Stereochemistry

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Organic Chemistry › Stereochemistry

Questions 1 - 10
1

Which statement is not true regarding stereoisomers?

Corresponding stereoisomers have the same biological properties

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

Corresponding stereoisomers have the same molecular formulas

Diastereomers are easier to separate than enantiomers

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.

2

S or r

How many configurational stereoisomers exist for this structure?

3

4

2

1

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.

3

Screen shot 2015 07 07 at 5.56.34 pm

Which of these molecules is a meso compound?

III only

I only

II only

IV only

II and III

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

4

Which statement is not true regarding stereoisomers?

Corresponding stereoisomers have the same biological properties

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

Corresponding stereoisomers have the same molecular formulas

Diastereomers are easier to separate than enantiomers

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.

5

S or r

How many configurational stereoisomers exist for this structure?

3

4

2

1

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.

6

Screen shot 2015 07 07 at 5.56.34 pm

Which of these molecules is a meso compound?

III only

I only

II only

IV only

II and III

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

7

S or r

How many configurational stereoisomers exist for this structure?

3

4

2

1

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.

8

Screen shot 2015 07 07 at 5.56.34 pm

Which of these molecules is a meso compound?

III only

I only

II only

IV only

II and III

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

9

Which statement is not true regarding stereoisomers?

Corresponding stereoisomers have the same biological properties

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

Corresponding stereoisomers have the same molecular formulas

Diastereomers are easier to separate than enantiomers

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.

10

If a given molecule has 5 chiral centers, what is the maximum number of stereoisomers that exist for the molecule.

Explanation

If a molecule has n sterocenters, it has up to possible steroisomers. Thus, the given molecule has or possible stereoisomers. Keep in mind the key word here is maximum, where the molecule may have less than 32 steroisomers, for instance when discussing meso compounds.

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