Chemistry 125: Lecture 25

Models in 3-D Space (1869-1877)

Optical Isomers Despite cautions from their conservative elders, young chemists like Paternó and van’t

Hoff began interpreting molecular graphs in terms of the arrangement of a molecule’s atoms

in 3-dimensional space. Benzene was one such case, but still more significant was the

prediction, based on puzzling isomerism involving “optical activity”, that molecules could be

“chiral”, that is, right- or left-handed. Two decades earlier Louis Pasteur had effected the first

artificial separation of racemic acid into tartaric acid and its mirror-image.

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Genealogy Top

How many isomers of C2H4Br2??

"three isomers of C2H4Br2, supposing that they actually exist, can easily be explained without assuming any difference among the

four valence of the carbon atom." Paternó (1869)

Lieben to Paterno

Click Here for Translation

Ghent (1865)

AlbertLadenburg

Kekulé

Koerner

Remarks on the Theory of AromaticityAlbert Ladenburg (1869)

Already several years ago I had the occasion to make Kekulé aware that his graphical formula for benzene is not adequate, since here 1.2 and 1.6 must be inequivalent, while one could have various views on the identity of positions 3 and 5.

Both conditions could however be fulfilled through alternative formulas which, so far as I know, have not been proposed before

Kekulé adopts two hypotheses…1. The 6 hydrogen atoms of benzene are equivalent2. Each hydrogen atom of benzene has two sets of two others which

are symmetrically arranged with respect to it… 1.2 = 1.61.3 = 1.5

Bonn (1872)

Kekulé

van't Hoff

Proof that the prism formula suffers from the same problem as the hexagon with fixed double bonds removes the previously asserted superiority from the former and makes the original Kekulé notation not only simpler, but also the presentation with which the facts conform best.

Ladenburg's Benzene FormulaJ. H. van't Hoff (1876, age 24)

The 1,2; 5,6; and 3,4 deriva-tives are completely similar, although differing from 4,5; 2,3; and 6,1.

Exactly the same thing happens in Ladenburg's formula. An adequate consideration shows that I and II are absolutely different.

A 1,3 product is different according to whether A or B occupies position 1.

Dashed linesto show 3D

On Benzene Formulas A. Ladenburg (1877)

I cannot agree with him in this. Van't Hoff is dragging some-thing into the formulas which I together with most chemists expressly exclude. I refer to arrangement in space…

van't Hoff finds the two formulas below "absolutely" different.

The formula takes account of the composition, molecular weight, and mode of union of the atoms.

CONSTITUTION

No BrokenLinesNot 3D, just a

Constitutional graph

On Benzene Formulas A. Ladenburg (1877)

Even van't Hoff will have to find that these two formulas are "absolutely" identical.

If van't Hoff's view of space really will not tolerate the two formulas above to be identical, then I invite him, for his own special purposes, to use a different benzene formula, which was proposed by me in the year 1869 (when I compared the hexagon and

the prism for the first time) together with the prism, to which it is for me certainly identical. It is the so-called Cross of David.

Identical:as

abstract

2D graph

Kohler 1920s/1

Harvard OrganicNotes (1920s)Prof. E. P. Kohler

(from R. M. Fuoss)

Greek alpha, , means on carbon next to C=OTypically easy to substitute

Kohler 1920s/2

Harvard OrganicNotes (1920s)Prof. E. P. Kohler

(from R. M. Fuoss)

"1276" cm-1

Vibration Calculated by MO Theory (Spartan)

Structure ofmimimum

potential energy

+

“Breathing” “Kekulé Distortion” of single minimum!

"1367" cm-1

Might “stressful” substitution favor "Kekulé Distortion"?

(alternating bond lengths at energy minimum)

Careful X-Ray at -170°C (R. Boese,1988)

1.402

1.402 1.391

1.402

1.387

1.387

(±0.001Å)

Not Here!

Genealogy Top

L. PASTEUR-Fresolution

It is hard to PROVE two samples identical(There could always be another test)

Carvone SamplesBoiling Points b.p. 230-231°C

Infrared Spectra "identical"

NMR Spectra "identical"

Odor rye bread (carraway)spearmint ?

Densities d20 0.9652 g/mL

Refractive Indices n20 1.4988D

Some Materials(like Karo Syrup)

Have the Ability to“Rotate” Polarized Light

colortemp

° / g/mL / dmD[]20

Specific Rotation

(solvent should be specified)

It is hard to PROVE two samples identical(There could always be another test)

Carvone SamplesBoiling Points b.p. 230-231°C

Infrared Spectra "identical"

NMR Spectra "identical"

Odor rye bread (carraway)spearmint ?

Densities d20 0.9652 g/mL

Refractive Indices n20 1.4988D

Specific Rotation+62 -62[]20

D

Constitutional Formulae of Carvone

Models withTetrahedral Carbon

longhand C

CC

C

CC OH

C

H

HH C

H

H

C

CH

HH

H

H

H

H

HO

shorthand

s?

unlikevs.

back

forward

Horientation

Is thisdifference

real?

Isomers of Tartaric Acid(CO2H)CH(OH).CH(OH)(CO2H)

0

[]20

13

D

meso-Tartaric Acid"Pyrotartaric"

140°

0

Racemic Acid 206°

Tartaric Acid 170°d-(+)-

mp(°C)

Berzelius (1830)

Problems with the ModelISOMER NUMBERS

Maleic & Fumaric Acids HOOCCH=CHCOOH

dextro & levo Lactic Acids (CH3)CH(OH)COOH

Tartaric & Racemic AcidsHOOCCH(OH)CH(OH)COOH

& meso-Tartaric & l-Tartaric Acids !

Too Many Predicted?Paternó’s dibromoethanes

Too Few Predicted:

Louis Pasteur(1822-1895)

budding chemistry-physics-

crystallographyinterdisciplinarian

"On the relations that can exist among crystalline form, chemical composition and the direction of rotatory polarization"

1848http://www.lmcp.jussieu.fr/~soyer/cristallo/pasteur_l.html

EilhardMitscherlich

(1794-1863)

Device to Measure Crystal Angles

Figure 9

(CO2Na)CH(OH).CH(OH)(CO2NH4)Tartrate: "The eight f edges should be modified in the same way,

Racemate:I carefully separated the right from the left crystals, and observing their dissolution separately with M. Biot's polarization apparatus, I saw with surprise and delight that the [right] crystals deviated the plane of polarization to the right, and the [left] to the left."

but always only [the same] four of them are facetted.""…as often to the right as to the left on different crystals…

Mitscherlich had reported that this optically inactive salt was isomorphous with the tartrate. Shouldn’t crystals from ‘untwisted’ molecules be “holohedral” and show all eight f-facets?

Isomers of Tartaric Acid(CO2H)CH(OH).CH(OH)(CO2H)

meso-Tartaric Acid"Pyrotartaric"

140°

0

Racemic Acid 206°

Tartaric Acid 170°d-(+)-

mp(°C)

0

[]20

+13

l-(-)-Tartaric Acid 170° -13d and l are mirror images

(50:50)

?

Resolution (by Pasteur - 1848)

Kekulé

van't Hoffage 20

Bonn (1872)

FastForward

24 Years

!

J. H. van't Hoff(1852-1911)

First Nobel Prize(1901)

Most Admired Trait:Imagination

(poets/artists)

Hero:Lord Byron

1874 - Student

THE

ARRANGEMENT OF ATOMS

IN

SPACE

1877

53 pp.

See website for:(1) van't Hoff's Tetrahedral Carbon

(2) Kolbe's Criticism of van't Hoff

(3) Your Teacher, a later-day Kolbe?

“It is completely impossible to criticize this booklet in any detail because the fancy trifles in it are totally devoid of any factual reality and are completely incomprehensible to any clear-minded researcher… The brochure begins with the words: "The modern chemical theory has two weak points. It says nothing either about the relative position or the motion of the atoms within the molecule.”

Hermann Kolbe(age 56)

End of Lecture 25Nov. 3, 2008

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