discovering molecular models #2: stereochemistry: e and … · discovering molecular models #2 | 1...
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Discovering Molecular Models #2 | 1
Discovering Molecular Models #2: Stereochemistry: E and Z / R and S There are no additional tutorial or laboratory notes (you don’t need them!). Read and bring your course notes (end of organic unit #2 and all of organic unit #3), as they provide all of the background material necessary to perform these exercises and problems. Bring Your Molecular Model Kits! There will be a quiz at the end of this exercise. If you are performing this in the lab, the mark will count towards your lab component of the course. Likewise, if you are performing this in the tutorial room, it will count towards the tutorial component. A. Familiarization with Molecular Models 1. Build molecular models of each of the two molecules in each pair illustrated below. For
each pair, compare the two models by trying to overlap them. Indicate if they isomers of each other or if they are the same molecule?
and
and
and
and
and
and
and and
and and
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2. Repeat the exercise below, which is from the lecture notes, to be sure you understand the outcome. For each pair of molecules that are mirror images of each other, are they the same or different (can you overlap the two models at every color… yes or no)?
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3. Next, we will look at converting different representations of molecules to those on the previous page, so that they can be manipulated to determine the stereochemistry. Convince yourself that all six of the representations below are of the same molecule.
Br
CH3FH
F
Br HCH3 H
H3C
FBr
CH3C Br CF CH3
Br
H
CH Br
CH3
F
F
H
Hint: assign each group on the chiral carbon a different colour and build the corresponding models. For example:
Br
CH3FH
Br
CH3FH
CH3C F
H
Br
CH3C F
H
Br
C
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Note: Fisher projections. These molecules could also have been drawn as Fisher Projections, where the molecule is drawn in the form of a cross, with the central carbon atom being the point of intersection. The horizontal lines signify bonds directed out towards you (wedges), while the vertical lines indicate bonds pointing away (dashes).
CH3C Br
F
H
CF CH3
Br
H
CH Br
CH3
F
FH3C Br
H
BrF CH3
H
CH3
H Br
F Exercise: For each of the two molecules below, describe them as being the same as those above, or different. If different, what is the relationship to the others (constitutional isomer or stereoisomer)?
HF CH3
Br CH3
HBrF
4. Assign each of the stereocentres in the molecules in question 3 as either R or S. (Assign
priorities 1, 2, 3, and 4, and use your models. Recall that you first have to build the model that is an accurate representation of the molecule. Second, assign the group priorities and then label appropriately colored ball. Third, rotate the model such that the lowest priority group is pointing away from you. Fourth, see which way the order of groups 1, 2 and 3 are: counterclockwise = S, clockwise = R).
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5. This is a more complex example, where there are two or more stereoisomers. Build the two models that are representations of molecules A and B below (shown in three different representations). Are A and B the same molecule or different? How about A and C?
CH3
Br H
CH3
H Cl
Br
ClH3CH
CH3H
CH3
CBr HC
CH3
H Cl
CH3
H Br
CH3
Cl H
H3C
ClBrH
HCH3
CH3
CH Br
C
CH3
Cl H
A
B
H
CH3BrH3C
HCl
C
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6. The molecules in question 5 each have two stereocentres. Each stereo centre can be either R or S, so the possible molecules could be R, R; S, S; R, S; or S, R (four combinations). How does one use models to assign R and S to larger more complex molecules? Treat each stereo centre separately, and the group containing the other stereo centre is simply regarded as a substituent. Assign R/S to each chiral carbon in molecules A, B, and C in #5.
CH3
Br H
CH3
H Cl
CH3
CBr HC
CH3
H Cl
CH3
H Br
CH3
Cl H
A
B
**
do top carbon firstand assign the priorities
treating the bottomcarbon as a single goup CH3
CBr HC
CH3
H Cl
1
2
3
4
then do bottom carbonfirst and assign the
prioritiestreating the top carbon as
a single goup
CH3
CBr HC
CH3
H Cl
R
CH3
CBr H
C
CH3
H Cl1
2
34 R
H
CH3BrH3C
HCl
C
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B. Practice Problems. Use your models as necessary! 7. Each of the following has the molecular formula C5H12O. Which ones are chiral?
OH
OH
OH
OH
OH
OH
8. Rank the substituents in each set in order of priority, from highest to lowest.
a. –Cl, –OH, –SH, –H
b. –H, –OH, –CHO (aldehyde), –CH3
c. –H, –N(CH3)2, –OCH3, –CH3
d. –CH3, –CH2Br, –CHCl2, –‐CH2OH
e. –CH(CH3)2, –‐C(CH3)3, –CH3, –CH=CH2
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9. Assign the configurations at the double bond of these compounds as E, Z, or neither.
CH2BrF
BrO
OH
HOCl CH2Br
FBr
CH3
CH2F
(a) (b) (c)
(d) (e) (f)
F3C CH2Cl
OCH3HO
(g) (h) (i)
O
F
HO
H2N
OH
OH
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10. Assign R/S to each of the stereo centres in the following molecules.
Cl CH3
HO H
HO H
H
CH3
ClF
CH2BrHO CH3
CHF2 HO CH3
Br
O OH
H
Br CH3
H
HH3C
H3CH2C
ClH3C
O
CH3
(a) (b)
(c) (d)
(e) (f)
(g) (h)
(i) (j)
H3CO HOH
H3CH2C
H
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11. Label the stereocentres in each of the following molecules. (Some of these are from your course notes). For each, what is the maximum number of stereoisomers possible?
HO
CCH
Onorethindrone
(an oral contraceptive)
N
S
O
OH
OHNO
HONH2
amoxicillin(an antibiotic)
O
O NH
HOOH
OH
OH
OOH
pancratistatin(an anti-cancer drug)
CH3
CH3
H3C
HO
O
Vitamin E
farnesene(found in the waxing
coating of apples)
HOH
Vitamin D2
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12. Assign R/S to each stereocentre in the following molecules.
CH3
H Cl
CH3
ClH
HBrH3C
FH3C
H
H3NNH
O
OCH3
HO
HO
O
NHCH3
HO H
H3C H
ephedrine(brochodilator) aspartame
(artificial sweetener)
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13. How are the compounds in each pair related? Are they identical, enantiomers, constitutional isomers, or not isomers of each other?
BrH2C CH3CH2OH
O OH
Cl
Br CH3
H
HH3C
Br
ICl
H
I
BrH
ClCH3
F3C CH3Br
CH3H3C CF3
Br
Br
BrCH3
BrH2C
HOH2CH
H
O
OH
H
H CH3
Br
ClH3C
OCH3 OH
CH3
H CH2CH3
OH
H
H CH3
CH3
HHO