Chemistry 125: Lecture 23

Valence Theory and Constitutional Structure

(1858) Youthful chemists Couper and Kekulé replaced radical and type theories with a new

approach involving atomic valence and molecular structure, and based on the tetravalence

and self-linking of carbon. Valence structures offered the first explanation for isomerism,

and led to the invention of nomenclature, notation, and molecular models closely related to

those in use today.

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Genealogy Top

Archibald Scott

Couper(1831-1892)

1851-1856 Dilettante Edinburgh/Berlin

Classics, MetaphysicsLogic, Moral Philosophy,

Concerts

1855 Chemist Berlin

1856 Chemist Paris (Wurtz) - Salicylic Acid

Fired

1859-1892 Mental Invalid

1858

Kirkintilloch

(near Glasgow)

Endrick Waterwhere Couper (age 28) suffered sunstroke

while fishing during recuperation

“Laurel Bank”Kirkintilloch

where his mother cared for him during his final 33 years

Theory and Types

p. 106 - on the French Type Theory

Should the principle which is therein adopted be applied to the common events of life, it will be found that it is simply absurd. Suppose that some one were to systematize the formation of letters into words that formed the contents of a book.

p. 106 - on the French Type Theory

Were he to begin by saying that he had discovered a certain word which would serve as a type, and from which by substitution and double decomposition all the others are to be derived, - that he by this means not only could form new words, but new books, and books almost an infinitum, - he would state certainly an empirical truth.

p. 106 - on the French Type Theory

At the same time, however, his method would, judged by the light of common sense, be an absurdity. But a principle which common sense brands with absurdity, is philosophically false and a scientific blunder.

p. 107 - on the German Radical Theory

I can only remark that it is not merely an unprofitable figure of language, but is injurious to science, inasmuch as it tends to arrest scientific inquiry by adopting the notion that these quasi elements contain some unknown and ultimate power which it is impossible to explain.

p. 107 - on the German Radical Theory

It stifles inquiry at the very point where an explanation is demanded, by putting the seal of elements, of ultimate powers, on bodies which are known to be anything but this.

but stated undiplomatically,especially for a newcomer.

Absolutely true

Science demands the strict adherence to a principle in direct contradiction to this view. That first principle, without which research cannot advance a step, dare not be ignored; namely, that a whole is simply a derivative of its parts.

p. 108 - Look to the Elements

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As a consequence of this, it follows that it is absolutely necessary to scientific unity and research to consider these bodies as entirely derivative, and as containing no secret ultimate power whatever, and that the properties which these so-called quasi elements possess are a direct consequence of the properties of the individual elements of which they are made up.

p. 108 - Look to the Elements

p. 109 - Focus on Carbon In applying this method, I propose at present to consider the single element carbon. This body is found to have two highly distinguishing characteristics: -

1. It combines with equal numbers of hydrogen, chlorine, oxygen, sulfur, &c.

2. It enters into chemical union with itself.

These two properties, in my opinion, explain all that is characteristic of organic chemistry.

1858

Tetravalenceand Self-Linking

of Carbon

C2 as nucleus H = 1 C = 6 O = 8 Cl = 35.5

(Cf. Wöhler/S.C.H.Windler hoax)

"Structural"Formulae

CH3CH2-OH

CH3-OH

Ethyl Ether

...

CH3CH2-O-CH2CH3

There is a Printer’s Error.(This was unfamiliar territory.)

Couper C 12

Glycerine

CH(OH)2

CH2OH

CH2

CH(OH)2

CO2H

CH2

2OH

OH

2OH

OHguessed

a bit wrong

English GlucoseCH2OH

CHOH

CHOH

CHOH

CHOH

CH(OH)2O + H2OTry 2- or 3-step HOMO/LUMO

scheme for H2O + R2C=O

guessedright forhydrate!

2OH

English GlucoseCH2OH

CHOH

CHOH

CHOH

CHOH

CH(Typo: Addition of H2, not H2O)

French version of same paper; lines for bonds

Old Aisle CemeteryKirkintillochOct. 28, 2008 Special thanks

to our agentSusan Frew

from K. HafnerAngew. Chem. Int. Ed.18, 641 (1979)

Drawing by Kekulé age 13

By Kekulé age 18 Giessen for architeture

from K. HafnerAngew. Chem. Int. Ed.18, 641 (1979)

Liebig's advice to Paris-bound graduate August Kekulé (1851)

There you will broaden your horizons,

there you will learn a new language,

there you will learn to knowthe life of a great city,

but there you will not learn chemistry!

Kekulé in Heidelberg (1856-58)

Performed Research on (CH3)2As(Prof. Bunsen's cacodyl radical)

in his apartment's kitchen!

1857 - Proposed a new type, "Marsh Gas"

HHHH

C

1858 - Proposed self-linking of Carbon

(tetravalence)

Kekulé Claims Priority (1858)Observations on Mr. Couper's New Chemical Theory

"In fact, in two memoirs which have appeared in Liebig's Annalen...I have put forward different views, which, in my opinion, should furnish a clearer insight into the constitution of chemical compounds.

I may be allowed to indicate that [my first paper] lays down the principle…which I have called the basicity of atoms [we would say 'valence']. If Mr. Couper thinks he has discovered the cause of this difference of basicity in the existence of a special kind of affinity, I am the first to admit that I have no right to contest his priority in this."

Ghent (1865) Structure of Benzene

Kekulé

Bonn (1872)

Kekulé

Nomenclature

Facts, Ideas, and Words

Notation

Models

Hofmann's

SystematicHydrocarbonNomenclaturefor Radicals

(1866)

Quartane, (C4H10)° Quartyl (C4H9)' Quartene, (C4H8)'' Quartenyl, (C4H7)''' Quartine, (C4H6)iv Quartinyl, (C4H5)v Quartone, (C4H4)vi Quartonyl, (C4H3)vii Quartune, (C4H2)viii Quartunyl, (C4H) ix

Kekulé on Notation("Benzene" Paper, 1865)

"For greater clarity I am presenting at the end of this note a table giving graphical formulae for most of the substances mentioned. The idea that these formulae are designed to express is rather well known now; so it will not be necessary to dwell upon it. I am keeping the form that I had adopted in 1859 when expressing for the first time my views on the atomic constitution of molecules. This form is nearly identical with that which M. Wurtz used in his beautiful lectures on chemical philosophy. It seems to me preferable to the modifications proposed by MM. Loschmidt and Crum-Brown."

What SHOULDa Formula Show?

Constitution:Nature & Sequence of Bonds

Composition:Elements & number of Atoms

Isomers

Berzelius is fine for this, but now we also need to show

Lohschmidt Acetic Acid (1861)

Crum-Brown (1861-1866)

isomers

benzene

reaction

What do you think Kekulé thought?

How Many Isomers?

Kekulé's "Preferable" Notation (1865)

WRONGCH3CH2CH2OH

CH3CCH3

O=CH3CHOH

CH3

- CH3CHCH3

OH-

Constitution same as 28!

Kekulé thought that his notation said

these are different.

MolecularModels

Gaines Models Now, if a class thus instructed be at the recitation bench, and one of them is requested to form carbonic acid, he will walk quickly to the table in front, where the box is open, and place on one of the shelves a cube marked 6, and two marked 8 – carbon, one, and oxygen, two. If requested to form lime, he will place on the shelf a cube marked 20, (calcium,) and another marked 8, (oxygen,) these being the elements of lime. These cubes being all placed in contact, and the pupil being asked, “What have we now?” answers, “Carbonate of lime.”

Another pupil may then be asked to give the proximate analysis for carbonate of lime, and the cubes before him will indicate the answer and fix it in his memory: “It is composed of one equivalent of carbonic acid, (22) and one of lime, (28).” Another may be requested to give the ultimate analysis, and by the same means he is furnished with the answer: “Carbonic acid is composed of one atom of carbon and two of oxygen, and the lime is composed of one atom of calcium and one of oxygen.”

The intelligent educator will see at a glance, that in this chemical alphabet he has the means of at once arresting the attention of his class, and converting what has hitherto been an irksome task into a pleasant recreation. The atomic theory, the characteristics of affinity, the law of multiple proportions, the nomenclature, isomerism, &c., may all be learned in half the time required heretofore, and learned so as not likely ever to be forgotten.

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James Dewar'sBrass Strip

Models(1866)

"to make the combination look like an atom, a thin round disc

of blackened brass can be placed under the central nut"

Prepared ~100 yearslater, and called

"Dewar Benzene"

4-center bond?

“CPK” Model

of Alaninean amino acid

Hofmann Croquet-Ball Models (1865)

Established the “standard” colors we still use!

Hofmann Croquet-Ball Models (1865)

Hofmann Croquet-Ball Models (1865)

O• •

• •

• •

_

Cl• •

• •

• •

• • • •

• ••

•O• •

• •

• •O

O• •

• • • •

• •

• •

• •O

Oxidizing agent for Gay-Lussac

“Chlorox” bleachExplosive

_NH4ClO4 (AP)

is a component of military explosives.

Hofmann Croquet-Ball Models (1865)

Isomers?

Constitutional Models Try

to Show Nature & Sequence of Bonds NOT Position in Space.

Successive Substitution Products Sc

ienc

e M

useu

m, L

ondo

n by

per

mis

sion

Science Museum, London by permission

Hofmann Croquet-Ball Models (1865)Reactivity!

“Unfinished” or “Non-Saturated”(now called “unsaturated”)

Cl2

additionnot

substitution

ClH2C-CH2ClCl2

Actually, as we’ve seen: H2C=CH2

End of Lecture 23Oct. 29, 2008

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