Chemistry 255 Workshop Exercise #10 (11/11/08) Fall 2008 Prof ...

Chemistry 255 Workshop Exercise #10 (11/11/08) Fall 2008
Prof. Loyd Bastin
1. Find all of the symmetry operations (rotation axes, mirror planes, and inversion centers)
present in each of the following molecules. Also state whether the molecule is chiral or
achiral. Assume the molecules are “frozen” in the conformations shown.
(a)
(b)
Cl
Cl
Al
Cl
Cl
Al
Cl
Cl
(c)
(d)
Cl
Cl
B
B
Cl
Cl
(e)
(f)
O
Cl
H
Br
Br
H
Cl
2. Identify with an asterick (*) all the stereogenic carbons in brucine (shown below) and
designate each as either (R) or (S).
H 3
CO
H
N
H
H 3
CO
O
N
H
H
H
O

3. Dorothy Sayers, the famous mystery writer, wrote a novel called Documents in Case in
which a murder was solved by a chemist using a polarimeter. The protagonist in the
novel, a mushroom expert, was found slumped dead over a plate of poisonous Amanita
muscaria mushrooms that he had apparently just fixed for supper on the stove. However,
his son was convinced that Dad would never mistake the poisonous mushrooms for
edible ones. On returning to England from America, the son demanded that the police
reopen the case. He persuaded the authorities to exhume the body. They extracted the
muscarine poison from the corpse and placed a solution of it in a polarimeter. The
detective looked through the eyepiece and saw no deviation of the plane of polarization.
“Aha”, he declared, “your father was murdered!” How did the detective come to this
conclusion?
H 3 C O
+
CH 2 N(CH 3 ) 3
HO
Muscarine
4. Draw three-dimensional pictures of all the stereoisomers of 3,4-dimethylheptane and 3,5-
dimethylheptane. It is probably easiest to draw them in the eclipsed arrangement, even
though this is not a low-energy conformation. Determine the absolute configuration (R or
S) of each stereogenic carbon. Designate the stereogenic carbons with an asterisk.
Provide the relationship of each pair of molecules you drew.
5. Tartaric acid [HO 2 CCH(OH)CH(OH)CO 2 H] was an important compound in the history
of stereochemistry. Two naturally occurring forms of tartaric acid are optically inactive.
One form has a melting point of 206ºC, the other a melting point of 140ºC. The inactive
tartartic acid with a melting point of 206ºC can be separated into two optically active
forms of tartaric acid with the same melting point (170ºC). One optically active tartaric
acid has [α] D
25
= +12º, and the other, [α] D
25
= –12º. All attempts to separate the other
inactive tartaric acid (melting point 140ºC) into optically active compounds fail. (a)
Write the three-dimensional structure of the tartaric acid with the melting point of 140ºC.
(b) What are the possible structures for the optically active tartaric acids with melting
points of 170ºC? (c) Can you be sure which tartaric acid in (b) has a positive rotation and
which has a negative rotation? (d) What is the nature of the form of tartaric acid with a
melting point of 206ºC?