Organic Chemistry 1 · Molecular shape and stereochemistry
Stereochemical relationships: practice problems
38 problems, each with its worked solution. Open one to read every step, or filter by tag.
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- Add HBr next to an existing stereocenter
- Br2 + (E)-oct-4-ene: anti addition to meso-4,5-dibromooctane
- Br2/H2O + (E)-but-2-ene: racemic 3-bromobutan-2-ol
- Br2 + (Z)-hex-3-ene gives an enantiomeric pair
- Br2 + (Z)-oct-4-ene: racemic 4,5-dibromooctane
- Anti E2 of (2S,3S)-2-bromo-3-phenylbutane
- Anti E2 turns diastereomers into E and Z alkenes
- H2/Pd on (S)-3-methylpent-1-ene: stereocenter lost
- H2/Pd on (Z)-3,4-dimethylhex-3-ene: meso alkane
Draw the mechanism 12
- Add HBr next to an existing stereocenterClassify the two products formed when a new stereocenter is created in a molecule that already contains one.
- Br2 + (E)-oct-4-ene: anti addition to meso-4,5-dibromooctaneDraw bromonium-ion formation from (E)-oct-4-ene and its anti opening by bromide, and explain why only the meso dibromide forms.
- Br2/H2O + (E)-but-2-ene: racemic 3-bromobutan-2-olDraw bromonium formation, anti opening by water and deprotonation for (E)-but-2-ene, and explain why the bromohydrin is chiral when the dibromide is meso.
- Br2 + (Z)-hex-3-ene gives an enantiomeric pairDraw bromonium-ion formation from (Z)-hex-3-ene and anti opening by bromide, and classify the stereoisomers formed.
- Br2 + (Z)-oct-4-ene: racemic 4,5-dibromooctaneDraw bromonium-ion formation from (Z)-oct-4-ene and anti opening by bromide, and relate the racemic product to the meso dibromide from the E alkene.
- Anti E2 of (2S,3S)-2-bromo-3-phenylbutaneDraw the anti-periplanar E2 of (2S,3S)-2-bromo-3-phenylbutane and predict the single alkene geometry it gives.
- Anti E2 turns diastereomers into E and Z alkenesDraw the anti-periplanar E2 elimination of HBr from meso-1,2-dibromo-1,2-diphenylethane and decide which alkene geometry each diastereomer gives.
- H2/Pd on (S)-3-methylpent-1-ene: stereocenter lostDraw hydrogen delivery to (S)-3-methylpent-1-ene and explain why the optically active alkene gives an optically inactive alkane.
- H2/Pd on (Z)-3,4-dimethylhex-3-ene: meso alkaneDraw syn delivery of H2 to (Z)-3,4-dimethylhex-3-ene on a metal surface and decide whether the alkane formed is meso or chiral.
- mCPBA + (Z)-hex-3-ene: meso cis-2,3-diethyloxiraneDraw the concerted mCPBA epoxidation of (Z)-hex-3-ene and explain why the cis epoxide is a single meso compound.
- OsO4 + (E)-pent-2-ene: the racemic threo diolDraw the concerted osmate-ester formation from (E)-pent-2-ene and classify the threo diol formed after hydrolysis.
- OsO4 on (Z)-pent-2-ene: the erythro diolDraw the concerted osmate-ester formation from (Z)-pent-2-ene and classify the diol formed after workup.
Predict the product 2
- Comparing a numbered two-center systemClassify partial inversion when molecular symmetry cannot identify the structures.
- NaBH4 reduction of (R)-3-hydroxybutan-2-onePredict the stereoisomeric products when a new stereocentre is created next to an existing one, and judge whether they form equally.
Classify 9
- Comparing two one-center projectionsRecognize an enantiomeric pair from explicit Fischer drawings.
- Two pictures of a cyclic moleculeAvoid counting ring-flipped chairs as distinct configurational stereoisomers.
- Which 1,3-dimethylcyclohexane is chiral?Decide which disubstituted cyclohexane isomer is chiral by looking for an internal mirror plane in the cis and trans 1,3 isomers.
- Chiral without a stereocentreRecognise axial chirality in a 1,3-disubstituted allene that has no stereocentre.
- (2R,3S)- vs (2S,3S)-2,3-dibromopentaneClassify two stereoisomers of 2,3-dibromopentane by comparing their R/S descriptors centre by centre.
- What makes two compounds epimers?Define epimers as diastereomers that differ at exactly one stereocentre.
- Pick the meso compoundIdentify a meso compound by finding an internal mirror plane in a molecule with stereocentres.
- Anti and gauche Newman projections of 2,3-dibromobutaneTell conformers of one compound apart from configurational stereoisomers by checking whether single-bond rotation interconverts them.
- Find the enantiomer of (2R,3R)-2,3-dibromopentanePick the enantiomer of a two-stereocentre compound by inverting every stereocentre.
Compare 3
- Changing one of two centersDistinguish a meso form from a nonsymmetric member of an enantiomeric pair.
- Which property differs between enantiomers?Distinguish properties that enantiomers share from those that differ.
- Rank compounds by number of stereoisomersRank five compounds by stereoisomer count, applying 2ⁿ and subtracting duplicates caused by meso symmetry.
Calculate 3
- Counting configurations with symmetryCorrect the maximum 2ⁿ count for a meso structure.
- Stereoisomers of 1,2-dimethylcyclohexaneCount stereoisomers of a disubstituted ring when one isomer is meso.
- Stereoisomers of pentane-2,3,4-triolCount stereoisomers of a symmetric chain with a pseudoasymmetric middle carbon, correcting 2ⁿ for meso forms.
Find the error 3
- Testing a molecule with two centersUse superposability rather than stereocenter count to determine molecular chirality.
- Inferring a relationship from physical dataRecognize the limits of stereochemical classification from ordinary property differences.
- Spot the error: meso-2,3-dibromobutane labelsDiagnose why two descriptors of a symmetric molecule that look like mirror images actually name one meso compound.
Interpret the evidence 2
- Compare two orientations of a projectionEstablish identical connectivity and superposability despite a different page orientation.
- Specific rotation of (2S,3S)-tartaric acidUse the enantiomer relationship to predict a specific rotation from the value of the mirror-image compound.
Work backward 1
- Which but-2-ene gives meso-2,3-dibromobutane?Work backward from a meso dibromide to the alkene geometry required by stereospecific anti addition of Br2.
Plan a synthesis 1
- Choosing a separation strategyExplain why an achiral physical measurement cannot generally resolve enantiomers.
Explain why 2
- An alkene without stereocentersRecognize geometric isomers as diastereomers despite the absence of tetrahedral centers.
- Why cis-1,2-dimethylcyclohexane is not optically activeExplain how a rapid ring flip between enantiomeric chairs makes cis-1,2-dimethylcyclohexane optically inactive.
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