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Beauchamp
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16
2
Key ideas to know for stereochemistry topic
1. Can you recognize stereogenic centers? Can you distinguish between chiral centers, achiral centers that
are stereogenic? (possible in double bonds and possible in di- or poly-substituted rings with certain
types of substitution) and regular achiral centers that are not stereogenic.
2. Can you assign absolute configurations as R or S? (related ability…can you assign E or Z of an alkene
pi bond?)
3. Can you name a compound with R or S configurations and E or Z configurations?
Can you draw a structure from a name with R/S absolute configurations or E/Z configurations?
4. Can you draw Fischer projections…and their mirror images?
5. In a similar manner, can you analyze ring structures (with a Haworth formula) and their mirror images?
6. Can you draw all of the stereoisomers for a particular formula and identify the types of stereoisomers as
enantiomers, diastereomers and meso compounds?
Cover the answers (below) and see if you can determine how many stereoisomers there are and what the
relationships are (enantiomers, diastereomers or meso compounds).
I hope the following answers are correct, but I did not check them and I probably made some errors. If
something seems wrong to you, you may be right (and I may be wrong).
Z:\classes\314\314 Special Handouts\Stereochem topics key.doc
Beauchamp
3
Example 1
top groups (
) are the same as the bottom groups (
X
2,3-di X butane
X
X
X
X
X
X
meso
A
)
A*
X
enantiomers
B
enantiomers = BB*
diastereomers = AB, AB* (same as A*B, A*B*)
meso compounds = (A = A*)
molecules with no chiral centers = all stereoisomers have 2 chiral centers
B*
A plane of symmetry is present.
Expect some meso structures. Fewer
than 2n stereoisomers < (22 = 4).
Example 2
top groups (
) are the same as the bottom groups (
X
X
Y
Y
)
2-X-3-Y-butane
X
X
Y
enantiomers
A
A*
top groups (
) are different from the bottom groups (
X
X
X
X
enantiomers
enantiomers
B
enantiomers = AA*, BB*
diastereomers = AB, AB*, A*B, A*B*
meso compounds = None, no mirror planes of symmetry.
molecules with no chiral centers = all stereoisomers have 2 chiral centers
C
Y
)
B*
A plane of symmetry is not present.
Expect 2n stereoisomers = 22 = 4.
2,3-di X butane
and
2-X-3-Y-butane
X
X
X
C*
D
X
enantiomers
enantiomers = AA*, BB*
A plane of symmetry is not present.
diastereomers = AB, AB*, A*B, A*B*
Expect 2n stereoisomers = 22 = 4.
meso compounds = None, no mirror planes of symmetry.
molecules with no chiral centers = all stereoisomers have 2 chiral centers
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Beauchamp
D*
4
Example 3
top groups (
) are the same as the bottom groups (
) (trisubstituted 2,3,4-tri-X, or disubstituted 2,4-di-X (same as example 1) have
a plane of symmetry and meso structures are present, so fewer than 2n stereoisomers are present.
X
X
X
X
X
X
X
X
X
X
X
(0)
(1,2,3)
A
(1)
B
A*
X
X
X
X
X
X
X
X
X
X
X
X
X
(2,3)
(2)
(1,3)
(3)
(1,2)
B*
C
C*
D
D*
enantiomers = (B/B*) (D = B* are the same as D* = B after they are rotated 180o in the plane of the page)
diastereomers = (A and B,B*,C) (B and A,C) (B* and A,C) (C and A,B,B*)
Meso compounds: A = A*, C = C*
Deletion of the duplicates only leaves four stereoisomers = A, B, B* and C in this example.
X
Similar to example 1 (only 3 stereoiosmers present).
X
top groups (
X
Y
X
A
) are the same as the bottom groups (
X
X
X
Y
Y
X
X
meso #1
), BUT: 3-"Y"-2,4-di"X" (has a plane of symmetry in some stereoisomers)
A*
B
Y
X
X
X
Y
X
same as D/D*
X
Y
B*
C
meso #2
X
Y
Y
X
C*
X
D
enantiomers = (B/B*) (D = B* are the same as D* = B after they are rotated 180o in the plane of the page)
diastereomers = (A and B,B*,C) (B and A,C) (B* and A,C) (C and A,B,B*)
Meso compounds: A = A*, C = C*
Deletion of the duplicates only leaves four stereoisomers = A, B, B* and C in this example.
Z:\classes\314\314 Special Handouts\Stereochem topics key.doc
X
Beauchamp
same as B/B*
D*
5
Example 4
top groups (
X
) No plane of symmetry so full 23 = 8 stereoisomers are present.
) are different from the bottom groups (
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
(0)
(1,2,3)
(1)
(2,3)
(2)
(1,3)
(3)
(1,2)
E
E*
F
F*
G
G*
H
H*
enantiomers = (E/E*) (F/F*) (G/G*) (H/H*)
diastereomers = (E and F,F*,G,G*,H,H*) (E* and F,F*,G,G*,H,H*) (F and E,E*,G,G*,H,H*) (F* and E,E*,G,G*,H,H*)
(G and E,E*,F,F*,H,H*) (G* and E,E*,F,F*,H,H*) (H and E,E*,F,F*,G,G*) (H* and E,E*,F,F*,G,G*)
No meso compounds, since no mirror plane can cut a moleucle in half with different groups on the top and bottom halves.
With 3 chiral centers there are 23 = 8 stereoisomers possible and all are observed. There are 4 pairs of enantiomers.
Also similar are (2-Y,3,4-di-X), (3-Y,2,4-di-X), (4-Y,2,3-di-X), (any combination of trisubstituted X,Y,Z)
because none of these have a plane of symmetry.
top groups (
Y
X
X
), BUT: 2-"Y"-3,4-di"X" (no plane of symmetry, 23 = 8 stereoisomers)
) are the same as the bottom groups (
Y
Y
Y
X
X
X
X
X
X
A
A*
B
Y
Y
X
X
X
B*
X
C
Y
X
X
X
C*
X
D
X
Similar to example 2 (4 stereoiosmers present).
Y
Z:\classes\314\314 Special Handouts\Stereochem topics key.doc
Y
Beauchamp
D*
6
Example 5
Top groups (
) are the same as the bottom groups (
than 16 stereoisomers. There are 10 in this group.
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
(0)
(1,2,3,4)
(2,3,4)
B
B*
same as E/E*
X
X
X
X
X
X
X
X
X
X
X
(4)
X
X
X
(1,2)
(1,2,3)
same as B/B*
X
X
(1,3,4)
(3)
C*
D
same as D/D*
same as C/C*
A* = 1,2,3,4
X
X
X
X
X
X
X
(1,4)
(2,4)
G
X
G*
H
B=1
C=2
D=3
E=4
F = 1,2
G = 1,3
H = 1,4
B* = 2,3,4
C* = 1,3,4
D* = 1,2,4
E* = 1,2,3
F* = 3,4
G* = 2,4
H* = 2,3
Possible answers, assuming I didn't make any mistakes
Are any of these "meso"? (yes, A=A* and H=H*)
Are any of these identical with others after 180o rotation in the plane? (yes, B/B* = E/E* and C/C* = D/D*).
Not counting equivalent structures:
Enantiomers = B/B*, C/C*, F/F*, G/G*,
Meso structures = A, H,
Diastereopairs (many) = (A and B,B*,C,C*,F,F*,G,G*,H), (B and C,C*,F,F*,G,G*,H), (B* and C,C*,F,F*,G,G*,H),
(C and F,F*,G,G*,H), (C* and F,F*,G,G*,H), (F and G,G*,H), (F* and G,G*,H), GH, G*H
Deletion of duplicates only leaves ten stereoisomers = A, B, B*, C, C*, F, F*, G, G*, H
X
Y
Y
X
Similar
to above.
X
Z:\classes\314\314 Special Handouts\Stereochem topics key.doc
D*
X
X
X
X
(1,2,4)
"Group" movements - Each number indicates the position of a group that was moved to the left side.
A = none
X
X
X
(1,3)
X
X
C
X
X
X
(2)
X
X
X
X
(3,4)
F*
F
E*
X
X
(1)
A*
meso #1
X
E
X
X
X
A
), There is a plane of symmetry in some stereoisomers. There will be fewer
X
Similar to
Example 1
X
X
Beauchamp
meso #2
(2,3)
H*
7
Example 6
Top groups (
) are different from bottom groups (
), There is no plane of symmetry. Four chiral centers indicate 16 stereoisomers.
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
(0)
X
(1,2,3,4)
A
X
(1)
B
A*
X
(1,2,4)
D
D*
X
X
X
(1,2)
(1,2,3)
E
X
X
X
X
X
X
X
X
X
X
X
X
X
(1,4)
(2,4)
G
X
X
X
(1,3)
(3,4)
F*
F
E*
X
(3)
X
(4)
X
C*
X
X
X
C
X
X
X
B*
X
X
X
(1,3,4)
X
X
X
X
(2)
X
X
X
(2,3,4)
X
X
X
G*
(2,3)
H
H*
"Group" movements - Each number indicates the position of a group that was moved to the left side.
A = none
A* = 1,2,3,4
B=1
C=2
D=3
E=4
F = 1,2
G = 1,3
H = 1,4
B* = 2,3,4
C* = 1,3,4
D* = 1,2,4
E* = 1,2,3
F* = 3,4
G* = 2,4
H* = 2,3
Possible answers, assuming I didn't make any mistakes
Are any of these "meso"? (No, since no mirror plane can cut a molecule in half with different groups on the top and bottom halves.)
Are any of these identical with others after 180o rotation in the plane? (No rotation is possible when the top half if different from the
bottom half.)
Not counting equivalent structures:
Enantiomers = A/A*, B/B*, C/C*, D/D*, E/E*, F/F*, G/G*, H/H* (There are 8 pairs of enantiomers)
Diastereomer pairs (many) = (A and B,B*,C,C*,D,D*E,E*,F,F*,G,G*,H,H*), (A* is similar to A), (B and C,C*,D,D*,E,E*,F,F*,G,G*,H,H*),
(B* is similar to B), (C and D,D*,E,E*,F,F*,G,G*,H,H*), (C* is similar to C), (D and E,E*,F,F*,G,G*,H,H*), (D* is similar to D),
(E and F,F*,G,G*,H,H*), (E* is similar to E), (F and G,G*,H,H*), (F* is similar to F), (G and H,H*), (G* is similar to G)
There are 24 = 16 stereoisomers (the maximum number) = A, A*, B, B*, C, C*, D, D*, E, E*, F, F*, G, G*, H, H*
X
X
X
X
X
X
X
Y
X
Y
Y
X
Y
X
Y
Y
Z:\classes\314\314 Special Handouts\Stereochem topics key.doc
Similar
to above.
X
X
X
X
X
Similar to
Example 3
X
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8
Additional Examples - Determine how many stereoisomers there are in each of the following examples and determine what the
relationships are (enantiomers, diastereomers or meso compounds) among the stereoisomers.
time averaged
"flat" structure
=
rapid equilibrium between two chair conformations
Possible disubstituted cyclohexane structures (di "X"):
C
B
A
(1,1)
(1,1), (1,2), (1,3), (1.4)
maximum stereoisomers = 2n where n = # chiral centers,
watch out for "meso" structures with a plane of symmetry
and 2 or more chiral centers.
D
X
X
enantiomers: None. No chiral centers.
diastereomers: None.
meso: None
no chiral centers
(1,2)
A
B
D
C
X
X
X
X
X
X
X
meso
same as A
enantiomers: C/D
X
enantiomers
diastereomers: AC, AD (same as BC, BD)
Maximum number of stereoisomers = 22 = 4
meso: A = B
Fewer than this number because of meso compound.
no chiral centers: Each structure has two chiral centers.
A
(1,3)
X
X
D
C
B
X
X
X
X
meso
X
enantiomers
X
same as A
enantiomers: C/D
diastereomers: AC, AD (same as BC, BD)
Maximum number of stereoisomers = 22 = 4
meso: A = B
Fewer than this number because of meso compound.
no chiral centers: Each structure has two chiral centers.
(1,4)
A
X
No chiral centers,
achiral molecules
are identical.
X
C
B
X
No chiral centers,
achiral molecules
are identical.
X
X
D
X
X
enantiomers: No chiral centers.
same as A
(not meso)
X
same as C
(not meso)
diastereomers: (A = B) are diastreomers of (C = D). Cis/trans isomers are also called geometric isomers.
meso: None
no chiral centers: Neither structure has any chiral centers.
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9
Additional Examples - Determine how many stereoisomers there are in each of the following examples and determine what the
relationships are (enantiomers, diastereomers or meso compounds) among the stereoisomers.
time averaged
"flat" structure
=
rapid equilibrium between two chair conformations
maximum stereoisomers = 2n where
n = # chiral centers,
Possible disubstituted cyclohexane structures ("X" and "Y"): (1,1), (1,2), (1,3), (1.4)
C
B
A
(1,1)
D
X
Y
enantiomers: None. No chiral centers.
diastereomers: None.
meso: None
no chiral centers
(1,2)
A
B
D
C
X
X
X
X
Y
Y
enantiomers
Y
Y
enantiomers
enantiomers: A/B and C/D
diastereomers: AC, AD, BC, BD
Maximum number of stereoisomers = 22 = 4
meso: None
Full number is observed. No mirror planes of symmetry.
no chiral centers: Each structure has two chiral centers.
A
(1,3)
X
X
D
C
B
X
Y
X
Y
meso
Y
enantiomers
Y
enantiomers: A/B and C/D
diastereomers: AC, AD, BC, BD
Maximum number of stereoisomers = 22 = 4
meso: None
Full number is observed. No mirror planes of symmetry.
no chiral centers: Each structure has two chiral centers.
(1,4)
A
Y
No chiral centers,
achiral molecules
are identical.
X
C
B
X
No chiral centers,
achiral molecules
are identical.
X
Y
D
X
Y
enantiomers: No chiral centers.
same as A
(not meso)
Y
same as C
(not meso)
diastereomers: (A = B) are diastreomers of (C = D). Cis/trans isomers are also called geometric isomers.
meso: None
no chiral centers: Neither structure has any chiral centers.
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Additional Examples - Determine how many stereoisomers there are in each of the following examples
and determine what the relationships are (enantiomers, diastereomers or meso compounds) among the
stereoisomers.
time averaged
"flat" structure
=
rapid equilibrium between two chair conformations
Three substituents in a ring.
Each of these trisubstituted possibilities would have a plane of symmetry. There would be meso structures and
the maximum number of stereoisomers would not be observed: stereoisomers < 8.
Similar to
Example 3.
(1-Y,2,3-di-X), (1-Y,2,4-di-X), (1-Y,2,5-di-X), (1,2,4-tri-X),
X
X
Y
X
X
X
Y
X
X
(2-Y-1,3-di-X)
X
(1-Y-3,5-di-X)
X
X
(1,2,3-tri-X)
(1,3,5-tri-X)
None of these trisubstituted possibilities would have a plane of symmetry. There would not be any meso structures and
the maximum number of stereoisomers would be observed: 23 = 8 stereoisomers.
Similar to
(1-Y,2,3-di-X), (1-Y,2,4-di-X), (1-Y,2,5-di-X), (1,2,4-tri-X),
Example 4.
X
Y
X
X
(1-Y-2,3-di-X)
X
Y
X
(1-Y-2,4-di-X)
Z:\classes\314\314 Special Handouts\Stereochem topics key.doc
Y
X
Y
X
(1-Y-2,5-di-X)
X
(1,2,4-tri-X)
Beauchamp
11
1
7
H3C
F
5
6
H
H3C
H
2
1
2
3R
3
H
4
4
2
3
3
2
3
4
2
6S
Br
H
1
Cl
1
3
4
H
3
H3C
5R
H
1 = S
H
HO
2
1
H
Br
Cl
H
3 = R
H
3
H
4
5
Br
NH2
4 = R
F
H
F
6
H
H
H
5 = S
2 = R
4
3
1
4
1 = R
Br
NH2
Cl
2
HOBr
2
4
2R
H
2 = S
6
1
1
4S
5
3 = R
4 = R
H
5 = S
Ring Example B
Ring Example A
6 = S
6 = S
Examples of stereochemical features and terms.
b
a
H
R
Br
H
S
H
R
Br
Br
H
i
h
H
S
Br
Br
Cl
R
Br
H
Br
S
R
R
R
H
H
Br
H Cl
Br
p
Br
l
H
Cl
H
R
Br
H
Br
H
o
R
Br
R
R
H
n
m
H
S
Cl
Br
Cl
f
k
R
H
Br
H
R
j
R
Cl
R
H
Cl
g
e
d
c
Br
H
H Br
Cl
q
Br
E
Br
Br
Br
Z
Cl
Questions:
1. Does the molecule have any stereoisomers? (a,g,p = none), (b,h,m,n,o,q = 1), (e,f,k,l = 2), (c,d,i,j = 3)
2. Does the molecule have any chiral centers? How many? (a,g,m,n,o,p,q = none), (b,h = 1), (c,d,e,f,i,j,k,l = 2)
3. Does the molecule have any stereogenic centers? How many? (a,g,p = none), (b,h,m,n = 1), (c,d,e,f,i,j,k,l,o*,q* = 2)
4. Does the molecule have any enantiomers? How many? (b,c,d,f,h,i,j,l = 1), (others = none)
5. Does the molecule have any diastereomers? How many? (a,b,g,h,p = none), (f,l,m,n,o,q = 1), (c,d,e,i,j,k = 2)
6. Is the molecule meso? (yes = e,k)
7. Is the molecule chiral? (yes = b,c,d,f,h,i,j,l) Is the molecule achiral? (no = a,e,g,k,m,n,o,p,q)
8. Can a mixture of the molecules rotate plane polarized light? (yes = b,c,d,f,h,i,j,l) Same as: "Is it chiral?"
What directions of rotation are possible and how are they indicated? (d = + = right rotation), (l = - = left rotation)
9. Can a mixture of the molecules be racemic? (yes = b,c,d,f,h,i,j,l) What would be the composition of the mixture(s)? (50/50)
10. Specify the absolute configuration of any stereogenic centers as "R" or "S" or "E" or "Z". (see above)
11. Does the molecule have any prochiral centers or faces? (NOT responsible for this in our course.) Not applicable for us.
* = only in relation to each other, changing one is the same as changing the other.
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Beauchamp
12
Problem
a. Draw a zig-zag 3D structure of (2S,3R)-3-methylpentan-2-ol.
H3C H
4
5
2
3
(2S,3R)-3-methylpentan-2-ol
1
H OH
b.
Draw a Sawhorse projection of the same structure using the C3ÆC2 bond.
CH3
2
HO
H
H
(2S,3R)-3-methylpentan-2-ol
CH3
3
CH2CH3
c.
Draw the same structure as a Newman projection using the C3ÆC2 bond.
CH3
CH3
2
HO
H
CH3
HO
H
H
CH3
3
CH2CH3
d.
= 3 = 3R
CH2CH3
Draw the same structure as a Fischer projection and assign the absolute configuration all chiral centers as R or S.
H
CH3
HO
H
OH
H
CH3
rotate back carbon
to eclipsed conformation
H
CH3
CH3
CH3
tilt structure
to "up" position
H
OH
H
CH3
CH2CH3
CH2CH3
e.
= 2 = 2S
H
CH2CH3
Draw the mirror image of your Fischer projection and assign all absolute configurations as R or S. What is the
stereochemistry term used to describe the relationship between these two structures? Switch the alcohol group and assign
all absolute configurations. What is the relationship of this stereoisomer with the first two? Are there any other
stereoisomers?
2S
CH3
CH3
H
OH
HO
H
H
CH3
H3C
H
3R
CH2CH3
diasteromers
2R
CH2CH3
3S
If just the top group is
switched, a diastereomer
is formed with the first
two structures. It would
have an enantiomer, also
a diastereomer with the
first two structures.
enantiomers
Z:\classes\314\314 Special Handouts\Stereochem topics key.doc
2R
HO
H
3R
CH3
CH3
H
CH3
H
H3C
CH2CH3
enantiomers
Beauchamp
2S
OH
H
CH2CH3
3S
13
Questions to ponder?
meso?
not an isomer?
identical?
optically acitve (dl) or (±)?
enantiomers?
diastereomers?
(enantiomers)
CH3
S
H
R
Br
Br
Cl
Cl
S
(enantiomers)
S
H
R
H
R
H
H
Br
H
H
R
R
CH3
mirror
plane
CH3
CH3
R
CH3
1a
Br
Br
Cl
Cl
Cl
Cl
CH3
Cl
Cl
S
2a
CH3
CH3
R
S
S
Br
H
H
H
H
R
R
H
H
H
R
2b
Br
H
Cl
Cl
H
Cl
Cl
Cl
H
H
CH3
6a
(2R,3R,4R)
mirror
plane
Br
Br
Br
H
H
R
CH3
4a
S
S
S
mirror
plane
(enantiomers) CH
3
CH3
Br
Br
H
H
H
H
Cl
H
Cl
Cl
H
Cl
S
H
H
Br
Br
Br
H
S
CH3
mirror
plane
(enantiomers)
S
R
S
4b
CH3
Br
Br
H
Cl
Cl
H
H
H
Cl
CH
CH3
CH3
mirror
7a
plane
(2S,3S,4R)
6b
(2S,3S,4S)
H
CH3
CH3
CH3
S
CH3
R
3b = 3a
CH3
H
Br
R
CH3
CH3
H
Br
CH3
3a = 3b
(enantiomers)
R
H
Br
CH3
mirror
plane
1b
5b
(2R,3S,4S)
(2S,3R,4R)
H
CH3
S
S
H
(enantiomers)
CH3
CH3
CH3
CH3
mirror
plane
5a
(meso)
(enantiomers)
CH3
H
racemic as mixture?
mixture does not rotate light?
3
mirror
plane
8a
8b
(2S,3R,4S)
(2R,3S,4R)
7b
(2R,3R,4S)
All eight stereoisomers have the same name, "2-bromo-3,4-dichloropentane" except for the identification of absolute configuration (shown above). For
example structure 1 is named as (2S,3R,4R)-2-bromo-3,4-dichloropentane.
(meso 1)
CH3
S
H
s
H
R
H
Cl
Cl
Cl
Cl
Cl
Cl
CH3
9a = 9b
(2S,3s,4R)
mirror
plane
(enantiomers)
CH3
CH3
R
H
Cl
H
H
H
H
R
H
H
Cl
Cl
Cl
Cl
CH3
CH3
S
(meso 2)
S
Cl
H
H
Cl
H
H
Cl
Cl
H
H
Cl
Cl
r
r
S
CH3
R
CH3
mirror
10a = 12b plane
(2R,4R)
9b = 9a
(2R,3r,4S)
S
R
CH3
mirror
11a = 11b plane
(2S,3r,4R)
CH3
10b = 12a
(2S,4S)
(same as 10a/10b)
R
s
S
CH3
CH3
CH3
H
H
Cl
Cl
H
Cl
H
Cl
Cl
H
H
Cl
H
H
Cl
CH
CH3
CH3
11b = 11a
(2R,3s,4S)
12a = 10b
(2S,4S)
3
mirror
plane 12b = 10a
(2R,4R)
All eight (really only four) stereoisomers have the same name, "2,3,4-trichloropentane" except for the identification of absolute configuration (shown above).
For example structure 1 is named as (2S,3s,4R)-2,3,4-trichloropentane and 10a is named as (2R,3R,4R) - 2,3,4-trichloropentane.
(meso)
(meso)
CH3
CH3
S
H
H
R
Br
Br
Br
Br
CH3
R
S
CH2
H
Br
H
Br
CH3
mirror
plane
3a
R
S
H
(CH2)n = rings
H
If odd number "C",
bisects an atom, if
even numbeer of "C",
bisects a bond.
CH2
3b
(enantiomers)
(enantiomers)
CH3
Br
H
R
CH3
S
H
H
Br
R
CH3
4a
CH2
Br
S
Br
(CH2)n = rings Br
H
H
CH3
mirror
plane
4b
Z:\classes\314\314 Special Handouts\Stereochem topics key.doc
R
R
CH2
H
H
Br
Br
CH2
S
S
CH2
mirror
plane
Br
(CH2)n = rings
H
If odd number "C",
bisects an atom, if
even numbeer of "C",
bisects a bond.
Beauchamp
14
The following carbohydrates are Nature’s choice for energy use, structural support and also serve as recognition targets. Nature
almost exclusively picks one enantiomer of a pair. When drawn as a Fischer projection it is the stereoisomer having the second to
the last OH on the right side. In Biochemistry, this is referred to as the D isomer. If the second to last OH were on the left side in a
Fischer projection it would be referred to as the L isomer. (There is no logical reason for these designations. They must be
memorized.) “Reducing carbohydrates” have an aldehyde carbon at C1 and can be 3Cs, 4Cs, 5Cs, 6Cs or more in length. Some of
these are shown below. How many chiral centers does each example have? How many stereoisomers are possible for each length
of carbon? What are the absolute configurations of any chiral centers (R/S)? Specify the first stereoisomer as A (then B, C, etc.)
and state what each relationship is to the others. Identify any meso structures. Which are enantiomers, diastereomers, meso
compounds?
O
H
O
H
C
C
CH2OH
CH2OH
The first structure in each group
starts with an H on the left side
and an OH on the right side of
horizontal positions.
represents a
mirror plane
D-glyceraldehyde
O
H
O
C
O
H
CH2OH
H
O
H
C
CH2OH
CH2OH
O
H
C
C
CH2OH
CH2OH
D-allose (0)
O
H
(#) represents which positions
moves an OH on the right side
to the left side to generate all
possible stereoisomers. (0) means
all OH groups are on the right side.
CH2OH
O
H
O
H
C
C
CH2OH
CH2OH
O
H
O
O
H
C
C
H
CH2OH
CH2OH
C
C
CH2OH
CH2OH
D-mannose (1,2)
O
H
O
H
C
CH2OH
CH2OH
O
H
O
O
H
C
O
H
C
C
CH2OH
CH2OH
C
CH2OH
CH2OH
O
D-galactose (2,3)
H
H
O
H
C
C
CH2OH
CH2OH
D-lyxose (1,2)
O
D-glucose (2)
D-altrose (1)
O
H
C
D-xylose (2)
D-arabinose (1)
D-ribose (0)
H
H
C
D-threose (1)
C
O
O
CH2OH
CH2OH
D-erythrose (0)
O
H
C
C
O
H
C
CH2OH
CH2OH
D-gluose (3)
O
H
C
C
CH2OH
CH2OH
D-idose (1,3)
H
C
O
H
O
H
C
C
CH2OH
CH2OH
D-talose (1,2,3)
How would each of these structures change if C1 became -CH2OH?
Z:\classes\314\314 Special Handouts\Stereochem topics key.doc
Beauchamp
15
CH2OH
CH2OH
HO
OH
The first structure in each group
starts with an H on the left side
and an OH on the right side of
horizontal positions.
represents a
mirror plane
CH2OH
CH2OH
D-carbohydrates - second from bottom "OH"
is on the right side of the Fischer projection.
glycerol
glycerol
CH2OH
CH2OH
CH2OH
OH
HO
OH
HO
CH2OH
CH2OH
HO
OH
OH
HO
CH2OH
CH2OH
(#) represents which positions
moves an OH on the right side
to the left side to generate all
possible stereoisomers. (0) means
all OH groups are on the right side.
CH2OH
(1)
(0)
CH2OH
CH2OH
CH2OH
CH2OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
CH2OH
CH2OH
OH
CH2OH
CH2OH
OH
OH
OH
CH2OH
CH2OH
CH2OH
OH
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
OH
HO
OH
OH
HO
OH
HO
OH
CH2OH
CH2OH
CH2OH
HO
OH
HO
OH
CH2OH
(1,2)
CH2OH
HO
OH
HO
OH
HO
CH2OH
HO
CH2OH
(2,3)
HO
OH
OH
HO
CH2OH
CH2OH
OH
HO
OH
HO
OH
HO
CH2OH
(1,2,3)
Top carbon is the same as the bottom carbon.
Z:\classes\314\314 Special Handouts\Stereochem topics key.doc
CH2OH
HO
OH
CH2OH
(1,3)
CH2OH
(3)
CH2OH
HO
CH2OH
OH
OH
OH
OH
CH2OH
HO
CH2OH
HO
HO
OH
OH
(2)
CH2OH
CH2OH
HO
CH2OH
OH
(1)
CH2OH
HO
CH2OH
HO
HO
CH2OH
CH2OH
CH2OH
OH
CH2OH
CH2OH
HO
(0)
HO
(1,2)
OH
CH2OH
OH
(2)
HO
CH2OH
HO
HO
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
HO
HO
(1)
(0)
CH2OH
CH2OH
Beauchamp
CH2OH
16
...and Rings (need at least 3 atoms)
Chains...
middle
middle
middle
middle
(CH2)n middle
mirror
plane
mirror
plane
top part ≠ bottom part
middle
(CH2)n middle
(CH2)n
mirror
plane
mirror
plane
top part ≠ bottom part
top part = bottom part
middle (CH2)n
top part = bottom part
Possible Specific Examples:
O
O
H
C
=
=
CH3
=
Middle Part
1 Carbon
H
Ring
C
=
Ring
Fischer Projection
C
H
H
C
Br
H
H
H
Br
2 Carbons
H
C
H
H
C
Br
H
C
H
H
C
Br
H
C
Br
H
C
H
H
C
H
H
C
Cl
H
C
Cl
H
C
Br
H
H
H
Br
H
H
H
Br
H
Br
H
H
H
H
H
Cl
H
Cl
H
Br
Fischer Projection
3 Carbons & Fischer Projections
H
C
Br
H
Br
H
C
Br
H
Br
H
C
Br
H
Br
H
C
Br
H
Br
H
C
Br
H
Br
H
C
Cl
H
Cl
H
C
Cl
H
Cl
H
C
Br
H
Br
H
C
Br
H
Br
Relevant considerations:
1. Represent stereoismers as Fischer projections (when chains) or Haworth structures (when rings) for easier analysis.
2. How many chiral centers are there? What is the maximum number of stereoisomers? (2n, where n = # chiral centers)
Are there less than this number because of meso compounds?
3. What is the absolute configuration of each chiral center? (R and S assignments)
4. Are stereoisomers enantiomers, diastereomers, meso compounds, identical? (not even isomers?)
5. Is there optical activity? (Is a compound chiral?) Optical rotations can be right (d = + = dextrorotatory) or left (l = _ = levorotatory).
6. Are racemic mixtures possible? (50/50 mixture of enantiomers having zero optical rotation).
Z:\classes\314\314 Special Handouts\Stereochem topics key.doc
Beauchamp