Chemistry 125: Lecture 26 van’t Hoff’s Tetrahedral Carbon and Chirality 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 involve inverting an odd number of coordinate axes interconvert mirror-images. Like printed words, only a small fraction of molecules are achiral. Verbal and pictorial notation for stereochemistry are discussed. Synchronize when the speaker finishes saying “…from Kolbe on the young van’t Hoff.” Synchrony can be adjusted by using the pause(||) and run(>) controls. For copyright notice see final page of this file
Transcript
Chemistry 125: Lecture 26
van’t Hoff’s Tetrahedral Carbon and Chirality
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 involve inverting an odd number of coordinate axes interconvert mirror-
images. Like printed words, only a small fraction of molecules are achiral. Verbal and
pictorial notation for stereochemistry are discussed.
Synchronize when the speaker finishes saying
“…from Kolbe on the young van’t Hoff.” Synchrony can be adjusted by using the pause(||) and run(>) controls.
For copyright notice see final page of this file
van't Hoff’s Optically Active Compounds
Malic Acid (CO2H)CH(OH).CH2. (CO2H)
Lactic Acid (CH3)CH(OH)(CO2H)
Tartaric Acid (CO2H)CH(OH).CH(OH)(CO2H)
Aspartic Acid (CO2H)CH(NH2).CH2. (CO2H)
Amyl Alcohol (CH3)(C2H5)CH(CH2OH)
Glucose COH(CHOH)4CH2(OH)(Levulose, Lactose)
Maleic & Fumaric Acids(CO2H)HC=CH(CO2H)
(CO2H)CH2CH2(CO2H)Succinic Acid
HI/P OH H “reduction” (Bremer & van't Hoff)
& Most of theirDerivatives, but not
inactive
ScheeleLiebigWislicenus
(Scheele, Berzelius, Pasteur, Wislicenus)
right, left, and inactive
Encyclopaedia Britannica, 1911:LACTIC ACID (hydroxypropionic acid), C3H6O3. Two lactic acids are known, differing from each other in the position occupied by the hydroxyl group in the molecule; they are known respectively as -hydroxypro-pionic acid (fermentation, or inactive lactic acid), CH3CH(OH) . CO2H, and 3-hydroxypropionic acid (hydracrylic acid), CH2(OH)CH2CO2H. Although on structural grounds there should be only two hydroxypro-pionic acids, as a matter of fact four lactic acids are known . The third isomer (sarcolactic acid) is found in meat extract (J. v. Liebig), and may be prepared by the action of Penicillium glaucum on a solution of ordinary ammonium lactate.
Constututional Isomers
It is identical with -hydroxypropionic acid in almost every respect, except with regard to its physical properties. The fourth isomer, formed by the action of Bacillus laevo-lacti on cane-sugar, resembles sarcolactic acid in every respect, except in its action on polarized light.
“…the fancy trifles in it are totally devoid of any factual reality…” Kolbe on van’t Hoff (1877)
!
van't Hoff Obituary (1911)
In his whole life he never made what would be called a very accurate measurement, and he never cared to. I remember his saying to me eighteen years ago, “How fortunate it is that there are people who will do that sort of work for us!”
(CO2H)CH2CH2(CO2H)Succinic Acid
“reduction” (Bremer & van't Hoff)& Most of theirDerivatives, but
inactive
van't Hoff’s Optically Active Compounds
Malic Acid (CO2H)CH(OH).CH2. (CO2H)
Lactic Acid (CH3)CH(OH)(CO2H)
Tartaric Acid (CO2H)CH(OH).CH(OH)(CO2H)
Aspartic Acid (CO2H)CH(NH2).CH2. (CO2H)
Amyl Alcohol (CH3)(C2H5)CH(CH2OH)
Glucose COH(CHOH)4CH2(OH)(Levulose, Lactose)
Maleic & Fumaric Acids(CO2H)HC=CH(CO2H)
"Every carbon compound which in solution can rotate the plane ofpolarized light contains one or more asymmetric carbon atoms."
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
Maleic and Fumaric 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
handedness 1874
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 (1894)
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"
distinction based onbonding 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
Facts
Ideas
Words (pictures too)
3D Clues in Model PicturesStereo, Rotation, Size, Perspective, Shading,
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