Organic Chemistry : Help with Enantiomers

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #381 : Organic Chemistry

Isomers that are nonsuperimposeable mirror images of each other are called __________.

Possible Answers:

enantiomers

constitutional isomers

conformers

diastereomers

Correct answer:

enantiomers

Explanation:

Enantiomers are chiral isomers of the same molecule that are mirror images of one another. Because of this characteristic, enantiomers cannot be placed on top of one another (superimposed) and yield the same molecule.

Example Question #11 : Isomers

Cholic acid

How many stereocenters does the molecular framework of cholic acid (shown) have?

Possible Answers:

Correct answer:

Explanation:

Cholic acid 3d

There are 11 stereocenters, because here there are 11 asymmetric carbons and no E/Z isomerisms, nor planes of symmetry.

Example Question #31 : Stereochemistry

Cholic acid

How many possible stereoisomers does the molecular framework of cholic acid (shown) have?

Possible Answers:

Correct answer:

Explanation:

Cholic acid 3d

There are 11 stereocenters, because here there are 11 asymmetric carbons and no E/Z isomerisms, nor planes of symmetry. For compounds with no meso isomers or E/Z isomerisms, the possible number of stereoisomers is  where  is the number of stereocenters.

Example Question #31 : Stereochemistry

Cholic acid derivative

How many stereocenters does this steroid derivative have?

Possible Answers:

Correct answer:

Explanation:

There are 11 asymmetric carbons and one E double bond, so there are 13 stereocenters in total.  For each E/Z isomerism, there are 2 stereocenters. 

Cholic acid derivative 3d

Example Question #11 : Help With Enantiomers

Cholic acid derivative

How many possible stereoisomers does the following steroid derivative have?

Possible Answers:

Correct answer:

Explanation:

Here there are 11 asymmetric carbons and one E/Z isomerism.

So, the number of possible stereocenters from asymmetric carbons is , and the number of possible stereoisomers from E/Z isomerisms is .

Thus the number of stereoisomers from asymmetric carbons is doubled, and   possible stereoisomers

Cholic acid derivative 3d

Example Question #11 : Isomers

What is the IUPAC name for the given organic molecule?

Screen shot 2015 09 21 at 1.19.58 pm

Possible Answers:

(3S, 5R)-2,2,3-trimethylhexan-5-ol

(2S, 4R)-4,5,5-trimethylhexan-2-ol

(3R, 5R)-2,2,3-trimethylhexan-5-ol

(2R, 4R)-4,5,5-trimethylhexan-2-ol

(2R, 4S)-4,5,5-trimethylhexan-2-ol

Correct answer:

(2R, 4S)-4,5,5-trimethylhexan-2-ol

Explanation:

The absolute configuration around carbons 2 and 4 is R and S, respectively. The molecule is a hexanol, and the numbering starts with the carbon closest to the hydroxyl group.

Example Question #21 : Isomers

Which of the following structures is equivalent to the enantiomer of the molecule whose Fischer Projection is shown below?

Screen shot 2015 09 21 at 2.08.29 pm

Screen shot 2015 09 21 at 2.20.32 pm

Possible Answers:

III

II

IV

V

I

Correct answer:

IV

Explanation:

The compound shown is SS, so we need to look for the compound that is RR. 

Example Question #22 : Isomers

What is the Fischer projection of the following molecule?

Screen shot 2015 11 09 at 3.48.42 pm

Screen shot 2015 11 09 at 3.59.36 pm

Possible Answers:

I

III

IV

II

Correct answer:

I

Explanation:

The absolute configuration of the given molecule is   (amino group is R, methyl group is S). Putting the methyl group on the top and the alcohol on the bottom, this puts the amino group on the left, and the methyl group on the right. Recall that in the Fischer projection, the vertical bonds are dashed and are going into the plane of the screen/paper, while the horizontal bonds are wedges and are coming out of the plane of the screen/paper. One way to remember this is to think of Fischer projections as a skeleton (dashed vertical) wearing a bowtie (wedged horizontal).

Example Question #22 : Isomers

What is the IUPAC name for the molecule shown?

Screen shot 2015 11 09 at 4.10.48 pm

Possible Answers:

R-2-methylbut-3-en-1-ol

S-2-methylbut-3-en-1-ol

R-3-methylbut-1-en-4-ol

S-3-methylbut-1-en-4-ol

Correct answer:

S-2-methylbut-3-en-1-ol

Explanation:

The molecule's highest priority functional group is the  group, therefore, numbering starts at the left-most carbon. The absolute configuration of the molecule at carbon  is S.

Example Question #16 : Help With Enantiomers

What is the IUPAC name for the molecule shown?

Screen shot 2015 11 12 at 6.01.33 pm

Possible Answers:

(2R,3R)-3-methyl-2-pentanol

(2R,3S)-3-methyl-2-pentanol

(2S,3R)-3-methyl-2-pentanol

(2S,3S)-3-methyl-2-pentanol

Correct answer:

(2R,3S)-3-methyl-2-pentanol

Explanation:

Carbon number : hydroxyl is first priority, The rest of the molecule is second, methyl is third. Placing  in back makes  go clockwise. Therefore, it is R. Carbon number : alkyl with hydroxyl is first priority, The rest of the molecule is second, methyl is third. Placing  in back makes  go counterclockwise. Therefore, it is S.

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