Organic Chemistry 1 · Unsaturated molecules and radicals
Alkyne transformations: practice problems
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Draw the mechanism 102
- Generate an acetylideTrack the C–H pair during terminal-alkyne deprotonation.
- Extend a carbon chainBuild a new C–C bond by acetylide SN2 substitution.
- Acetylide alkylation — propyne to pent-2-yneRemove the terminal alkyne proton, then use the carbon lone pair in SN2 to make a longer chain.
- Use each acetylene terminus onceCarry out staged double alkylation without assuming that the first neutral alkyne still has an acetylide lone pair.
- Extend an acetylide by two carbonsCombine epoxide opening with an explicit aqueous protonation stage to obtain a homopropargylic alcohol.
- Add an acetylide to a ketoneMake a propargylic alcohol by carbonyl addition while distinguishing the retained alkyne π bonds from the changing carbonyl pair.
- Recognize a failed chain-extension designUse an acetylide as a base at a tertiary electrophile and finish the competing elimination rather than forcing SN2.
- Finish the enol formed by alkyne hydrationComplete acid-catalyzed enol tautomerization with solvent-mediated proton exchange and a carbonyl octet.
- Finish an aldehyde-forming enol in baseUse an enolate pathway to complete the tautomerization stage after terminal-alkyne hydroboration–oxidation.
- Use both alkyne pi bonds in successive additionsTrack two HX equivalents through vinyl and saturated intermediates to a geminal dihalide.
- Replace the terminal isotope without reducing the alkyneFollow terminal deprotonation and a separately introduced deuterium quench while preserving every original hydrogen atom.
- Build an ynone by acyl substitutionDistinguish acetylide acylation from simple carbonyl addition by completing tetrahedral collapse and chloride loss.
- Reduce an internal alkyne to a trans alkeneTrack single-electron transfers from sodium and proton transfers from ammonia through radical-anion and vinyl-radical intermediates to explain trans-alkene selectivity.
- Add one equivalent of bromine antiForm and open a bridged bromonium ion on an alkyne to explain why one Br₂ equivalent gives the E-dibromoalkene.
- Add a second equivalent of bromineConsume the second π bond of an alkyne with excess bromine through a bromonium ion on the dibromoalkene and recognize that the tetrabromide has no stereocenters.
- Hydrate a terminal alkyne with mercury(II)Carry Hg²⁺-catalyzed Markovnikov hydration of a terminal alkyne from mercury addition through water capture and removal of mercury to the neutral enol.
- Add HBr anti-Markovnikov to a terminal alkyneComplete both radical-chain propagation steps on a terminal alkyne and explain terminal bromine placement through the more substituted vinyl radical.
- Make an alkyne from a geminal dibromideCarry a geminal dihalide through two anti-periplanar E2 eliminations, the second from a vinyl bromide, to a terminal alkyne.
- Spend and recover the third amide equivalentExplain why terminal-alkyne synthesis by double dehydrohalogenation consumes a third NaNH₂ equivalent and needs an ammonium chloride workup to restore the terminal C–H bond.
- Hydroborate and oxidize a terminal alkyneFollow concerted syn hydroboration of a terminal alkyne by a bulky borane and the peroxide-driven boron-to-oxygen shift to the anti-Markovnikov enol.
- 2-Bromopropene to propyne with NaNH₂Draw the E2 elimination in which amide removes the =CH₂ proton anti to bromine in 2-bromopropene, forming propyne.
- Cyclohexylacetaldehyde from ethynylcyclohexane and 9-BBNConvert ethynylcyclohexane to cyclohexylacetaldehyde by 9-BBN hydroboration, basic peroxide oxidation and enolate tautomerization.
- Octanal from oct-1-yne by 9-BBN hydroboration–oxidationHydroborate oct-1-yne with 9-BBN, oxidize the alkenylborane with basic H2O2 and tautomerize the enolate to octanal.
- Propynide with 2-bromopropane: E2 winsDraw propynide acting as a base on 2-bromopropane: removal of a methyl proton anti to bromine to give propene and propyne (substrate: 2-bromopropane).
- 2-Methylbut-3-yn-2-ol from ethynide and acetoneAdd ethynide to the carbonyl carbon of acetone and protonate the alkoxide to give 2-methylbut-3-yn-2-ol.
- 1-Phenylprop-2-yn-1-ol from ethynide and benzaldehydeAdd ethynide to benzaldehyde and protonate the alkoxide to make 1-phenylprop-2-yn-1-ol.
- Prop-2-yn-1-ol from ethynide and formaldehydeAdd ethynide to formaldehyde and protonate the alkoxide to make the primary propargylic alcohol prop-2-yn-1-ol.
- Dec-5-yne from hex-1-yne and 1-bromobutaneDeprotonate hex-1-yne with amide, then draw the SN2 attack of the hexynide carbon on 1-bromobutane that builds dec-5-yne.
- Base-catalysed tautomerization of prop-1-en-2-olDraw hydroxide removing the enol O–H proton of prop-1-en-2-ol and water protonating the enolate at carbon to give acetone.
- Hydroboration–oxidation of hex-3-yneDraw syn hydroboration of the symmetric internal alkyne hex-3-yne, oxidation of the alkenylborane to the enolate, and protonation at carbon to hexan-3-one.
- Pentan-2-one and pentan-3-one from pent-2-yne with BH3Hydroborate pent-2-yne in both orientations, oxidize each alkenylborane and tautomerize the two enolates to pentan-3-one and pentan-2-one.
- Br₂ (1 equiv) adds anti to but-2-yneDraw bromine adding to but-2-yne through a cyclic bromonium ion and bromide opening it from the back face to give (E)-2,3-dibromobut-2-ene.
- (E)-2,3-Dibromohex-2-ene from hex-2-yne and one Br2Use the bridged bromonium ion from hex-2-yne and its anti opening to explain the E geometry of 2,3-dibromohex-2-ene.
- (E)-3,4-Dibromohex-3-ene from one Br2Form and open the bridged bromonium ion from hex-3-yne to explain why one Br2 gives (E)-3,4-dibromohex-3-ene.
- (E)-2,3-Dibromopent-2-ene from pent-2-yne and one Br2Form and open the bromonium ion from pent-2-yne to show why one equivalent of Br2 gives (E)-2,3-dibromopent-2-ene.
- 2,2,3,3-Tetrabromohexane from hex-2-yne and two Br2Add two equivalents of Br2 to hex-2-yne through two bromonium ions to reach 2,2,3,3-tetrabromohexane.
- 3,3,4,4-Tetrabromohexane from two Br2Add two equivalents of Br2 to hex-3-yne through two bromonium ions to reach 3,3,4,4-tetrabromohexane.
- Hept-3-yne from but-1-yne and 1-bromopropaneTurn but-1-yne into butynide with amide and draw the SN2 attack on 1-bromopropane that joins the two chains into hept-3-yne.
- (E)-2,3-Dichloro-4-methylpent-2-ene from Cl2Draw the chloronium ion from 4-methylpent-2-yne and Cl2 and its anti opening by chloride to (E)-2,3-dichloro-4-methylpent-2-ene.
- Cyclohexene with Br₂ then NaNH₂ gives a diene, not an alkyneDraw bromination of cyclohexene to trans-1,2-dibromocyclohexane and two trans-diaxial E2 eliminations by amide that give cyclohexa-1,3-diene instead of an alkyne.
- Prop-1-ynylcyclohexane from ethynylcyclohexane and CH3IDeprotonate ethynylcyclohexane with amide and draw the SN2 methylation by iodomethane that gives prop-1-ynylcyclohexane.
- 1,2-Dibromobutane to but-1-yne with NaNH₂Draw two E2 eliminations of HBr from 1,2-dibromobutane by amide, deprotonation of but-1-yne by the third equivalent, and protonation by water on workup.
- 1-Phenylpropyne from (E)-1-phenylpropene via Br₂, NaNH₂Draw anti addition of bromine to (E)-1-phenylpropene, an E2 that removes the benzylic hydrogen, and a second E2 from the vinyl bromide that gives 1-phenylpropyne.
- Hex-3-yne from (E)-hex-3-ene via Br₂ then NaNH₂Draw anti addition of bromine to (E)-hex-3-ene giving meso-3,4-dibromohexane, then two E2 eliminations by amide through (E)-3-bromohex-3-ene to hex-3-yne.
- Diphenylacetylene from (E)-stilbene via Br₂ then KOHDraw anti addition of bromine to (E)-stilbene giving meso-stilbene dibromide, then two dehydrobrominations by hydroxide through (E)-1-bromo-1,2-diphenylethene to diphenylacetylene.
- Hex-3-yne from ethyne by two alkylationsUse each C–H of ethyne in turn: deprotonate, ethylate, deprotonate again and ethylate again to reach hex-3-yne.
- 3-Phenylpropyne from acetylene and benzyl chlorideRemove one proton from acetylene with one equivalent of amide, then draw the SN2 attack of ethynide on benzyl chloride that gives 3-phenylpropyne.
- 2,2-Dibromopentane to pent-1-yneDraw two E2 eliminations from geminal 2,2-dibromopentane toward C1, deprotonation of pent-1-yne by amide, and protonation on aqueous workup.
- Hexane from hex-1-yne with H2 and Pd/CShow two equivalents of H2 adding across hex-1-yne on palladium, first to hex-1-ene and then to hexane.
- Two (Z)-bromoalkenes from prop-1-ynylcyclohexaneAdd one HBr anti to prop-1-ynylcyclohexane in both orientations and identify the two (Z)-(bromopropenyl)cyclohexanes that result.
- (1-Bromoprop-1-en-1-yl)benzene from 1-phenylpropyneProtonate 1-phenylpropyne so the vinyl cation sits on the benzylic carbon, then trap it with bromide to give (1-bromoprop-1-en-1-yl)benzene.
- HBr adds anti to pent-2-yne: (Z)-2-bromopent-2-eneDraw the termolecular anti addition of HBr and bromide across pent-2-yne that puts bromine on C2 and gives (Z)-2-bromopent-2-ene, and name the regioisomer that forms alongside it.
- 2-Bromopropene from propyne and one HBrProtonate propyne at the terminal carbon and trap the vinyl cation with bromide to make 2-bromopropene.
- (1,1-Dibromopropyl)benzene from 1-phenylpropyne and 2 HBrAdd two equivalents of HBr to 1-phenylpropyne and show why both bromines of (1,1-dibromopropyl)benzene end on the benzylic carbon.
- Two gem-dibromides from 4-methylpent-2-yne and 2 HBrAdd two HBr to 4-methylpent-2-yne in both orientations and explain why 2,2-dibromo-4-methylpentane and 3,3-dibromo-2-methylpentane both form.
- 2,2- and 3,3-Dibromopentane from pent-2-yne and 2 HBrAdd two equivalents of HBr to pent-2-yne along both possible orientations and show why each path ends in a geminal dibromide.
- 2,2-Dibromopropane from propyne and two HBrAdd two equivalents of HBr to propyne and explain why both bromines end on the same carbon of 2,2-dibromopropane.
- (Z)-2-Bromobut-2-ene by anti addition of HBrDraw the termolecular anti addition of HBr and bromide across but-2-yne that gives (Z)-2-bromobut-2-ene.
- HBr with peroxide adds anti-Markovnikov to hex-1-yneDraw the initiation by an alkoxyl radical and both propagation steps that put bromine on the terminal carbon of hex-1-yne (substrate: hex-1-yne).
- Two (Z)-chloroalkenes from 4-methylpent-2-yne and HClDraw both anti additions of HCl across 4-methylpent-2-yne and name the two (Z)-chloroalkenes the mixture contains.
- 2-Chlorohex-1-ene from hex-1-yne and one HClAdd one HCl to hex-1-yne with Markovnikov orientation through the secondary vinyl cation to give 2-chlorohex-1-ene.
- HCl adds anti to hex-2-yne: (Z)-2-chlorohex-2-eneDraw the termolecular anti addition of HCl and chloride across hex-2-yne that puts chlorine on C2 and gives (Z)-2-chlorohex-2-ene, and name the regioisomer that also forms.
- 2,2- and 3,3-Dichlorohexane from hex-2-yne and 2 HClFollow two equivalents of HCl adding to hex-2-yne in both orientations to 2,2-dichlorohexane and 3,3-dichlorohexane.
- (Z)-3-Chlorohex-3-ene by anti addition of HClDraw the termolecular anti addition of HCl and chloride across hex-3-yne that gives (Z)-3-chlorohex-3-ene.
- Non-1-en-4-yne from hex-1-yne and allyl bromideForm hexynide with sodium amide, then draw its SN2 attack on the CH2Br carbon of allyl bromide to build the skipped enyne non-1-en-4-yne.
- Hex-1-ynide with bromocyclohexane: E2 gives cyclohexeneDraw hex-1-ynide acting as a base on bromocyclohexane: anti-periplanar removal of a β-hydrogen that gives cyclohexene and hex-1-yne instead of an alkylated alkyne.
- Hex-1-ynide quenched by ethanol instead of alkylatedDraw the proton transfer from ethanol to hex-1-ynide that destroys the acetylide (giving hex-1-yne and ethoxide) before any alkylation can occur.
- Oct-3-yne from hex-1-yne and ethyl tosylateDeprotonate hex-1-yne and draw the SN2 attack of hexynide on the ethyl carbon of ethyl tosylate, with tosylate as the leaving group, to give oct-3-yne.
- Mercury-catalysed hydration of hex-1-yneDraw mercury(II)-catalysed Markovnikov hydration of hex-1-yne through the enol and its acid-catalysed tautomerization to hexan-2-one.
- Hydration of prop-1-ynylcyclohexane gives two ketonesDraw HgSO4-catalysed hydration of prop-1-ynylcyclohexane with water adding at the methyl-bearing carbon, giving 1-cyclohexylpropan-2-one, and name the second ketone.
- Hydration of 1-phenylpropyne to propiophenoneDraw HgSO4-catalysed hydration of 1-phenylpropyne, with water adding at the ring-bearing carbon, through the enol to propiophenone.
- Hydration of 4,4-dimethylpent-2-yne to two ketonesDraw HgSO4-catalysed hydration of 4,4-dimethylpent-2-yne with water adding at C2, giving 4,4-dimethylpentan-2-one, and name the ketone from the other orientation.
- Hydration of 4-methylpent-2-yne to two ketonesDraw HgSO4-catalysed hydration of 4-methylpent-2-yne with oxygen adding at C2, giving 4-methylpentan-2-one, and name the second ketone that forms.
- Hydration of but-2-yne to butanoneDraw Hg²⁺-catalysed hydration of the symmetric internal alkyne but-2-yne to its enol and the tautomerization to butan-2-one.
- Hydration of hept-3-yne: heptan-3-one and heptan-4-oneDraw HgSO4-catalysed hydration of hept-3-yne with water adding at C3, through the enol to heptan-3-one, and explain why heptan-4-one forms too.
- Hydration of hex-2-yne: hexan-2-one and hexan-3-oneDraw HgSO4-catalysed hydration of hex-2-yne with water adding at C2, through the enol to hexan-2-one, and explain why hexan-3-one also forms.
- Hydration of pent-2-yne to pentan-2-one and pentan-3-oneDraw HgSO4-catalysed hydration of pent-2-yne along the pathway that puts oxygen on C2, through the enol to pentan-2-one, and recognize that pentan-3-one forms alongside it.
- Hydration of phenylacetylene to acetophenoneDraw Hg²⁺-catalysed hydration of phenylacetylene, with oxygen on the benzylic carbon, and the tautomerization of the enol to acetophenone.
- Hydration of propyne to acetoneDraw Hg²⁺-catalysed Markovnikov hydration of propyne, the mercury-bearing enol, its demercuration and tautomerization to acetone.
- (Z)-Prop-1-enylbenzene by Lindlar hydrogenationDeliver two hydrogen atoms syn to 1-phenylpropyne on a Lindlar catalyst to form (Z)-prop-1-enylbenzene (cis-β-methylstyrene).
- Hex-1-ene from hex-1-yne by Lindlar hydrogenationAdd one equivalent of H2 to hex-1-yne on a Lindlar catalyst and explain why the reduction stops at hex-1-ene.
- (Z)-Hex-3-ene by Lindlar hydrogenationShow both hydrogen atoms of one H2 being delivered to the same face of hex-3-yne so that the cis alkene, (Z)-hex-3-ene, forms.
- Anti E2 of meso-stilbene dibromide to (E)-bromostilbeneDraw the single anti-periplanar E2 of meso-1,2-dibromo-1,2-diphenylethane with ethoxide and deduce that it gives (E)-1-bromo-1,2-diphenylethene.
- Pent-1-yne from pent-1-ene via Br₂ then NaNH₂Draw bromine adding to pent-1-ene through a bromonium ion, two E2 eliminations of HBr by amide, deprotonation of pent-1-yne by the third equivalent and its reprotonation by water.
- 2-Methylnon-4-yne from pent-1-yne and isopentyl bromideDeprotonate pent-1-yne and draw the SN2 attack of pentynide on 1-bromo-3-methylbutane (isopentyl bromide) that gives 2-methylnon-4-yne.
- Phenylacetylide with 2-bromobutane: E2 to (E)-but-2-eneDraw phenylacetylide acting as a base on 2-bromobutane: Zaitsev E2 removal of the C3 hydrogen anti to bromine that gives (E)-but-2-ene and phenylacetylene.
- 1,4-Diphenylbut-1-yne from phenethyl bromideDeprotonate phenylacetylene and draw the SN2 attack of phenylacetylide on (2-bromoethyl)benzene that gives 1,4-diphenylbut-1-yne.
- Phenylacetylide + allyl chloride: pent-4-en-1-ynylbenzeneDeprotonate phenylacetylene and draw the SN2 attack of phenylacetylide on the CH2Cl carbon of allyl chloride that gives pent-4-en-1-ynylbenzene.
- 1-Phenylpropyne from phenylacetylene and iodomethaneDeprotonate phenylacetylene with amide and draw the SN2 methylation of phenylacetylide by iodomethane that gives 1-phenylpropyne.
- 6-Chlorohex-2-yne: propynide picks bromide over chlorideDeprotonate propyne and draw the chemoselective SN2 attack of propynide at the C–Br carbon of 1-bromo-3-chloropropane, leaving the C–Cl bond intact in 6-chlorohex-2-yne.
- 1-Phenylbut-2-yne from propyne and benzyl bromideDeprotonate propyne with sodium amide, then draw the SN2 attack of propynide on the benzylic carbon of benzyl bromide that gives 1-phenylbut-2-yne.
- Pent-3-yn-1-ol from propyne and ethylene oxideDeprotonate propyne, open ethylene oxide with the propynide carbon, and protonate the alkoxide to give pent-3-yn-1-ol.
- 3,3-Dimethylbutanal from 3,3-dimethylbut-1-yne and Sia2BHUse disiamylborane and basic H2O2 to turn 3,3-dimethylbut-1-yne into 3,3-dimethylbutanal through an enolate.
- Disiamylborane hydroboration–oxidation of hex-1-yneDraw syn hydroboration of hex-1-yne by disiamylborane, the peroxide oxidation to the enolate, and its protonation at carbon to hexanal.
- Octan-4-one by Sia₂BH hydroboration–oxidation of oct-4-yneDraw syn hydroboration of oct-4-yne by disiamylborane, the peroxide oxidation that replaces boron by oxygen, and protonation of the enolate at carbon to give octan-4-one.
- (Z)-Oct-4-ene from oct-4-yne with Sia₂BH and acetic acidDraw syn hydroboration of oct-4-yne by disiamylborane and protonolysis of the alkenylborane by acetic acid, which replaces boron by hydrogen with retention to give (Z)-oct-4-ene.
- Phenylacetaldehyde from phenylacetylene with Sia2BHHydroborate phenylacetylene with disiamylborane and oxidize it to phenylacetaldehyde, explaining why no acetophenone forms.
- Anti E2 of (S,S)-stilbene dibromide to (Z)-bromostilbeneDraw the single anti-periplanar E2 of (1S,2S)-1,2-dibromo-1,2-diphenylethane with ethoxide and deduce that it gives (Z)-1-bromo-1,2-diphenylethene.
- Phenylacetylene from styrene via Br₂ then NaNH₂Draw bromine adding to styrene through a bromonium ion opened at the benzylic carbon, two E2 eliminations by amide starting at the benzylic C–H, and the acetylide formed and reprotonated on the way to phenylacetylene.
- 2,2-Dimethylhex-3-yne from 3,3-dimethylbut-1-yne + EtBrDeprotonate 3,3-dimethylbut-1-yne and draw the SN2 attack of tert-butylacetylide on bromoethane that gives 2,2-dimethylhex-3-yne.
- 3,3-Dimethylbut-1-yne from 3,3-dimethylbut-1-ene via Br₂Draw bromination of 3,3-dimethylbut-1-ene through a bridged bromonium ion that cannot rearrange, then two E2 eliminations by amide, acetylide formation and the water workup giving 3,3-dimethylbut-1-yne.
- Ethynylcyclohexane from vinylcyclohexane via Br₂, NaNH₂Draw bromination of vinylcyclohexane to (1,2-dibromoethyl)cyclohexane, two E2 eliminations by amide, trapping of ethynylcyclohexane as its acetylide, and the water workup.
Predict the product 5
- Stop after one halogen equivalentUse bromine stoichiometry and retained unsaturation to predict a dibromoalkene rather than a tetrabromide.
- Translate an enol into a ketonePreserve oxygen-bearing carbon during keto–enol tautomerization.
- Ketones from mercury-catalysed hydration of 2-methylhex-3-ynePredict that hydration of an unsymmetrical internal alkyne gives a mixture of two ketones.
- Ozonolysis products of hex-2-ynePredict the two carboxylic acids formed when an internal alkyne is cleaved by ozone.
- Dissolving-metal reduction productPredict the alkene geometry from Na/NH3 reduction of hex-3-yne and contrast it with Lindlar hydrogenation.
Classify 4
- Identify the removable protonLocate an sp-carbon hydrogen rather than a propargylic hydrogen.
- Name the alkyneName a branched alkyne by numbering from the end nearest the triple bond.
- Which base makes an acetylide?Choose a base strong enough to deprotonate a terminal alkyne by comparing conjugate-acid pKa values.
- Name the enyne CH≡C–CH₂–CH=CH₂Name a five-carbon enyne, breaking the numbering tie between the double and triple bond in favour of the double bond.
Compare 3
- Choose opposite alkene geometriesCompare net selectivity of two alkyne reduction methods.
- Rank C–H acidity by hybridizationRank the C–H acidity of ethane, ethene and ethyne from carbon hybridization.
- Rank vinyl and alkyl carbocation stabilityRank two vinyl cations against two alkyl carbocations to explain why HX adds more slowly to alkynes than to alkenes.
Calculate 3
- Estimate whether a base is sufficientCalculate an acid–base equilibrium from supplied pKa values.
- Equivalents of HBrCount the equivalents of HBr needed to turn a terminal alkyne into a geminal dibromide.
- NaNH₂ equivalents to make hex-1-yne from 1,2-dibromohexaneCount the equivalents of NaNH₂ consumed when a vicinal dibromide is converted into a terminal alkyne.
Find the error 2
- Reject an unsuitable coupling partnerDiagnose elimination rather than SN2 at a tertiary alkyl halide.
- Diagnose a wrong vinyl bromide from hex-1-yne and HBrTrace an anti-Markovnikov vinyl bromide back to peroxide contamination and a radical-chain HBr addition.
Interpret the evidence 2
- Which intermediate makes Na/NH₃ give the trans alkeneIdentify the trans vinyl radical and vinyl anion as the intermediates that fix the E geometry in a dissolving-metal alkyne reduction.
- Identify a C₆H₁₀ alkyne from H₂, NaNH₂ and hydration dataCombine hydrogen uptake, acidity toward NaNH₂ and the hydration products to identify an alkyne isomer.
Work backward 4
- Choose a workable disconnectionReverse acetylide alkylation while selecting an SN2-compatible fragment.
- Alkyne from its cleavage productsDeduce an internal alkyne's structure from the two carboxylic acids its ozonolysis gives.
- Plan a Z-alkene from acetyleneWork backward from (Z)-hex-3-ene to acetylene using acetylide alkylation and stereoselective reduction.
- Choose the dibromide that gives 3,3-dimethylbut-1-yneWork backwards from a terminal alkyne to the vicinal dibromide that a double E2 with NaNH₂ converts into it.
Plan a synthesis 3
- Make a terminal carbonylSelect hydroboration oxidation of a terminal alkyne for an aldehyde.
- Order the steps: but-1-yne to hexan-3-oneSequence acetylide formation, alkylation and hydration so that a symmetric alkyne gives one ketone.
- Aldehyde from a terminal alkynePlan reagents that convert hex-1-yne to hexanal rather than hexan-2-one.
Explain why 3
- Know when one ketone is not guaranteedIdentify regioisomer ambiguity for an unsymmetrical internal alkyne.
- Why use Sia₂BH or 9-BBN to make hexanal from hex-1-yneExplain why a bulky dialkylborane, not BH₃, is used to turn a terminal alkyne into an aldehyde.
- Why Lindlar hydrogenation gives a cis alkene and stops thereExplain both the syn stereochemistry and the stop-at-the-alkene selectivity of Lindlar hydrogenation.
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