Organic Chemistry 1 · Molecular shape and stereochemistry
Stereocenters and configuration: practice problems
43 problems, each with its worked solution. Open one to read every step, or filter by tag.
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- Assign R/S through activation and displacement
- Invert a ring carbon: cis becomes trans
- E2 of (2R,3R)-2-bromo-3-methylpentane gives the E alkene
- Base closes (2S,3R)-3-bromobutan-2-ol to a trans epoxide
- Methoxide opens (R)-2-methyloxirane: R is kept
- HBr adds next to an existing stereocenter
- NaBH4 reduction makes a new stereocenter
- SN1 hydrolysis of (R)-3-chloro-3-methylhexane racemizes
- SN2 on (S)-2-bromobutane: track R/S
Draw the mechanism 13
- Assign R/S through activation and displacementAssign the descriptor at each stage of an alcohol → tosylate → nitrile sequence, separating a retained C–O bond from backside C–O cleavage.
- Invert a ring carbon: cis becomes transTranslate backside substitution at a ring carbon into a change of cis/trans configuration when no R/S descriptor applies.
- E2 of (2R,3R)-2-bromo-3-methylpentane gives the E alkeneDraw the anti-periplanar E2 elimination of a diastereomerically pure bromide and assign E/Z to the single alkene isomer it forms.
- Base closes (2S,3R)-3-bromobutan-2-ol to a trans epoxideDraw the base-promoted ring closure of a bromohydrin and use the anti-periplanar requirement to assign the trans epoxide and its (2S,3S) configuration.
- Methoxide opens (R)-2-methyloxirane: R is keptDraw the methoxide opening of an enantiopure epoxide at its less hindered carbon and explain why the stereocenter keeps its R configuration.
- HBr adds next to an existing stereocenterDraw protonation of (S)-4-phenylpent-1-ene and bromide capture of the secondary cation, and decide how the products formed at the new C2 center are related.
- NaBH4 reduction makes a new stereocenterDraw hydride addition from borohydride to 2-methylcyclohexanone and protonation of the alkoxide, and explain why two diastereomeric alcohols form.
- SN1 hydrolysis of (R)-3-chloro-3-methylhexane racemizesDraw the SN1 hydrolysis of an enantiopure tertiary chloride and explain why the planar carbocation gives both enantiomers of the alcohol.
- SN2 on (S)-2-bromobutane: track R/SDraw the SN2 displacement of bromide from (S)-2-bromobutane by hydrosulfide and assign the configuration of the thiol product.
- SN2 of (S)-2-bromopentane with cyanide: R/S after inversionDraw the SN2 displacement of bromide by cyanide on (S)-2-bromopentane, and assign R/S to the inverted nitrile, (R)-2-methylpentanenitrile.
- SN2 inverts yet stays S: cyanide on a methoxy tosylateDraw the SN2 displacement of a secondary tosylate by cyanide and show why inversion leaves the S label unchanged.
- SN2 on trans-1-bromo-2-methylcyclohexane gives cisDraw the SN2 attack of methanethiolate on a trans ring bromide and explain why only one of the two stereocenters inverts, turning trans into cis.
- Tosylation of (R)-1-phenylethanol keeps RDraw the tosylation of a chiral secondary alcohol and explain why the stereocenter keeps its configuration and its R label.
Predict the product 3
- Assigning a substituted alkeneRank substituents separately at the two alkene carbons.
- Two swaps in a Fischer projectionPredict how exchanging substituents in a Fischer projection changes the configuration.
- R or S from a Newman projectionAssign R/S to a stereocentre shown as the front carbon of a Newman projection.
Classify 8
- Testing an alcohol carbonIdentify a tetrahedral stereocenter by comparing its four ligands.
- Comparing two directions around a ringRecognize identical cyclic ligands in an unsubstituted ring.
- E or Z for 3-methylpent-2-eneAssign E/Z to a trisubstituted alkene by ranking substituents on each alkene carbon.
- Assign both centers of L-threonineAssign R or S at both stereocenters of an amino acid and tell it apart from its diastereomers.
- Name with stereodescriptorsWrite a full name that combines R/S and E/Z descriptors in the correct format.
- Pick (R)-butan-2-olIdentify the R enantiomer of a secondary alcohol from wedge-dash drawings.
- Pick (S)-ibuprofen, the active enantiomerIdentify the S enantiomer of a chiral drug from wedge-dash structures by ranking its four groups.
- E or Z for 2-bromobut-2-ene with trans methylsAssign E/Z to an alkene where the cis/trans relationship of like groups points to the wrong answer.
Compare 4
- Ranking isotopically labeled ligandsApply isotope priority without confusing mass with atomic number.
- Rank ethynyl, tert-butyl, vinyl and isopropyl by CIPRank carbon substituents that tie at the first atom by exploring further spheres and counting multiple bonds as duplicate atoms.
- Rank six groups by CIP priorityRank substituents by CIP priority using atomic number, first point of difference and duplicate atoms.
- Which can be resolved: a sulfoxide or an amine?Compare pyramidal inversion at nitrogen and sulfur to decide which heteroatom stereocenter gives isolable enantiomers.
Calculate 2
- Count the stereocenters in cholesterolCount the stereocenters of a polycyclic natural product, including ring-fusion carbons without hydrogen and a side-chain carbon.
- Count stereocentres in mentholCount stereocentres by finding carbons with four different substituents, including ring carbons.
Find the error 3
- Repairing a projection argumentPredict the effect of exchanging exactly two ligands at one center.
- Checking whether E/Z is definedIdentify the structural prerequisite missing from a terminal alkene.
- Fixing an R/S call made with H on a wedgeDiagnose an R/S assignment that ignored a lowest-priority group pointing toward the viewer.
Interpret the evidence 3
- Assigning the drawn acidUse the Fischer viewing convention to obtain an absolute descriptor.
- R or S for L-alanine from its Fischer projectionAssign R or S to an amino acid drawn as a Fischer projection, correcting the trace when the lowest-priority group is on a horizontal line.
- Name a tartaric acid isomer from its Fischer projectionAssign both stereocenters of a two-center molecule from a Fischer projection and name the stereoisomer.
Work backward 1
- Draw (R)-carvone, the spearmint enantiomerChoose the structure that matches a given name and descriptor by ranking ring branches at a ring stereocenter.
Plan a synthesis 2
- Choosing a legal Fischer manipulationPreserve configuration while reorienting a projection.
- Order the steps for assigning R or SSequence the steps of the CIP procedure for assigning R or S to a stereocenter.
Explain why 4
- First-point-of-difference comparisonUse duplicate-atom bookkeeping to compare three carbon ligands.
- A descriptor after rotating the moleculeDistinguish a physical rotation in space from ligand exchange or a changed projection convention.
- Why L-cysteine is R while other L amino acids are SExplain how a heavy atom one bond away from the stereocenter changes the CIP ranking and the R/S label of an amino acid.
- Why can't this amine be resolved?Explain why tertiary amines with three different groups are not isolable as enantiomers.
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