Organic Chemistry 1 · Stereochemistry
Stereochemistry exam review: practice problems
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- OsO4 dihydroxylation of cyclopentene gives cis-cyclopentane-1,2-diol
- Anti addition of bromine to cis-2-butene gives the racemic dibromide
- Anti addition of bromine to trans-2-butene gives the meso dibromide
- Chlorohydrin formation from cyclopentene: anti addition of Cl and OH
- Hydroxide opens (2R,3R)-2,3-dimethyloxirane to meso-butane-2,3-diol
- NaBH4 reduces butan-2-one to racemic butan-2-ol
- SN1 methanolysis of (S)-2-chloro-2-phenylbutane with racemisation
- Azide converts cis-1-bromo-3-methylcyclohexane into the trans azide
- Cyanide displaces bromide from (R)-2-bromobutane with inversion
Draw the mechanism 52
- Anti addition of bromine to cis-2-butene gives the racemic dibromideDraw bromonium-ion formation from (Z)-but-2-ene and its backside opening by bromide, and explain why the product is racemic (2R,3R)/(2S,3S)-2,3-dibromobutane.
- Anti addition of bromine to trans-2-butene gives the meso dibromideDraw bromonium-ion formation from (E)-but-2-ene and its backside opening by bromide, and show that anti addition gives meso-2,3-dibromobutane.
- Chlorohydrin formation from cyclopentene: anti addition of Cl and OHDraw the chloronium ion from cyclopentene and chlorine, its backside opening by water, and the deprotonation that gives trans-2-chlorocyclopentan-1-ol.
- Hydroxide opens (2R,3R)-2,3-dimethyloxirane to meso-butane-2,3-diolDraw the backside SN2 opening of (2R,3R)-2,3-dimethyloxirane by hydroxide and protonation of the alkoxide by water, and explain why a single enantiomer of the epoxide gives the achiral meso diol.
- NaBH4 reduces butan-2-one to racemic butan-2-olDraw hydride transfer from borohydride to butan-2-one and protonation of the alkoxide by methanol, and explain why the new stereocentre forms as a racemate.
- SN1 methanolysis of (S)-2-chloro-2-phenylbutane with racemisationDraw the SN1 methanolysis of (S)-2-chloro-2-phenylbutane through a planar benzylic cation and explain why the ether forms largely racemic.
- Azide converts cis-1-bromo-3-methylcyclohexane into the trans azideDraw the backside SN2 attack of azide on cis-1-bromo-3-methylcyclohexane and explain why the product is trans-1-azido-3-methylcyclohexane.
- Cyanide displaces bromide from (R)-2-bromobutane with inversionDraw the backside SN2 attack of cyanide on (R)-2-bromobutane and assign the configuration of the (S)-2-methylbutanenitrile formed.
- Br2 addition to 1-methylcyclohexene: trans dibromideDraw the bromonium mechanism for Br2 adding to 1-methylcyclohexene and explain why the two bromines end up trans.
- Br2 addition to (E)-2,5-dimethylhex-3-ene: meso dibromideDraw the anti addition of Br2 to (E)-2,5-dimethylhex-3-ene and show why the dibromide is meso.
- Bromine adds anti to (Z)-pent-2-ene to give (2S,3S)-2,3-dibromopentaneDraw bromonium-ion formation from (Z)-pent-2-ene and its anti opening by bromide, and explain why racemic (2R,3R)/(2S,3S)-2,3-dibromopentane forms.
- Trans bromohydrin from 1-ethylcyclohexene with NBS and waterDraw bromonium-ion formation from 1-ethylcyclohexene and NBS, its opening by water at the tertiary carbon, and explain why the OH and Br end up trans.
- Cl2 in methanol turns cyclohexene into a trans chloro etherDraw chloronium-ion formation from cyclohexene and its anti opening by methanol, and explain why trans-1-chloro-2-methoxycyclohexane forms instead of the dichloride.
- Chlorohydrin from (Z)-but-2-ene: Cl2 and water anti additionDraw the chlorohydrin mechanism for (Z)-but-2-ene with Cl2 in water and assign the stereochemistry of 3-chlorobutan-2-ol.
- D2 adds syn to (Z)-but-2-ene: meso-2,3-dideuteriobutaneDraw deuterium addition to (Z)-but-2-ene on palladium and explain why syn addition to the Z alkene gives the meso dideuteriobutane.
- E2 of (2S,3S)-2-bromo-3-cyclohexylbutane with ethoxideDraw the anti-periplanar E2 of (2S,3S)-2-bromo-3-cyclohexylbutane and explain why (E)-but-2-en-2-ylcyclohexane is formed.
- E2 of (3R,4R)-3-bromo-4-phenylhexane: (E)-3-phenylhex-3-eneDraw the anti-periplanar E2 of (3R,4R)-3-bromo-4-phenylhexane and deduce why (E)-3-phenylhex-3-ene is the only alkene.
- E2 of cis-1-chloro-2-ethylcyclohexane: 1-ethylcyclohexeneDraw the E2 elimination of cis-1-chloro-2-ethylcyclohexane with ethoxide and explain why the Zaitsev alkene 1-ethylcyclohexene is major.
- E2 of menthyl chloride with ethoxide: 2-menthene onlyDraw the E2 elimination of menthyl chloride with ethoxide and explain why 2-menthene is the only alkene.
- E2 of neomenthyl chloride with ethoxide: 3-mentheneDraw the E2 elimination of neomenthyl chloride with ethoxide and explain why the Zaitsev alkene 3-menthene is major.
- (2S,3S)-2,3-Dibromobutane eliminates to (Z)-2-bromobut-2-eneDraw the anti-periplanar E2 of (2S,3S)-2,3-dibromobutane with one equivalent of hydroxide and explain why only (Z)-2-bromobut-2-ene forms.
- E2 of trans-1-bromo-2-isopropylcyclohexane with ethoxideDraw the E2 elimination of trans-1-bromo-2-isopropylcyclohexane and explain why 3-isopropylcyclohexene forms instead of the Zaitsev alkene.
- E2 of trans-1-chloro-2-ethylcyclohexane: 3-ethylcyclohexeneDraw the E2 elimination of trans-1-chloro-2-ethylcyclohexane with ethoxide and explain why 3-ethylcyclohexene is the product.
- Acidic methanol opens 1-methylcyclohexene oxideDraw the acid-catalysed opening of 1-methylcyclohexene oxide by methanol and explain why the methoxy group goes to the tertiary carbon, trans to the OH.
- NaOMe opens cyclohexene oxide to trans-2-methoxycyclohexanolDraw the backside attack of methoxide on cyclohexene oxide and the protonation of the alkoxide, and explain why the methoxy and hydroxyl groups end up trans.
- Hydrogenation of 1,2-dimethylcyclopentene gives cis-1,2-dimethylcyclopentaneDraw the hydrogenation of 1,2-dimethylcyclopentene on palladium and explain why both hydrogens add to the same face to give meso cis-1,2-dimethylcyclopentane.
- HBr adds to 1,2-dimethylcyclohexene through a planar cationDraw protonation of 1,2-dimethylcyclohexene by HBr and bromide capture of the tertiary cation, and explain why both cis and trans 1-bromo-1,2-dimethylcyclohexane form.
- HBr adds to (E)-hex-3-ene to give racemic 3-bromohexaneDraw protonation of (E)-hex-3-ene by HBr and capture of the carbocation by bromide, and explain why 3-bromohexane forms as a racemate.
- Hydroboration–oxidation of 1-ethylcyclohexene gives trans-2-ethylcyclohexan-1-olDraw the syn addition of borane to 1-ethylcyclohexene and the oxidation with basic hydrogen peroxide, and explain why the alcohol is trans to the ethyl group.
- Hydroboration of 1-isopropylcyclohexene: the trans alcoholDraw the syn addition of borane to 1-isopropylcyclohexene and the basic peroxide oxidation, and explain why the OH ends up trans to the isopropyl group.
- Cold permanganate turns (E)-stilbene into (1R,2R)-hydrobenzoinDraw the cycloaddition of permanganate to (E)-stilbene and the hydrolysis of the cyclic manganate ester, and explain why syn addition gives racemic (1R,2R)/(1S,2S)-1,2-diphenylethane-1,2-diol.
- mCPBA converts (E)-pent-2-ene into (2R,3R)-2-ethyl-3-methyloxiraneDraw the one-step transfer of oxygen from mCPBA to (E)-pent-2-ene and explain why the epoxide keeps the trans relationship of the alkene.
- mCPBA epoxidizes (Z)-hex-3-ene to cis-2,3-diethyloxiraneDraw the concerted oxygen transfer from mCPBA to (Z)-hex-3-ene and explain why the cis alkene gives the meso cis epoxide.
- Hydroxide opens meso-2,3-dimethyloxirane to (2S,3S)-butane-2,3-diolDraw the backside opening of cis-2,3-dimethyloxirane by hydroxide and the protonation of the alkoxide, and explain why the meso epoxide gives a racemic diol.
- OsO4 turns 1,2-dimethylcyclohexene into the cis diolDraw the cyclic osmate ester from 1,2-dimethylcyclohexene and its hydrolysis, and explain why both OH groups end up cis in a meso diol.
- OsO4 dihydroxylation of cyclopentene gives cis-cyclopentane-1,2-diolDraw the cycloaddition of OsO4 to cyclopentene and the hydrolysis of the osmate ester, and explain why the diol is cis and meso.
- Simmons–Smith on (E)-1-phenylpropene: trans cyclopropaneDraw the concerted CH2 transfer from the Simmons–Smith carbenoid to (E)-1-phenylpropene and explain why the methyl and phenyl groups stay trans.
- Simmons–Smith cyclopropanation of (E)-but-2-ene gives trans-1,2-dimethylcyclopropaneDraw the concerted transfer of CH2 from the Simmons–Smith reagent to (E)-but-2-ene and explain why the methyl groups stay trans.
- Ethanolysis of (R)-3-chloro-3-methylhexane gives racemic 3-ethoxy-3-methylhexaneDraw the SN1 ethanolysis of (R)-3-chloro-3-methylhexane through the planar tertiary carbocation and explain why the ether is racemic.
- SN1 hydrolysis of (1S,3R)-1-chloro-1,3-dimethylcyclohexaneDraw the SN1 hydrolysis of (1S,3R)-1-chloro-1,3-dimethylcyclohexane and explain why the two alcohols are diastereomers.
- SN1 hydrolysis of (R)-3-bromo-3-methylheptaneDraw the SN1 hydrolysis of (R)-3-bromo-3-methylheptane and explain why 3-methylheptan-3-ol forms as a racemate.
- SN1 hydrolysis of (S)-1-(4-methoxyphenyl)ethyl chlorideDraw the SN1 hydrolysis of (S)-1-(4-methoxyphenyl)ethyl chloride, including the methoxy resonance form, and explain the loss of optical activity.
- SN1 methanolysis of 1-bromo-1,4-dimethylcyclohexaneDraw the SN1 methanolysis of 1-bromo-1,4-dimethylcyclohexane and explain why both cis and trans ethers form.
- SN2 of cis-4-tert-butylcyclohexyl tosylate with acetateDraw acetate displacing the axial tosylate of cis-4-tert-butylcyclohexyl tosylate and explain why the ester is trans.
- SN2 of (S)-2-bromopentane with azide: (R)-2-azidopentaneDraw the backside attack of azide on (S)-2-bromopentane and show why the product is (R)-2-azidopentane.
- SN2 of trans-2-methylcyclohexyl mesylate with azideDraw azide displacing the mesylate of trans-2-methylcyclohexyl mesylate and explain why the azide is cis to the methyl.
- SN2 of cis-1-bromo-2-methylcyclopentane with cyanideDraw cyanide displacing bromide from cis-1-bromo-2-methylcyclopentane and explain why the nitrile is trans.
- Hydrosulfide displaces tosylate to give (S)-butane-2-thiolDraw the SN2 displacement of tosylate from (R)-butan-2-yl tosylate by hydrosulfide and assign the configuration of the thiol.
- Hydroxide converts (S)-2-bromooctane into (R)-octan-2-olDraw the backside attack of hydroxide on (S)-2-bromooctane and show that (R)-octan-2-ol forms with inversion of configuration.
- Iodide converts cis-1-bromo-4-methylcyclohexane into trans-1-iodo-4-methylcyclohexaneDraw the SN2 attack of iodide on cis-1-bromo-4-methylcyclohexane and explain why the product is the trans iodide.
- Finkelstein SN2 of (S)-2-bromohexane: (R)-2-iodohexaneDraw the Finkelstein SN2 reaction of (S)-2-bromohexane with iodide and assign the configuration of (R)-2-iodohexane.
- SN2 of (R)-2-iodobutane with methanethiolateDraw the SN2 attack of methanethiolate on (R)-2-iodobutane and assign the configuration of the sulfide formed.
Predict the product 5
- Rotating a Fischer projection by 90°Predict how an in-plane rotation of a Fischer projection changes the configuration.
- Inversion without a change of labelPredict the R/S label of an SN2 product when the nucleophile does not take the leaving group's priority rank.
- OsO₄ dihydroxylation of cis-2-butenePredict the stereoisomer formed by syn dihydroxylation of a cis alkene.
- Exchanging groups twicePredict the configuration after two exchanges of substituents on a stereocentre.
- Br2 adds to (E)-but-2-enePredict the stereoisomer formed by anti addition of bromine to an E alkene.
Classify 15
- Configuration of natural alanineAssign R or S to the stereocentre of an amino acid drawn with wedge bonds.
- cis- and trans-1,2-dimethylcyclohexaneDecide which ring diastereomer is meso and which is chiral by looking for an internal mirror plane.
- Which compound has a stereocentre?Identify a stereocentre by checking that a carbon carries four different groups, including ring paths.
- R or S from a Fischer projectionAssign R or S from a Fischer projection in which the hydrogen lies on a horizontal bond.
- Which compound is meso?Recognise a meso compound as one with stereocentres and an internal mirror plane, distinguishing it from chiral look-alikes.
- (2R,3S) versus (2S,3R)-2,3-dibromobutaneRecognise that inverting every centre of a meso compound returns the same compound, not an enantiomer.
- Find (2R,3S)-2-bromo-3-chlorobutaneMatch a two-centre name to its structure by assigning each stereocentre independently.
- (2R,3R) versus (2R,3S)Classify two stereoisomers that differ at only one of two stereocentres and state a consequence for their properties.
- (2R,3R) versus (2S,3S)Classify two stereoisomers whose every stereocentre is inverted as enantiomers.
- Configuration of a ring stereocentreAssign R or S to a ring stereocentre by exploring both ring branches until the first point of difference.
- The mirror image of L-threonineChoose the enantiomer of a two-centre amino acid by inverting every stereocentre.
- R or S: the stereocentre of spearmint (−)-carvoneAssign R or S to a ring stereocentre whose two ring branches differ only at the second atom out.
- Assign R or S to 2-bromobutaneAssign a CIP descriptor by ranking substituents and reading the direction with H away.
- Relationship between (2R,3R)- and (2R,3S)-2,3-dibromopentaneClassify a stereoisomer pair from its descriptors.
- (1R,2R)- versus (1S,2S)-1,2-dimethylcyclohexaneClassify two trans-1,2-disubstituted cyclohexanes whose descriptors are all inverted.
Compare 6
- Rank the conformations of butaneOrder the staggered and eclipsed Newman conformations of butane by energy and name the interaction responsible for each.
- Rank six carbon-based groups by CIP priorityRank alkyl, alkenyl, alkynyl and hydroxymethyl groups by exploring CIP priorities sphere by sphere with duplicated atoms.
- Rank five common substituents by CIP priorityRank heteroatom and carbon substituents by CIP priority using atomic number and duplicated atoms for C=O bonds.
- Which 1,3-dimethylcyclohexane can be diequatorial?Use chair geometry to decide which 1,3-disubstituted cyclohexane diastereomer can place both groups equatorial.
- What differs between enantiomers?Compare the physical properties of two enantiomers and identify the one that differs.
- Order four chloroalkanes by number of stereoisomersRank chloroalkanes by stereoisomer count, recognising when symmetry creates a meso form.
Calculate 10
- Stereoisomers of an open-chain aldohexoseApply the 2ⁿ rule to a chain whose ends differ, after locating every stereocentre.
- Count the stereoisomers of 2,3-dichlorobutaneCount the stereoisomers of a symmetric compound with two stereocentres, allowing for a meso form.
- Enantiomeric excess from optical rotationCalculate enantiomeric excess from the observed specific rotation of a partly resolved sample.
- Composition of an 80% ee sampleConvert an enantiomeric excess into the percentage of the major enantiomer.
- Equatorial methylcyclohexane at equilibriumConvert an axial–equatorial free-energy difference into an equilibrium percentage of the equatorial chair.
- Calculate a specific rotationCalculate a specific rotation from an observed rotation, concentration and path length in the correct units.
- Stereoisomers of 2,3,4-trihydroxyglutaric acidCount stereoisomers of a symmetric chain with a pseudoasymmetric middle carbon.
- Stereoisomers of 2,3-dichlorobutaneCount stereoisomers when symmetry creates a meso compound.
- ee after mixing 3.0 g of (R) with 1.0 g of (S)Calculate enantiomeric excess from the masses of two enantiomers mixed together.
- ee of a butan-2-ol sample from its rotationCalculate enantiomeric excess from observed and pure specific rotations.
Find the error 2
- Find the error in an R/S assignmentDiagnose an R/S assignment made with the lowest-priority group pointing toward the viewer.
- Is cis-1,2-dichlorocyclohexane optically active?Find the error in claiming a meso cyclohexane is optically active because one chair conformer is chiral.
Interpret the evidence 4
- Reading tartaric acid in a Fischer projectionInterpret a two-centre Fischer projection by looking for a horizontal mirror plane.
- Newman projection of meso-2,3-dibromobutaneInterpret a Newman projection of a meso compound to locate the remaining groups once two groups are placed anti.
- R or S from a Newman projection of 2-bromobutaneAssign R or S at the front carbon of a Newman projection, where the group pointing away is not the lowest priority.
- Anti-bromine conformer of (2R,3R)-2,3-dibromobutaneRelate a staggered Newman conformer to the configuration of a chiral versus meso diastereomer.
Work backward 2
- Which bromide gives (R)-2-azidobutane?Work backwards from an SN2 product to the enantiomer of the alkyl halide required, allowing for inversion.
- Which hexene gives racemic hexane-3,4-diol with OsO4?Work backwards from a racemic syn-dihydroxylation product to the alkene geometry it requires.
Plan a synthesis 2
- Resolve a racemic carboxylic acidPlan the resolution of a racemic acid by converting the enantiomers into separable diastereomeric salts.
- Convert (R)-butan-2-ol into (S)-2-bromobutaneChoose a one-step reagent that turns a chiral secondary alcohol into the alkyl bromide with clean inversion.
Explain why 2
- Why a racemate shows no rotationExplain why a racemic mixture of chiral molecules is optically inactive.
- Why SN1 on (R)-3-bromo-3-methylhexane racemisesExplain loss of configuration in an SN1 hydrolysis from the geometry of the carbocation.
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