Chemistry 125: Lecture 24October 30, 2009

Structure Proof and Models in 3-D Space (1869-1877)

Wilhelm (Guglielmo) Koerner developed a strictly logical system for proving the structure of

benzene and its derivatives based on isomer counting and chemical transformation. His

elegant proof that the six hydrogen positions in benzene are equivalent was widely ignored

because, in Palermo, he was far from the seats of chemical authority. Despite cautions from

their conservative elders, young chemists like Paternó (and later van’t Hoff) began

interpreting molecular graphs in terms of the arrangement of a molecule’s atoms in 3-

dimensional space.

For copyright notice see final page of this file

Kekulé

Ghent (1865)

rheumatism

Koerner

Proving Structure

PalermoSicily

Liberated in 1860after 2700 yearsof occupation by

PhoeniciansRomansVandals

ByzantinesArabs

NormansSwabiansAngevinsAragoneseHabsburgsBourbons

Garibaldi

Palermo

Laboratory ofStanislao

Cannizzaro

In 1860 hehelped bothGaribaldi

&

Avogadro(Gay-Lussac)

Revolutionaries

but in his way cautious…

Cannizzaro on Chemistry Teaching (1872)

Above all we should take care that the pupils do not form to themselves any mechanical or geometrical conception of the cause and effects of the quantivalence of atoms; they must be frequently reminded that chemical facts neither prove nor disprove anything relating to the size, form, continuity, distance, relative disposition, &c., &c., of the atoms.

Cannizzaro on Chemistry Teaching (1872)

If we are sometimes obliged to speak of the relative positions of the atoms in the molecules, and even to give graphic representations of these positions, we must hasten to remark that these figures are nothing but artifices of the mind, intended to represent to the sight that which in reality we perceive only through the veil of transformations; but that we do not really know what it is that corresponds to that which we call position, either in space, or in the mutual actions of different portions of matter.

Facts Serving to Determine Chemical Position in Aromatic Substances

Wilhelm Koerner (1869) The dogma of the impossibility of determining the atomic constitution of substances, which until recently was advocated with such fervor by the most able chemists, is beginning to be abandoned and forgotten; and one can predict that the day is not far in the future when a sufficient collection of facts will permit determination of the internal architecture of molecules. A series of experiments directed toward such a goal is the object of this paper…

(paper introduced and warmly endorsed by Cannizzaro!)

Add a third (identical) substituentand count isomers.

Establishes identity of both di- and tri-substituted benzenes,

Koerner : Which is which?

if the 6 substituent positions in benzene are equivalent.Proven by Koerner in 1869.

ortho meta paraKoerner

used thesesame names,

but differently

(less deserving individuals weremore influential)

Are the Four Valences of C Equivalent?

Only one isomer of methyl chloride is known!

Is this proof ?

Evidence?

C Cor

Koerner’s Assumptions (1867)

(2) Experimental Distinguishability

“…most of the demonstrations are based on establishing the identity or difference of

several substances of the same composition,but obtained by different reactions…”

(1) Direct Replacement

“If one grants that in simple transformation the new substituent assumes the position of

the element displaced…”

(parsimonious, but rearrangements do sometimes occur)

(depends on experimental skill - crystals are best)Ac. Salicilico

(Koerner)

2,6-Dinitro-3-bromo-1-toluene

(Koerner)

THE SAME PRODUCT!

Graebe1866

HCl

for example

Koerner Equivalence Proof (1869)

The difference between these two isomers is not in the position of OH.

PhenolC6H5OH

Graebe1866

HCl

Salicylic Acid Hydroxybenzoic Acid Parahydroxybenzoic Acid

Three known isomers of C6H4 OH COOH

Anisic AcidC6H4 OCH3 COOH

Methyl AnisateC6H4 OCH3 COOCH3 Graebe

1) Na 2) CO2

BromoanisoleC6H4 OCH3 Br

Koerner

thus not like these three isomers of

Dewar’s OH

OH

OH

The site(s) occupied by OH are either identical or equivalent.(since the O survives in identical compounds)

HOOC

HOOC

OH

HOOC

but maybe like these three isomers of

Dewar’s HO

HOOC

HO OH

Are these two truly equivalent?

Superimposable?

among the three

HO

three

OH

OH

Is it silly to treat disks and bars as behind, in front, or sandwiched?

How “real” is the constitutional model?

NO! YES!

almost

Koerner Equivalence Proof (cont)

The three isomers do not differ because of the absolute position of COOH.

Salicylic Acid Hydroxybenzoic Acid Parahydroxybenzoic Acid

Use the same three isomers of C6H4 OH COOH

1) Na/Hg

2) H2O

PCl5

C6H4 Cl COOHKoerner ?

Benzoic AcidC6H5 COOH

Beilstein1864

1) Na/Hg

2) H2O

PCl5

C6H4 Cl COOH

PCl5?

C6H4 Cl COOH

Koerner?

1) Na/Hg

2) H2O

Thus the 3 sites of COOH are equivalent Hx = Hy = Hz

as would be the case with

Dewar’s

for example

XY

Z

(but they differ regarding H)

Call OH position H

Koerner Equivalence Proof (cont)

Replace NH2 by Br

Change NO2 to NH2

BromochlorobenzeneC6H4 BrCly

Replace NH2 by Cl

BromochlorobenzeneC6H4 ClBry

Replace NH2 by Cl

Change NO2 to NH2

Replace NH2 by Br

“Arppe’s Nitroaniline”C6H4 NH2

NO2y

So there are at least four equivalent substituent positionsH = Hy = Hx = Hz

These compounds are identical

Hydroxybenzoic AcidC6H4 OHCOOHy

/ y established by interconversion with hydroxybenzoic acidas would be the case with

Dewar’s

for example

XY

Z

NOT

for example(which cannot be superimposed)

Br

Cly

Cl

Bry

Koerner Equivalence Proof (cont)

C6H4OHAzxO2

C6H4OHBrx

Start with known“meta” substituentsas in salicylic acid

C6H2 AzO2 • Br • OH • AzxO2

(The new substituents are in unknown positions. but and x are known.)

Br2

KNO3 H2SO4

“bromonitro-metanitrophenol”

NaNO3 H2SO4

“binitro-metabromophenol”

C6H2 AzO2 • AzO2 • OH • Brx

Thus there must be two positions, x and x’ that have the same relationship to (not the y or z relationship). The products are both C6H2 AzO2 • Brx’ • OH • AzxO2

So there are at least five equivalent substituent positionsH = Hx’ = Hx = Hy = Hz

The two compounds are identical!

Now none of Dewar’s models work, although perfect hexagon and equilateral

triangular prism will still do. (Note: Koerner used “meta” differently from how we do.)

Koerner’s ProofAlthough we cited specific examples, the the proof so far was abstract and made no reference to a geometric model such as:

Koerner’s argument for equivalence of the sixth substituentposition in benzene was faulty, because it hinged in a subtleway on a hidden assumption that benzene was hexagonal.

It is possible, using group theory, to construct a logically rigorous abstract argument on the basis of Koerner’s evidence to show that all six positions must be equivalent, so he was right in his intuition and his conclusion.

or

Koerner deserves enormous credit for formulating the logic of the first true structural proof in organic chemistry.

How many isomers of C2H4Br2?Rubber tubing joining actual 3D tetrahedral

models?

"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)

“It is superfluous to say that this is only a way of representing the facts, and

that all these ideas need to be tested experimentally.”

Lieben to Paterno

Click Here for Translation

Cf. Hofmann Models

Ghent (1865)

AlbertLadenburg

Kekulé

Koerner

End of Lecture 24Oct. 30, 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