Author Question: Are the following structures chiral as drawn? When placed in solution at 298 K, which structures ... (Read 78 times)

WWatsford · **on:** Aug 23, 2018

Are the following structures chiral as drawn? When placed in solution at 298 K, which structures will show
an optical rotation? Explain.

Question 2

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Analyzing the planar structures reveals that compounds (b)

Question 3

Will the following compound show any optical activity if there is restricted rotation along the central C-C
bond? What will happen to the optical activity at elevated temperatures as the rotation becomes less restricted.

Question 4

Which of the following compounds are chiral? Which, if any, are meso? Which, if any, does not have apossible diastereomer?

Question 5

A long polymer chain, such as polyethylene (-CH2CH2-)n, can potentially exist in solution as a chiral object.
Give two examples of chiral structures that a polyethylene chain could adopt.

Question 6

One reason we can be sure that sp3-hybridized carbon atoms are tetrahedral is the number of stereoisomers
that can exist for different organic compounds.
(a) How many stereoisomers are possible for CHCl3, CH2Cl2, and CHClBrF if the four bonds to carbon have a
tetrahedral arrangement?

(b) How many stereoisomers would be possible for each of these compounds if the four bonds to the carbon had a square
planar geometry?

Question 7

Think about the helical coil of a telephone cord or a spiral binding and suppose that you view the spiral from
one end and find that it is a left-handed twist. If you view the same spiral from the other end, is it a right-handed or a lefthanded twist?

cadimas · **Reply #1 on:** Aug 23, 2018

Answer to Question 1

The above structures are chiral as drawn if the chair conformation shown has no plane or center of symmetry.
Therefore, structures (a), (b), (d), and (f) are chiral. Structure (c) has a plane of symmetry, and structure (e)
has a center of symmetry. To show optical rotation when placed in solution, the molecule must retain chirality
even when considering both of the equilibrating chair structures. The easiest way to identify the chiral
molecules is to examine the planar representations as shown below.

Answer to Question 2

(c)

Answer to Question 3

Atropisomers are stereoisomers that occur when rotation restriction around a central bond is sufficient to allow
isolation of individual conformers. The two atropisomers shown below do not have a plane of symmetry , so
pure samples of each would rotate the plane of plane polarized light.

Two different atropisomers that could be isolated at lower temperature
At higher temperature, as rotation becomes less restricted, the molecule becomes a meso compound because the
two chiral centers have identical substituents on them and possess R and S chirality, respectively (Section
3.4B). Therefore, as rotation of the central bond becomes less restricted, the optical rotation of either isolated
conformer will be reduced until it reaches a value of zero when full rotation is observed.

Answer to Question 4

To identify chiral molecules, look for planes or centers of symmetry. Compound (c) and (d) each have a plane
of symmetry, so only molecules (a) and (b) are chiral. Because (d) has no chiral centers, it cannot have
diastereomers. Molecules (a), (b), and (c) all have chiral centers (marked with asterisks ()), so they do h