Organic Chemistry : Help with Meso Compounds

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #1 : Help With Meso Compounds

Which of the following are meso compounds?

I. 2,4-dichloropentane

II. 1,3-dimethylcyclopentane

III. 2,3-dichloropentane

Possible Answers:

II and III

I only

II only

I and III

I and II

Correct answer:

I and II

Explanation:

A meso compound has at least two stereocenters, but is not chiral due to an axis of symmetry. Each of the given molecules have two stereocenters. However, if you cut the first molecule in half, you would get two identical half molecules. If you cut the second molecule in half, the same would occur. Thus, I and II have meso stereoisomers. To solve this question, it is easiest to draw out the given molecules.

Example Question #2 : Help With Meso Compounds

Screen shot 2015 07 19 at 10.49.30 am

Which of the given chair conformations represents a meso compound?

Possible Answers:

II

None of these

I

IV

III

Correct answer:

III

Explanation:

Meso compounds are characterized by an internal plane of symmetry that renders them achiral despite the presence of chiral center(s). For the given six member rings, the key to identifying the meso compound is finding the structure in which the two chlorine atoms are on the same side of the ring. It is also crucial to recognize that six member rings undergo rapid chair-flipping. Identical substituents on the same side of the ring quickly alternate between equatorial and axial positions such that they are on average of the same orientation. Compound III is the only structure given in which the chlorine atoms are facing in the same direction (up in the given conformation). Although one is equatorial and the other is axial, observation of the corresponding Haworth projection (see below) shows that there is indeed an internal plane of symmetry. Thus, compound III is meso.

Screen shot 2015 07 19 at 10.49.43 am

Example Question #3 : Help With Meso Compounds

Screen shot 2015 07 07 at 5.56.34 pm

Which of these molecules is a meso compound?

Possible Answers:

II and III

II only

IV only

I only

III only

Correct answer:

III only

Explanation:

A molecule is meso if it contains at least two stereocenters, but is rendered optically inactive by internal structural symmetry. In other words, a meso compound may be split in half in some way such that portions on either side of an imaginary line are mirror images. Note: The absolute configurations of a meso compound with two stereocenters are opposite (R/S). The internal symmetry that makes molecule III a meso compound is best conveyed through a Haworth projection:

Screen shot 2015 07 07 at 6.23.10 pm

Example Question #4 : Help With Meso Compounds

How many possible stereoisomers does the product of the following reaction have?

Cl cl meso compound

Possible Answers:

Correct answer:

Explanation:

Cl cl meso answer

This is the product of the given reaction. Remember, anti elimination is favored over the Zaitsev product. All possible stereoisomers with methyl groups syn are meso compounds. Of the possible stereoisomers with methyl groups anti, there are two pairs of identical structures. Thus there are only 2 possible stereoisomers.

Example Question #1 : Help With Meso Compounds

S or r

How many of the existing configurational stereoisomers are chiral? 

Possible Answers:

3

1

2

None

Correct answer:

2

Explanation:

There are three configurational stereoisomers. These include the RS, SS, and RR isomers. Since one of them, the RS isomer, has a plane of symmetry, it is achiral, and the other two are chiral.

Example Question #2 : Help With Meso Compounds

S or r

How many configurational stereoisomers exist for this structure? 

Possible Answers:

4

2

1

3

Correct answer:

3

Explanation:

There are two tetrahedral asymmetrical stereocenters in this molecule (the carbon atoms attached to each of the chlorine atoms). Thus, the combinations of R and S include RR, RS, SR, and SS. Note the plane of symmetry in the molecule; RS and SR are the same molecule (meso compounds). Thus, there are three distinct configurational stereoisomers of this compound. 

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