EXAM II

***** Answer Key *****

Problem 1 (total 20 points, each question worth 2 points)

Which of the following statements are true (write "true or "false", as applicable, in front of the question on the same line):

  1. All compounds which have carbon atom with four different ligands are chiral.
  2. Enantiomers are stereoisomers of a chiral compound.
  3. Enantiomers of the same compound have different physical properties.
  4. Tereoisomers with different configurations are enantiomers.
  5. Diastereomers are compounds of the same constitution, but not related through the mirror reflection operation.
  6. Meso-compounds have chiral carbon atoms and are therefore chiral.
  7. Compounds which only have an axis of symmetry are always chiral.
  8. cis- and trans-isomers are diastereomers.
  9. All structures which are related to each other as an object to its mirror image are enantiomers.
  10. The d-prefix means the compound has the same configuration as d-glyceraldehyde.

ANSWER:

  1. False, because meso compounds which are achiral have chiral carbon atoms( for different ligands).
  2. True, by definition of enantiomers.
  3. False, the physical properties of enantiomers such as melting point, boiling point, reflective index ..etc are the same only fluroptical properties are different.
  4. False, for example, diastereomers have different configurations, yet they are not related by mirror plane reflection.
  5. True, by definition of diastereomers.
  6. False, meso compounds have a plane of symmetry and are hence achiral.
  7. True, only compounds which have a plane or center of symmetry are achiral; all others are chiral.
  8. True, cis/trans isomers are compounds of the same constitution, but are not related by mirror plane of reflection according to the definition of diastereomer.
  9. False, because to be enantiomers such structures must also be nonsuperimposable.
  10. False, the lower case of d-refers to the sign of optical rotation not configuration. Compounds which have the same optical rotation do not necessarily have the same configuration.

Problem 2 (total 20 points, 4 points each)

  1. List symmetry elements (n-fold axis, a plane, or a center of symmetry) of the structures shown below.
  2. Draw where approximately in the structure the element may be applied.
  3. State in what relation it is to the structure (e.g. in plane of the structure, vertical in plane of projection, horizontal perpendicular to the plane of projection, etc.).
  4. If no symmetry is detected, state "NONE", however, in such case identify the chiral center by an asterisk (*).

ANSWER:

Problem 3

Part I: Designate the RS configurations of each chiral centers in the structures shown below (12 points, 3 for each chiral center). Be sure to label each ligand of the chiral carbon atoms with the appropriate precedence numbers (from 1 to 4). If two chiral centers share the same ligand, redraw the structure to show separately precedence at each center.

ANSWER:

Part II: Draw the Fisher projection of 2S,3R-dibromobutane (10 pts).

ANSWER:

Problem 4

Part I: (12 points, 3 each) Define the relation of each of the the structures B through E to the structure A shown below as:

ANSWER:

  1. Determine configuration of each compound A to E.
  2. Compounds C and E have the same configuration. Thus C and E are ID.
  3. Compound B has both configurations opposite to those of A. Thus A and B are ENT.
  4. Compound D has one configuration opposite and one the same as A. Thus A and D are DIA.

Part II: (8 points total, 2 for each problem). Which of the four pairs of the structures shown below are:

ANSWER:

Problem 5

Define which of the conformations in each row shown below is more stable (18 points, 6 points for each problem).

ANSWER:

For Part I

For Part II