chemistry 125: lecture 26 november 4, 2009 van’t hoff’s carbon & chirality; communicating...
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Chemistry 125: Lecture 26November 4, 2009
van’t Hoff’s Carbon & Chirality; Communicating Configuration
With his tetrahedral carbon models van’t Hoff explained the mysteries of known optical
isomers possessing stereogenic centers and predicted the existence of chiral allenes, a
class of molecules that that would not be observed for another 61 years. Symmetry
operations that invert an odd number of coordinate axes interconvert mirror-images. Like
printed words, only a small fraction of molecules are achiral. It is important for chemists to
agree on notation and nomenclature in order to communicate molecular constitution and
configuration. It is best when a diagram is as faithful as possible to the 3D shape of a
molecule, but the conventional 1891 Fischer projection, which has been indispensable in
understanding sugar configurations for over a century, involves highly distorted bonds.
For copyright notice see final page of this file
van’t Hoff Cardboard
Models(Bremer’s set, in Museum Boerhaave, Leiden)
(from T. M. van der Spek, Annals of Science, 2006)
Colored Faces
Colored Vertices
Ladenburg Benzenes
Fumaric and Maleic Acids
Free Rotation : Tartaric / meso
van’t Hoff made this diagram to show that free rotation about the central bond results in rapid interconversion (and thus inseparability and irrelevance) of “Paternó isomers.” Note that R1,R2,R3 (and R4,R5,R6) can be arranged clockwise or counterclockwise about the C-C axis. This sequence is permanent and unaffected by C-C rotation. If R 1=R4, R2=R5, R3=R6, as in tartaric acid, there are three possible isomers: cw-cw, ccw-ccw, and cw-ccw. If 123≠456, there is a fourth isomer ccw-cw. For mesotartaric acid, there is no net effect on polarized light, because the ccw half cancels the cw half.
van’t Hoffpredicts
handednessof allenes
1874
C
C
C
R
RH
H
C10H7
C6H5
C6H5
C10H7
61 years after
van’t Hoffprediction
of 1874
C6H5
OCOCH2COOH
C C C
C6H5
C10H7
C10H7
C6H5
tetrahedralbonds
coplanarbonds?
must break bond to
isomerize to mirror image
C6H5
OCOCH2COOH
C C C
C6H5
C10H7
Isomer Numbers for “Dewar's” 3D Benzene Structures
C
C
C
CC
C
H H
HH
H
H
"Prismane"or
"LadenburgBenzene"
"Dewar Benzene"
C
C
H H
C
C
C C
H HHH
C C
CC
C
CHH
H H
H H
C C
C
C
C C
H
H
H
H
H
H
C C
CC
CC
HH
H H
HH
C C
C C
CC
HH
H HHH
C
C
H H
C
C
CC
H
HH
H
2(1)
3(2)
3(1)6(3)
3(3)
5(4)3(2)
19(6)
2(1)
3(2)
3(1)6(3)
2(1)
5(3)1
6(3)
2(1)
3(2)3(1)
6(3)
3(2)
25(2)
1
3(1)
MONO
DI
(additional number if mirror-images count for these nonplanar structures)
(subtotals with total in Red)
3(1)assuming
free rotationof CH3
0top
0mid
0bottom
0total
?
which our intuition interprets as rotation about the vertical axis (exchanging right and left), because people pivot, but don’t "invert".
It exchanges front and back,
Mirror Images
Q. Why does a mirror exchange right . and left, but not top and bottom?
A. Actually it changes neither.
Right
is Top
on top
is on right
special.lib.gla.ac.uk/ exhibns/month/mar2000.html
Right Arm
Oxford Mathematician
blind in one eye
special.lib.gla.ac.uk/ exhibns/month/mar2000.html
Right ArmLeft Arm
“Well then, the books are something like our books, only the words go the wrong way; I know that, because I've held up one of our books to the glass, and then they hold up one in the other room…
"Now, if you'll only attend, Kitty, and not talk so much, I'll tell you all my ideas about Looking-glass House…
"How would you like to live in Looking-glass House, Kitty? I wonder if they'd give you milk, there?
“Perhaps Looking-glass milk isn't good to drink.
E. Heilbronner, J.D. Dunitz, Reflections on Symmetry, 1993, p. 86
(sarcolactic?)
“I call any geometrical figure, or group of points, chiral, and say that it has chirality, if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself.”
Lord Kelvin (1893)
Chirality (Greek “” hand)
Chirality (Greek “” hand)Change Sign of all
X Coordinates?
Reflection in yz MirrorChange Sign of all
Y Coordinates?
Reflection in xz MirrorChange Sign of all
Z Coordinates?
Reflection in xy MirrorChange Sign of all X & Y Coordinates?
Rotation about z AxisChange Sign of all X & Z Coordinates?
Rotation about y AxisChange Sign of all Y & Z Coordinates?
Rotation about x AxisChange Sign of all
X & Y & Z Coordinates?
Inversion through Center of Symmetry
Chirality - Non-superimposable Mirror Images
The right hand has only one mirror image, but different mirrors(or the inversion center) generate it in different orientations.
How Specialis Chirality?
Democratic Answer:Class voted overwhelmingly that there should be
more achiral than chiral molecules
(Vox Populi, Vox Dei)
Pretty special
2-D Chirality of Words
MUMis its own
mirror image.Thus it is“achiral”or “meso”
Mirror bychangingsign of X
NOONis not its ownmirror image.
Thus it is“chiral”
like mostof the words.
2-D Chirality of WordsRotate bychangingsigns ofX and Y
NOONdoes have rotationalsymmetry,but still itis chiral,
like a propeller
2-D Chirality of Words
DECODEis also an“achiral”
“meso” word,but it is harderto recognize,
because horizontal mirrors areunfamiliar.
Mirror bychangingsign of Y
How Special is Chirality?
Almost all words are chiral.
Achiral or meso words, such asMUM and DECODE are very rare.
It is the same with molecules.
Almost all molecules are chiral.
Not at all.
(But when we deal with very simple molecules,we often encounter achiral or meso ones.)
Beyond Constitutional Isomerism(Stereochemistry)
Composition
Constitution
"Stereoisomers"
The distinction between configuration & conformation is based on our bonding model.
Change requires breakingbonds (van't Hoff)
Change by rotatingabout single bonds
(Paternó)
Isom
ers
Configuration
Conformation
HARD
EASY
All "isomers" representlocal energy minima
(not just differentphases of vibration)
Stereochemical Relationships
Two molecules with the same constitution can be:
Identical
Completely Different
Mirror Images
(Homomers)
Diastereomers
Enantiomers(enantios - opposite)
(dia - across)
Facts
Ideas
Words(pictures too)
Stereochemical
3D Clues in Model PicturesSize, Perspective, Obstruction
Rotation , Highlight, Shading, etc., Stereo-Pair
Notation: Using Dashes and Wedgesto Show Tetrahedral Carbons
Planar
<90°Angle
90° & 180°Angles
OK(Conventional)
OK109° angles
"Fischer Projection" (1891) With the help of Friedländer's convenient rubber models, one can construct molecules of right-handed tartaric acid, left-handed tartaric acid, and inactive tartaric acid and lay them in the plane of the paper so that the four carbon atoms are in a straight line and the attached hydrogens and hydroxyls lie above the plane of the paper:
Bent to go back from BOTH atoms
COOH
C OHH
CHOH
COOH
Rotate 90°
"Fischer Projection" (1891)
May rotate in-plane by 180°but NOT by 90°
because horizontal bondsare taken as wedges
enantiomers diastereomers
diastereomers
“meso”
names of relationships Its own mirror image
name wasgeneralized
“inactive”(between + & - ?)
Configurational Isomer Counting
n "Stereogenic Centers" 2n isomers?
No, because of meso compounds
"Next we consider a symmetrical formula:
C(R1 R2 R3)C(R4 R4)C(R1 R2 R3) , and
C(R1 R2 R3)C(R4 R5)C(R1 R2 R3) as well.
As is easily conceived from the forgoingdiscussion, they lead to only three isomers."
van't Hoff (1877)(Baeyer & Fischer gave up on using bread rolls.)
End of Lecture 26Nov. 4, 2009
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