Stereoisomerism: Quiz

Stereoisomerism: Quiz

11 questions

1. Question 1EasyButane and methylpropane are:

   Acis–trans isomersBenantiomersCstructural isomersDthe same compound
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   Answer: structural isomers

   Both are C₄H₁₀ but the carbon atoms are joined differently (straight chain versus branched).

2. Question 2EasyWhich compound shows cis–trans isomerism?

   APropene, CH₂=CHCH₃BBut-2-ene, CH₃CH=CHCH₃CEthene, CH₂=CH₂DBut-1-ene, CH₂=CHCH₂CH₃
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   Answer: But-2-ene, CH₃CH=CHCH₃

   Each carbon of the double bond in but-2-ene carries two different groups (H and CH₃). The others have a carbon with two H atoms.

3. Question 3EasyWhy do cis–trans isomers exist?

   ASingle bonds cannot rotateBThe π bond in C=C prevents rotationCThe molecules have different formulasDCarbon has four bonds
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   Answer: The π bond in C=C prevents rotation

   Rotating about a double bond would break the side-on overlap of the π bond, so the groups are locked in place.

4. Question 4MediumWhich molecule contains a chiral centre?

   ACH₃CH₂OHBCH₃CH(OH)CH₂CH₃CCH₂Cl₂D(CH₃)₂CHOH
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   Answer: CH₃CH(OH)CH₂CH₃

   In butan-2-ol, carbon 2 has four different groups (H, OH, CH₃, C₂H₅). In propan-2-ol the central carbon has two identical CH₃ groups.

5. Question 5MediumIn CHCl=CHBr, the Cl and Br are on opposite sides of the double bond. Which isomer is it?

   AZBECNeither: it has no stereoisomersDOptical isomer
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   Answer: E

   On each carbon the halogen outranks H. The two higher-priority atoms (Cl and Br) are on opposite sides: E.

6. Question 6MediumWhich property is different for two enantiomers?

   ABoiling pointBMolar massCDirection of rotation of plane-polarized lightDDensity
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   Answer: Direction of rotation of plane-polarized light

   Enantiomers have identical physical properties except that they rotate plane-polarized light in opposite directions (and interact differently with other chiral molecules).

7. Question 7HardWhy does a racemic mixture not rotate plane-polarized light?

   AIt contains no chiral moleculesBEqual amounts of the two enantiomers rotate the light equally in opposite directionsCIt is not a solutionDRacemic molecules are planar
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   Answer: Equal amounts of the two enantiomers rotate the light equally in opposite directions

   The two equal and opposite rotations cancel out.

8. Question 8EasyWhich group has the highest CIP priority?

   A–CH₃B–CH₂CH₃C–OHD–NH₂
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   Answer: –OH

   Compare the attached atoms: O (8) beats N (7), which beats C (6). So –OH is highest.

9. Question 9MediumWhich group ranks higher: –CH₂OH or –CH(CH₃)₂?

   A–CH(CH₃)₂, because it has more carbon atomsB–CH₂OH, because O beats C at the first point of differenceCThey have equal priorityDIt cannot be decided
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   Answer: –CH₂OH, because O beats C at the first point of difference

   Both start with C. Compare the attached sets: (O, H, H) versus (C, C, H). The first atoms differ, O against C, so –CH₂OH wins. Atoms are not added up.

10. Question 10MediumGroup 4 points away from you, and the path 1 → 2 → 3 runs anticlockwise. What is the configuration?

    ARBSCEDZ
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    Answer: S

    With the lowest-priority group at the back, anticlockwise means S (sinister).

11. Question 11HardGroup 4 is on a solid wedge (towards you), and 1 → 2 → 3 runs clockwise as drawn. What is the configuration?

    ARBSCIt cannot be assignedDRacemic
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    Answer: S

    When group 4 points towards you, the direction you see must be reversed: clockwise as drawn means S.