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

Copyright © J. M. McBride 2009. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).

Use of this content constitutes your acceptance of the noted license and the terms and conditions of use.

Materials from Wikimedia Commons are denoted by the symbol .

Third party materials may be subject to additional intellectual property notices, information, or restrictions. 

The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0