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Química Inorgânica · Main-group elements

Química dos elementos representativos: do boro aos gases nobres

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O que você vai aprender

  • Explain the Lewis acidity of three-coordinate boron compounds (BH₃, boron trihalides, boric and boronic acids) from the empty p orbital, and predict which Lewis base binds or displaces another.
  • Use the strength of Si–O and Si–F bonds and the ability of silicon to become five-coordinate to explain silyl-ether chemistry, the hydrolysis of chlorosilanes, the Brook rearrangement and the β-silicon effect.
  • Follow phosphorus(III) compounds as nucleophiles that end as strong P=O bonds (Arbuzov, Staudinger, Appel) and assign oxidation states to phosphorus and sulfur.
  • Explain the oxidising power and acidity trends of the halogens and their oxoacids, and predict the shapes and oxidation states of xenon fluorides and polyhalide ions with VSEPR.
  • Describe how a frustrated Lewis pair, a bulky base and a bulky acid that cannot bind each other, splits H₂ and captures CO₂.

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Boron: six electrons and an empty p orbital

Three-coordinate boron has only six valence electrons and an empty 2p orbital, so BH₃, BCl₃ and B(OR)₃ are Lewis acids. BH₃ is never stored as such: in the gas phase it dimerises to diborane, B₂H₆, held together by two three-centre two-electron B–H–B bridges, and in solution it is sold as the THF complex (shown) or the dimethyl sulfide complex. A stronger Lewis base such as an amine displaces THF to give an amine–borane: boron in BH₃·THF already has an octet, so the amine bonds to boron from the side opposite the B–O bond as THF leaves, in one SN2-like step. Toward a strong base such as pyridine the trihalides rank BF₃ < BCl₃ < BBr₃: fluorine donates its lone pairs into boron's empty p orbital best, and that π bonding must be given up when boron becomes tetrahedral. Boric acid, B(OH)₃ (pKa ≈ 9.2), is an acid because boron accepts hydroxide from water, giving B(OH)₄⁻ and H₃O⁺, not because an O–H proton ionises. Boronic acids, RB(OH)₂, behave the same way and condense with 1,2-diols such as pinacol to give five-membered cyclic boronic esters. In hydroboration the empty p orbital takes the alkene π electrons, and boron ends on the less hindered carbon.

Silicon: strong bonds to oxygen and fluorine

Silicon is larger and less electronegative than carbon. Its bonds to electronegative atoms are very strong (approximate bond enthalpies: Si–F 565, Si–O 452, Si–Cl 381, Si–C 318 kJ/mol), while Si=Si and Si=C π bonds are weak because 3p orbitals overlap poorly; disilenes survive only when bulky groups such as mesityl block oligomerisation. Because silicon is large, a nucleophile can add to it to give a five-coordinate silicate before the leaving group goes. That is why Me₃SiCl and SiCl₄ fume in moist air while CCl₄ is inert to water, and why fluoride (TBAF) or methoxide can remove silyl groups. Silyl ethers survive acid in the order TMS < TES < TBS < TIPS as the groups on silicon get bulkier. An α-silyl alkoxide moves silicon from carbon to oxygen (the Brook rearrangement) because it trades a Si–C bond for a stronger Si–O bond; with a catalytic base the carbanion formed takes a proton from another molecule of the alcohol, so an α-silyl alcohol isomerises to a silyl ether. A carbocation β to silicon is stabilised by the C–Si σ bond (the β-silicon effect), so allylsilanes react with electrophiles at the carbon remote from silicon.

Phosphorus and sulfur: nucleophiles that end oxidised

Phosphorus(III) compounds such as PPh₃ and P(OEt)₃ are soft, polarisable nucleophiles, and the very strong P=O bond makes phosphorus(V) oxides the thermodynamic sink of many reactions. In the Michaelis–Arbuzov reaction a phosphite alkylates on phosphorus and then loses an ethyl group to bromide, giving a phosphonate. The phosphonate carbanion adds to aldehydes and eliminates a phosphate in the Horner–Wadsworth–Emmons reaction, giving mainly the E alkene. PPh₃ reduces azides to amines (the Staudinger reaction) and turns alcohols into alkyl bromides with CBr₄ (the Appel reaction, with inversion); each ends as triphenylphosphine oxide, which ³¹P NMR shows at δ ≈ +29 ppm against δ ≈ −6 ppm for PPh₃. A phosphorus ylide, such as Ph₃P⁺–CH₂⁻, is a carbanion held next to a phosphonium ion. Sulfur spans oxidation states from −2 (H₂S, sulfides) through 0 (S₈, sulfoxides such as dimethyl sulfoxide) and +4 (SO₂) to +6 (SO₃, H₂SO₄). A sulfide's lone pair attacks a peracid oxygen to give a sulfoxide; a second, slower oxidation gives the sulfone. A sulfoxide is pyramidal with a lone pair on sulfur and inverts only with difficulty (barrier roughly 145–175 kJ/mol), so methyl phenyl sulfoxide (shown) can be resolved into enantiomers.

Halogens and noble gases

Fluorine is the strongest elemental oxidant (E° F₂/F⁻ ≈ +2.87 V, against +1.36 V for Cl₂) even though chlorine has the larger electron affinity: the F–F bond is weak (about 158 kJ/mol, against 242 kJ/mol for Cl–Cl) and the small fluoride ion is hydrated far more strongly. HF is a weak acid in water (pKa ≈ 3.2) because the H–F bond is very strong (about 570 kJ/mol), while HCl, HBr and HI are strong acids. The chlorine oxoacids grow stronger as oxygens are added (HOCl pKa ≈ 7.5, HClO₂ ≈ 2.0, HClO₃ ≈ −1, HClO₄ ≈ −10) because each extra oxygen spreads the negative charge of the anion. In interhalogens such as ICl the heavier, less electronegative halogen is the δ+ end. Iodine is a Lewis acid: I₂ takes iodide to give the linear triiodide ion, whose central iodine, like xenon in XeF₂, carries three lone pairs. Xenon, whose first ionisation energy (about 12.1 eV) is far below that of helium (24.6 eV), forms fluorides and oxofluorides. VSEPR predicts their shapes: XeF₂ is linear with three lone pairs on xenon, XeF₄ (shown) is square planar with two lone pairs trans to each other, and XeOF₄ is square pyramidal with xenon in the +6 oxidation state.

Frustrated Lewis pairs

A Lewis base and a Lewis acid normally neutralise each other by forming an adduct. If both are very bulky, for example tri-tert-butylphosphine and tris(pentafluorophenyl)borane (shown), they cannot reach each other and stay frustrated. The unquenched pair can then act together on a small molecule: the phosphorus lone pair takes a proton from H₂ while boron takes the hydride, giving [tBu₃PH]⁺[HB(C₆F₅)₃]⁻ at room temperature. CO₂ is captured the same way, with phosphorus bonded to carbon and boron to an oxygen. The C₆F₅ groups make boron strongly Lewis acidic and also provide the bulk; the small trimethylphosphine simply forms a classical adduct with the same borane and does nothing with H₂.

What to carry forward

Read main-group reactivity from electron counts and bond strengths: three-coordinate boron wants a lone pair, silicon becomes five-coordinate and craves oxygen and fluorine, phosphorus(III) compounds and sulfides attack as nucleophiles and end oxidised, and the halogens and xenon follow electronegativity, bond strength and VSEPR. When a bulky acid and a bulky base cannot quench each other, the pair together can break H–H bonds.

Problemas de prática (40)

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Desenhe o mecanismo 10

Boron and Lewis acidity

  • Trimethylamine displaces THF from boraneDraw the exchange of Lewis bases on boron as a one-step, SN2-like displacement: the nitrogen lone pair of trimethylamine bonds to boron in BH₃·THF as THF leaves, giving trimethylamine–borane and free THF.
  • Phenylboronic acid and pinacol give the pinacol boronic esterDraw how phenylboronic acid condenses with pinacol: two cycles of oxygen addition to the empty p orbital of boron, proton transfer and loss of water, the second closing the five-membered dioxaborolane ring.

Phosphorus and sulfur

  • Michaelis–Arbuzov reaction of triethyl phosphite with benzyl bromideDraw the SN2 alkylation of triethyl phosphite on phosphorus by benzyl bromide and the dealkylation of the phosphonium ion by bromide that forms the P=O bond of diethyl benzylphosphonate.
  • Staudinger reduction of benzyl azide with triphenylphosphineDraw triphenylphosphine attacking the terminal nitrogen of benzyl azide, ring closure and loss of N₂ to the iminophosphorane, and its hydrolysis by water to benzylamine and triphenylphosphine oxide.
  • mCPBA oxidises thioanisole to methyl phenyl sulfoxideDraw the sulfur lone pair of thioanisole attacking the terminal oxygen of mCPBA with cleavage of the O–O bond, and the proton transfer that gives methyl phenyl sulfoxide and 3-chlorobenzoic acid.

Silicon

  • Base-catalysed Brook rearrangement of phenyl(trimethylsilyl)methanolDraw the base-catalysed [1,2]-Brook rearrangement: deprotonation of the α-silyl alcohol, attack of the alkoxide on the neighbouring silicon, cleavage of the Si–C bond to a benzylic carbanion, and proton transfer from a second molecule of the alcohol that gives benzyl trimethylsilyl ether and carries the chain.
  • Hydrolysis of chlorotrimethylsilaneDraw water adding to silicon in chlorotrimethylsilane to form a five-coordinate silicate, loss of chloride, and deprotonation by a second water molecule to give trimethylsilanol and hydrochloric acid.
  • Removing TMS from trimethyl(phenylethynyl)silane with K₂CO₃ in methanolDraw methoxide adding to silicon, the silicate expelling the phenylacetylide anion, and methanol protonating the acetylide to give phenylacetylene and methoxytrimethylsilane while regenerating methoxide.

Halogens, noble gases and frustrated Lewis pairs

  • A frustrated Lewis pair captures CO₂Draw how tri-tert-butylphosphine and tris(pentafluorophenyl)borane bind CO₂ together: phosphorus bonds to carbon and boron bonds to one oxygen, giving a neutral zwitterionic adduct.
  • A frustrated Lewis pair splits H₂Draw the heterolytic cleavage of H₂ by tri-tert-butylphosphine and tris(pentafluorophenyl)borane: the phosphorus lone pair bonds to one hydrogen while the H–H bonding pair moves to boron with the other hydrogen.

Preveja o produto 4

Phosphorus and sulfur

  • Appel bromination of (R)-octan-2-olPredict the configuration of the alkyl bromide formed from a chiral secondary alcohol in the Appel reaction.

Boron and Lewis acidity

  • What boric acid becomes in waterPredict the borate species formed when boric acid acts as an acid in water.

Silicon

  • Product of a Brook rearrangementPredict the silyl ether formed when an α-silyl alcohol undergoes a base-catalysed Brook rearrangement.

Halogens, noble gases and frustrated Lewis pairs

  • Iodine monochloride adds to propenePredict the regiochemistry of interhalogen addition to an alkene from the polarity of the I–Cl bond.

Classificar 3

Boron and Lewis acidity

  • Boron before and after binding THFClassify the geometry and hybridisation of boron in BH₃ and in its THF complex from electron-pair counts.
  • Which one accepts a lone pair?Identify a Lewis acid as the species whose central atom has an empty valence orbital.

Halogens, noble gases and frustrated Lewis pairs

  • Spot the frustrated Lewis pairClassify Lewis acid–base pairs as frustrated or adduct-forming from the steric bulk of both partners.

Comparar 5

Boron and Lewis acidity

  • Rank the boron trihalides as Lewis acidsRank BF₃, BCl₃ and BBr₃ by Lewis acidity using π back-donation from the halogen to boron.

Halogens, noble gases and frustrated Lewis pairs

  • Rank the chlorine oxoacidsRank the chlorine oxoacids by acid strength from the number of oxygens on chlorine.

Silicon

  • SiCl₄ fumes in water; CCl₄ does notCompare the hydrolysis of SiCl₄ and CCl₄ and attribute the difference to kinetics at the central atom.
  • Rank silyl ethers by acid stabilityRank common silyl protecting groups by their resistance to acidic cleavage from relative rate data.

Phosphorus and sulfur

  • Rank sulfur by oxidation stateAssign oxidation states to sulfur in four common compounds and rank them.

Calcular 5

Boron and Lewis acidity

  • How many alkenes per BH₃?Calculate how many equivalents of an unhindered alkene one borane can hydroborate.

Phosphorus and sulfur

  • Oxidation state of sulfur in DMSOCalculate the oxidation state of sulfur in dimethyl sulfoxide.

Silicon

  • Estimate the Si–O to Si–F exchange enthalpyEstimate the enthalpy of exchanging a Si–O bond for a Si–F bond from average bond enthalpies.

Halogens, noble gases and frustrated Lewis pairs

  • Lone pairs on xenon in XeF₂Count the lone pairs on the central atom of xenon difluoride from its valence electrons.
  • Oxidation state of xenon in XeOF₄Calculate the oxidation state of xenon in xenon oxytetrafluoride.

Encontre o erro 1

  • Find the error in a VSEPR answerDiagnose a VSEPR error that ignores lone pairs on the central atom of xenon tetrafluoride.

Interprete as evidências 1

  • Read a ³¹P NMR reaction checkInterpret ³¹P NMR shifts to follow the oxidation of triphenylphosphine during a reaction.

Planeje uma síntese 1

  • Reduce the azide and nothing elseChoose a reducing method that converts an azide into an amine without touching an alkene or an ester.

Explique por quê 10

Boron and Lewis acidity

  • Why ammonia borane is a solidExplain the very different physical states of ammonia borane and its isoelectronic analogue ethane.
  • How diborane holds togetherExplain the bonding in diborane with three-centre two-electron bridges from its valence electron count.
  • Why 9-BBN is more selective than BH₃Explain how the steric bulk of a dialkylborane raises anti-Markovnikov selectivity in hydroboration.

Silicon

  • Where allylsilanes reactExplain the regiochemistry of electrophilic attack on allylsilanes with the β-silicon effect.
  • What pays for the Brook rearrangement?Explain the driving force of the Brook rearrangement and the conditions that favour it.
  • Why disilenes need bulky groupsExplain why compounds with Si=Si double bonds are isolable only with bulky substituents.
  • Why fluoride removes silyl groupsExplain why fluoride selectively cleaves silyl ethers using Si–F and Si–O bond strengths.

Halogens, noble gases and frustrated Lewis pairs

  • Why F₂ out-oxidises Cl₂Explain why fluorine is a stronger oxidant than chlorine although chlorine has the larger electron affinity.
  • Why HF is a weak acidExplain why hydrofluoric acid is weak while hydrochloric acid is strong.

Phosphorus and sulfur

  • Why a sulfoxide can be resolvedExplain why sulfoxides with two different carbon groups are configurationally stable stereocentres while comparable amines are not.

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