Organic Chemistry : Help with Enantiomers

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #31 : Help With Enantiomers

What is the absolute configuration of the molecule shown?

Img 1227

Possible Answers:

1Z, 2S

R

This molecule is achiral

S

Correct answer:

S

Explanation:

The only stereocenter in this molecule is carbon number . The chlorine group is first priority, the carboxyl goup is second priority, the alkane is third priority, and the hydrogen (not drawn) is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a counterclockwise directionality, so the absolute configuration is S.

Example Question #32 : Help With Enantiomers

What is the absolute configuration of the molecule shown?

Img 1232

Possible Answers:

S

This molecule is achiral

R

2R, 3Z

Correct answer:

S

Explanation:

In the molecule shown, carbon  is the only stereocenter. The hydroxyl group is first priority, the alkyne group is second priority, the methyl group is third priority, and the hydrogen (not drawn) is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a counterclockwise directionality, so the absolute configuration is S.

Example Question #33 : Help With Enantiomers

What is the absolute configuration of the molecule shown?

Img 1234

Possible Answers:

R

S

This molecule is achiral

1S, 2R

Correct answer:

S

Explanation:

 In the molecule shown, carbon  is the only stereocenter. The amino group is first priority, the carboxyl group is second priority, the methyl group is third priority, and the hydrogen is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a counterclockwise directionality, so the absolute configuration is S.

Example Question #34 : Help With Enantiomers

What is the absolute configuration of the molecule shown?

Img 1235

Possible Answers:

R, Z

S, E

S, Z

R, E

Correct answer:

R, E

Explanation:

In the molecule shown are two stereocenters, carbon  and the alkene bond (double bond). For carbon , the hydroxyl group is first priority, the alkene group is second priority, the methyl group is third priority, and the hydrogen (not drawn) is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a clockwise directionality, so the absolute configuration is R.

For the alkene bond, the higher priority substituents are on opposite sides of the double bond. The one-letter designation for this type of geometric arrangement of substituents across a double bond is E.

Example Question #37 : Isomers

What is the absolute configuration of the molecule shown?

Img 1236

Possible Answers:

R

3R, 4S

S

This molecule is achiral

Correct answer:

This molecule is achiral

Explanation:

This molecule has no stereocenters (usually identified as a carbon atom attached to four different substituents. Therefore, the molecule is described as achiral.

Example Question #38 : Isomers

What is the absolute configuration of the molecule shown?

Img 1239

Possible Answers:

This molecule is achiral

S

2R, 3S

R

Correct answer:

R

Explanation:

In the molecule shown, carbon  is the only stereocenter. The hydroxyl group is first priority, the aldehyde group (on top of the molecule) is second priority, the rest of the molecule (the group) is third priority, and the hydrogen is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a clockwise directionality, so the absolute configuration is R.

Example Question #63 : Stereochemistry

What is the absolute configuration of the molecule shown?

Img 1228

Possible Answers:

1R, 3S

1R, 3R

1S, 3S

1S, 3R

Correct answer:

1S, 3S

Explanation:

This molecule contains two stereocenters, at carbon number  (containing the hydroxyl group), and carbon number  (containing the methyl group). 

For carbon , the hydroxyl group is first priority, the alkane group going across the top of the molecule is second priority, the alkane group going down the left side of the molecule is third priority, and the hydrogen (not drawn) is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a counterclockwise directionality, so the absolute configuration is S.

For carbon  , the alkane group moving across the top of the molecule toward the hydroxyl group is first priority, the alkane group going down the right side of the molecule is second priority, the methyl group is third priority, and the hydrogen (not drawn) is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a counterclockwise directionality, so the absolute configuration is S.

Example Question #37 : Help With Enantiomers

What is the absolute configuration of the molecule shown?

Img 1230

Possible Answers:

This molecule is achiral

S

R

2E, 3S

Correct answer:

S

Explanation:

In the molecule shown, carbon number  (numbering with the alkene terminal carbon as number ), is the only stereocenter. The alkene group is first priority, the ethyl group is second priority, the methyl group is third priority, and the hydrogen (not drawn) is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a counterclockwise directionality, so the absolute configuration is S.

Example Question #38 : Help With Enantiomers

What is the absolute configuration of the molecule shown?

Img 1231

Possible Answers:

S

This molecule is achiral.

1R, 2S

R

Correct answer:

R

Explanation:

In the molecule shown, carbon  is the only stereocenter. The fluorine group is first priority, the alkyl branch containing the amino group is second priority, the methyl group is third priority, and the hydrogen (not drawn) is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a clockwise directionality, so the absolute configuration is R.

Example Question #43 : Isomers

What is the absolute configuration of the molecule shown?

Img 1233

Possible Answers:

1S, 2S

2R, 3R

2R, 3S

2S, 3R

Correct answer:

2R, 3S

Explanation:

For the top chiral carbon (carbon number ), The hydroxyl group is first priority, the cyanide group is second priority, the branch forming the rest of the backbone is third priority, and the hydrogen is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a clockwise directionality, so the absolute configuration is R.

For the bottom chiral carbon (carbon number ), the hydroxyl group is first priority, the branch forming the top of the backbone is second priority, the  group is third priority, and the hydrogen is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a counterclockwise directionality, so the absolute configuration is S.

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