Organic Chemistry : Help with Diastereomers

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

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Example Question #1 : Help With Diastereomers

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How are the given molecules related?

Possible Answers:

Identical

Enantiomers

Diastereomers

Structural isomers

They have different molecular formulas

Correct answer:

Identical

Explanation:

The given images are Fisher projections of the molecules. To compare them, we must mentally rotate the substituents around the carbon-carbon bond through the molecule's center, as well as consider flipping the projection end-over-end.

If the molecule to the left were flipped directly end-over-end, it will match the molecule to the right, with the methyl pointed upward in the front plane and the ethyl pointed downward in the rear plane. These molecules are identical.

Example Question #71 : Stereochemistry

Img 20150723 132858

Label each stereocenter in the molecule above as R or S.

Possible Answers:

1R, 3S

2R, 3R

2S, 3S

1S, 2S, 3S

Correct answer:

2S, 3S

Explanation:

Carbon 1 is not a stereocenter as both constituents on the carbon are identical. For carbon 2, the bromine is the attachment with the most priority, followed by carbon 3, then carbon 1. Because bromine is in the back, the stereocenter is designated as an S. For carbon 3, the alcohol group is the attachment with the most priority, followed by carbon 2, then carbon 4. Because alcohol is in the front, the stereocenter is designated as an S.

Example Question #72 : Stereochemistry

What is the stereochemical relationship between these molecules?

4

Possible Answers:

Diastereomers

Conformational isomers

Enantiomers

Identical compounds

Correct answer:

Diastereomers

Explanation:

The molecules shown contain three stereocenters as evidenced by the bonds of carbons 2, 3, and 4 to four unique groups. Rotating the molecule on the right 180 degrees in the horizontal plane reveals that only carbon 2 differs in absolute configuration (R/S). As a general rule, switching the absolute configurations of all stereocenters present in a compound yields its enantiomer. Switching the configuration at least one stereocenter, but not all, yields diastereomers, non-superimposable stereoisomers that are not mirror images. The two molecules shown are diastereomers. A molecular modeling kit can prove extremely useful in visualizing the difference in such situations.

 

Example Question #2 : Help With Diastereomers

Img 0635

Label each stereocenter in the given molecule as R or S.

Possible Answers:

1R, 2R

1S, 2S

1S, 2R

1R, 2S

Correct answer:

1S, 2S

Explanation:

To label each stereocenter as R or S, we must use the Cahn-Ingold-Prelog priority system.

Lets start with stereocenter 1: The  is given first priority because it is the most massive constituent. The carbon to the right is given second priority because it is closer to the alcohol group and the carbon to the left is given third priority. These atoms lay clockwise by priority. However, because the hydrogen (not depicted) is in the front, we know that the stereocenter should be labeled as S.

Stereocenter 2: The  is given first priority. The carbon on top is given second priority while the carbon on the bottom is given third priority. These atoms lay counterclockwise by priority. Because the hydrogen (not depicted) already faces toward the rear, we know that the steroecenter should be labeled as S.

Example Question #74 : Stereochemistry

Img 0663

How many stereoisomers does the given molecule, deoxyribofuranose, have?

Possible Answers:

Correct answer:

Explanation:

To find the number of stereo isomers in a given molecule, we must count the number of stereocenters first.

Img 0664 (indicated by red dots)

Then, use the formula  to get the number of stereoisomers ( being the number of stereocenters).

Example Question #3 : Help With Diastereomers

Which statement is not true regarding stereoisomers?

Possible Answers:

Corresponding stereoisomers have the similar physical properties except for the rotation of light

Diastereomers are easier to separate than enantiomers

Corresponding stereoisomers have the same biological properties

Corresponding stereoisomers have the same molecular formulas

Correct answer:

Corresponding stereoisomers have the same biological properties

Explanation:

Corresponding stereoisomers differ in biological properties. The inversion of just one stereocenter on a large, complex molecule can alter the biological properties of the entire molecule. All other statements are true.

Example Question #76 : Stereochemistry

Img 0709

Consider the pictured molecule. How many diastereomers does this molecule have (including the molecule itself)?

Possible Answers:

Correct answer:

Explanation:

To find the number of diastereomers, we must first find the number of stereocenters, then the number of stereoisomers. The stereocenters are the carbon atoms with the red dots next to them, we have three:

Img 0710

To find the number of stereoisomers, we use the formula , where  is the number of stereocenters. We have 8 stereoisomers. Finally, the definition of a diastereomer is a stereoisomer that is not an enantiomer. Optically active molecules have only one enantiomer, so that leaves us with 7 remaining diastereomers (including the original molecule itself).

Example Question #77 : Stereochemistry

How many stereoisomers exist for this molecule?

Screen shot 2015 10 24 at 10.13.06 am

Possible Answers:

16

32

2

4

8

Correct answer:

16

Explanation:

There are 4 chiral carbons on the molecule shown below:

Screen shot 2015 10 24 at 10.13.10 am

The number of stereoisomers for a given molecule = 2n where n equals the number of chiral centers (in this case 4). Also there are no internal planes of symmetry, so there is no possibility for meso compounds.

Example Question #4 : Help With Diastereomers

How many stereoisomers result from this reaction?

Screen shot 2015 10 24 at 10.13.16 am

Possible Answers:

2

4

3

0

1

Correct answer:

4

Explanation:

The following products are a mix of constitutional isomers and stereoisomers.

Screen shot 2015 10 24 at 10.13.20 am

Example Question #5 : Help With Diastereomers

How many stereoisomers are possible for methylcyclopropane?

Possible Answers:

Correct answer:

Explanation:

The correct answer is 2 stereoisomers. To determine the number of stereoisomers that a compound may be present in, the number of stereocenters must be determined. In the case of methylcyclopropane, we have only one stereocenter, which is the carbon in the ring that is bound to the methyl group. Once the number of stereocenters is determined, we can use the following formula to determine the number of stereoisomers:

Where  is equal to the number of stereocenters. Therefore,

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