard Fe³⁺ through nitrogen. In nature, soft Hg, Pb and Cu occur as sulfide ores (cinnabar, galena, chalcopyrite), while hard Al and Mg occur as oxides, silicates and carbonates. The same idea guides organic selectivity: soft cuprates and thiolates add 1,4 to enones but hard organolithiums add 1,2; an enolate's soft carbon end reacts with CH₃I, its hard oxygen end with Me₃SiCl. Lewis acids that cleave ethers pair the two ideas: hard boron or silicon takes the oxygen and a soft bromide or iodide attacks the methyl carbon.

How strong is a Lewis acid?

Lewis acid strength depends on the base, so it is measured against a reference. Toward NH₃ or pyridine the boron halides (drawn) rank BF₃ < BCl₃ < BBr₃, the reverse of what halogen electronegativity suggests: fluorine's lone pairs donate most strongly into the empty boron 2p orbital, and that π bonding, together with the energy needed to bend planar BX₃ into a pyramid, must be paid when an adduct forms. Aluminium is larger, so AlCl₃ completes its octet by dimerising through two chloride bridges to Al₂Cl₆, in which each aluminium is four-coordinate; BCl₃ stays monomeric. The Gutmann–Beckett acceptor number, AN = 2.21 × (δ − 41.0), uses the ³¹P shift of Et₃PO bound to the acid. The Drago equation, −ΔH = E_A E_B + C_A C_B, separates electrostatic (E) and covalent (C) contributions to adduct enthalpy, a quantitative version of hard (large E) and soft (large C).

Lewis acid catalysis

Binding a Lewis acid to a carbonyl oxygen (drawn: BF₃ on benzaldehyde) puts a positive charge next to the carbonyl carbon and lowers the LUMO, so weak nucleophiles such as allylsilanes, silyl enol ethers and dienes react. AlCl₃ accelerates Diels–Alder reactions of acrylates and sharpens their regioselectivity; BF₃·OEt₂ opens epoxides to carbocations that rearrange to carbonyl compounds; TiCl₄ promotes the Hosomi–Sakurai allylation, in which the allylsilane bonds through the alkene carbon remote from silicon so that the cation formed is stabilised by the β C–Si bond. Because TiCl₄ can accept two donors, it chelates an α- or β-alkoxy carbonyl compound and fixes which face is attacked, whereas BF₃ binds only one oxygen. A Lewis acid can also turn a poor leaving group into a good one: ZnCl₂ in concentrated HCl (the Lucas reagent) binds the oxygen of an alcohol so that chloride can displace it; at a primary carbon this is an SN2 reaction. Soft Ag⁺ pulls bromide or iodide off alkyl halides and speeds their ionisation. A Lewis acid is a catalyst only if the product releases it; aryl ketones bind AlCl₃ so firmly that Friedel–Crafts acylation needs a full equivalent. Hydrolytically stable triflates such as Sc(OTf)₃ even work in water, where AlCl₃ and TiCl₄ are destroyed.

Superacids

A superacid is more acidic than 100% sulfuric acid, whose Hammett acidity function is H₀ ≈ −12. Each unit lower H₀ is ten times more protonating power. Fluorosulfuric acid (H₀ ≈ −15) and triflic acid (H₀ ≈ −14) are Brønsted superacids; adding the Lewis acid SbF₅ to HSO₃F gives magic acid, far stronger still, because SbF₅ binds the fluorosulfate anion and leaves a large, weakly basic counterion. In such media at low temperature in SO₂ or SO₂ClF, alcohols and alkyl fluorides ionise to persistent carbocations that can be studied by NMR: the tert-butyl cation (drawn) shows its cationic carbon near δ 335 ppm. Even alkanes are protonated, at a C–H σ bond, through three-centre two-electron bonding. In concentrated sulfuric acid, a tert-butyl cation made from 2-methylpropene is captured by the carbon lone pair of CO to give an acylium ion, which water converts to pivalic acid (the Koch carbonylation).

What to carry forward

Find the empty orbital and the lone pair, then ask whether the pair is hard or soft. Hard acids and hard bases bind mainly by charge, soft acids and soft bases by covalent overlap, and the matched pair wins. A Lewis acid catalyses a reaction by binding a lone pair of the substrate, which makes the neighbouring atom more electrophilic or a bond more ready to break; it must then be released to turn over. The weaker the conjugate base, the stronger the acid, which is why SbF₅-assisted superacids can protonate almost anything.

Problemas de prática (40)

Praticar é grátis com uma conta.

Desenhe o mecanismo 8

Lewis acid catalysis in synthesis

  • AlCl₃-catalysed Diels–Alder reaction of butadiene with methyl acrylateDraw how AlCl₃ binds the ester carbonyl of methyl acrylate, the concerted cycloaddition with buta-1,3-diene, and the release of AlCl₃ from methyl cyclohex-3-ene-1-carboxylate.
  • BF₃·OEt₂-catalysed rearrangement of styrene oxide to phenylacetaldehydeDraw the Lewis-acid-catalysed Meinwald rearrangement of styrene oxide: BF₃ binds the epoxide oxygen, the benzylic C–O bond breaks, a hydride shifts, and phenylacetaldehyde forms as BF₃ is released.
  • Trapping the cyclohexanone enolate on oxygen with Me₃SiClDraw the deprotonation of cyclohexanone by LDA and the reaction of the ambident enolate with chlorotrimethylsilane at oxygen to give the silyl enol ether.
  • TiCl₄-promoted Hosomi–Sakurai allylation of benzaldehydeDraw the TiCl₄-promoted allylation of benzaldehyde by allyltrimethylsilane: activation of the carbonyl, attack through the alkene terminus to a β-silyl carbocation, and loss of the silyl group to give the homoallylic titanium alkoxide.

Superacids and carbocations

  • Koch carbonylation of 2-methylpropene to pivalic acidDraw the strong-acid Koch carbonylation: protonation of 2-methylpropene to the tert-butyl cation, capture by the carbon end of CO to give an acylium ion, and hydration to 2,2-dimethylpropanoic acid.

Hard and soft acids and bases

  • Silver-assisted ethanolysis of 2-bromo-2-methylpropaneDraw how Ag⁺ pulls bromide off 2-bromo-2-methylpropane, how ethanol captures the tert-butyl cation, and how proton transfer gives 2-ethoxy-2-methylpropane and solid AgBr.

Lewis acid strength and adducts

  • Cleaving methyl benzoate with iodotrimethylsilaneDraw the cleavage of methyl benzoate by iodotrimethylsilane: silicon binds the carbonyl oxygen, iodide is displaced, and iodide removes the methyl group by SN2 to give trimethylsilyl benzoate and iodomethane.
  • ZnCl₂-promoted conversion of butan-1-ol to 1-chlorobutaneDraw how ZnCl₂ binds the hydroxyl oxygen of butan-1-ol, how chloride displaces the zinc-bound oxygen from the primary carbon by SN2, and how protonation releases water and regenerates ZnCl₂.

Preveja o produto 6

Lewis acid catalysis in synthesis

  • A soft nucleophile and an enonePredict the product of a soft organocopper nucleophile with an α,β-unsaturated ketone.
  • Carbon or oxygen methylation?Predict the site of alkylation of an ambident enolate with a soft electrophile.
  • Rearranging α-methylstyrene oxidePredict the carbonyl product of a Lewis-acid-promoted epoxide rearrangement from the most stable carbocation and the group that shifts.
  • Regiochemistry of a catalysed Diels–AlderPredict the major regioisomer of a Lewis-acid-catalysed Diels–Alder reaction of a 2-substituted diene.

Hard and soft acids and bases

  • Which way does the exchange run?Predict the direction of a double-exchange reaction between two salts using hard–soft matching.
  • Which end of thiocyanate binds?Predict the donor atom of the ambidentate thiocyanate ion toward a soft and a hard metal ion.

Classificar 4

Lewis acid strength and adducts

  • Draw boron trifluoride etherateIdentify the correct Lewis structure and formal charges of the BF₃–diethyl ether adduct.

Hard and soft acids and bases

  • Pick the hard acidClassify metal ions as hard or soft Lewis acids from their size, charge and polarisability.
  • Pick the soft baseClassify common ligands as hard or soft Lewis bases from the size and polarisability of the donor atom.

Superacids and carbocations

  • Which acid is a superacid?Classify acids as superacids using the 100% sulfuric acid benchmark.

Comparar 5

Lewis acid strength and adducts

  • Rank the boron trihalides as Lewis acidsOrder BF₃, BCl₃ and BBr₃ by Lewis acidity toward a nitrogen base.

Hard and soft acids and bases

  • Rank cations by hardnessOrder metal cations by absolute hardness and relate the order to charge, size and electron configuration.
  • Cu⁺ or Cu²⁺: which is softer?Compare the softness of one element in two oxidation states and give the reason.
  • Rank halide affinity of Hg²⁺Order the halide complexes of mercury(II) by stability and connect the order to softness.

Superacids and carbocations

  • Rank superacids by H₀Order strong and superacid media by acidity using Hammett acidity function values.

Calcular 4

Hard and soft acids and bases

  • Absolute hardness of fluorineCalculate the absolute hardness of an atom from its ionisation energy and electron affinity.

Lewis acid strength and adducts

  • Drago E and C: phenol and pyridineCalculate an adduct formation enthalpy from Drago E and C parameters and identify its electrostatic and covalent parts.
  • Acceptor number from a ³¹P shiftCalculate a Gutmann–Beckett acceptor number from the ³¹P shift of triethylphosphine oxide bound to a Lewis acid.
  • Electrons around aluminium in AlCl₄⁻Count the electrons around aluminium after AlCl₃ accepts chloride.

Encontre o erro 1

  • Basicity is not the whole storyDiagnose the error of predicting soft-acid binding from Brønsted basicity alone.

Interprete as evidências 1

  • Reading the spectrum of a carbocationInterpret ¹H and ¹³C NMR data to identify a persistent carbocation in superacid.

Trabalhe de trás para frente 1

  • Which allylsilane made this alcohol?Work back from a homoallylic alcohol to the allylsilane and carbonyl partner using the allylic transposition of the Sakurai reaction.

Planeje uma síntese 2

Hard and soft acids and bases

  • Design a ligand for Fe³⁺Choose the donor set for a chelating agent that must bind iron(III) selectively in water.

Lewis acid strength and adducts

  • A Lewis acid that survives waterChoose a Lewis acid catalyst that remains active in aqueous solution.

Explique por quê 8

Lewis acid catalysis in synthesis

  • Why a full equivalent of AlCl₃?Explain why a Lewis acid is consumed stoichiometrically when the product is a better Lewis base than the starting material.
  • Why TiCl₄ controls the diastereoselectivityExplain how the ability of TiCl₄ to chelate gives high diastereoselectivity in addition to an α-alkoxy aldehyde.

Lewis acid strength and adducts

  • Why does AlCl₃ dimerise but BCl₃ not?Explain the dimerisation of aluminium chloride and the monomeric structure of boron trichloride.
  • Why is BF₃ the weakest boron halide acid?Explain why boron trifluoride is a weaker Lewis acid than boron tribromide despite fluorine's higher electronegativity.

Hard and soft acids and bases

  • Opposite solubility trendsExplain why silver and lithium halides show opposite solubility trends down the halogen group.
  • Cinnabar and bauxiteExplain the different natural ores of mercury and aluminium with the hard–soft rule.

Superacids and carbocations

  • How SbF₅ makes magic acidExplain how a Lewis acid added to a Brønsted acid increases its acidity.
  • How can methane act as a base?Explain how a σ bond can act as the electron-pair donor when a superacid protonates an alkane.

Travou em ácidos e bases duros e moles, catálise por ácidos de Lewis e superácidos? 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.

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