Chemistry 125: Lecture 22October 25, 2010

Radical and Type Theories (1832-1850)

Valence Theory (1858)

Work by Wöhler and Liebig on benzaldehyde inspired a general theory of organic chemistry focusing on

so-called “radicals” - collections of atoms that appeared to behave as “posltive” or “negative” elements and

to persist unchanged through organic reactions. For a time Liebig’s French rival, Dumas, also advocated

radicals and dualism, but he converted to the competing ”unitary” theory of “types”, to accommodate

substitution reactions. These decades teach more about the psychology, sociology, and short-sightedness

of hot-shot chemists than about fundamental chemistry, but both discredited theories survive in competing

schemes of modern organic nomenclature. The HOMO-LUMO mechanism of addition to alkenes and the

SOMO mechanism of free-radical chain reactions are introduced. By mid-century there was serious

conflict between the French advocates of type theory and the Germanic advocates of radical theory.

Youthful chemists Couper and Kekulé replaced both radical and type theories with a new approach

involving atomic valence and molecular structure, and rooted in the tetravalence and self-linking of carbon.

For copyright notice see final page of this file

1832

Radical Theory

Correspondence of Liebig & Wöhler

12 July 1832 - Wöhler: The oil of bitter almonds has comewith the books from Paris. I've kept half of it and am herewith sending you the rest. I've already started all kind of experiments with it, without being able to obtain any precise results. It seems to be a hard nut. I'm coming soon to you and will be able to report.

30 August 1832 - Wöhler: Here I am again in my gloomylonesomeness not knowing how to thank you for all the love with which you took me in and kept me for so long. How happy I was to work with you from moment to moment. Herewith I'm sending the paper on oil of bitter almonds.

Oil of Bitter

Almonds

Toward Structure With Analysis?

Oil ofBitter

Almonds

C7H6O2O2

C7H5OClCl2

Liebig & Wöhler (1832)

C7H6O

C7H5OBr

Br2

C7H5OIKI

C7H7ONNH3

C14H10O2S

PbS

So?Persistence of C7H5O Benzoyl Radical

Bz • H

Bz • OH

Bz • Cl

Bz • Br

Bz • I

Bz • NH2

Bz2 • S

Organic Dualism

A radical can be the “base” of more than just an acid!

During the 1830s compound radicals were discovered

everywhere:Liebig: Acetyl

Bunsen: Cacodyl (Me2As •)

Piria: Salicyl

Dumas: Methyl Cetyl Cinnamyl Ethylene

Ethyl (Berzelius)

e.g. Ethyl Chloride

-yl from (wood , matter)

ether from (to shine) 1700s

hence eth yl (Liebig, 1834)

Dualistic Radical Theory Survives

Two Words (+ and -)in our Nomenclature

sky

colorless liquid

meth- from (wine, spirit)

-yl from (wood, matter)

-ene Greek Feminine

Patronymic"daughter of"

from

(1840) CH3 methyl from methylene

"Daughter of wood spirits"

hence meth yl ene (Dumas, 1835)

CH3OH = CH2 + H2O

(1852) ethylene C2H4

: ethyl :: methylene : methyl: C2H5 :: CH2 : CH3

C3 C4

C3H7 PropylC3H6 Prop(yl)ene

C4H9 ButylC4H8 But(yl)ene

From C3 Propionic Acid (1847)

(protos first) (pion fat)Derivatives of acids with >C2 were like fats,

unlike C1 (formic) and C2 (acetic), which mixed freely with water.

From C4 Butyric Acid (1826)

Lat. butyrum (butter)from rancid butter

Jean-Baptiste

André Dumas

(7/14/1800-1884)Post-Napoleonic

Guardian ofFrench Chemistry

Persistent Opponentof Liebig and

Berzelius

Chaired Professor:Sorbonne 1841-

École Polytechnique 1835-École de Médicine 1839-

http

://c

lend

enin

g.ku

mc.

edu

Note on the Present State of Organic Chemistry by MM. Dumas and Liebig (1837)

Sixty years have hardly passed since the ever memorable time when this same assembly heard the first discussions of the fertile chemical doctrine which we owe to the genius of Lavoisier. This short span of time has sufficed to examine fully the most delicate questions of inorganic chemistry, and anyone can easily convince himself that this branch of our knowledge possesses almost everything that it can with the methods of observation available.

…there barely remain a few cracks here and there to fill in.

MYOPIA!

(cf. Lavoisier, “in our own time”)

In a word, how with the help of the laws of inorganic chemistry can one explain and classify such varied sub-stances as one obtains from organic bodies, and which nearly always are formed only of carbon, hydrogen, and oxygen, to which elements nitrogen is sometimes joined?

This was the great and beautiful question of natural philosophy,

prin

tart

ist

Note on the Present State of Organic Chemistry by MM. Dumas and Liebig (1837)

Note on the Present State of Organic Chemistry by MM. Dumas and Liebig (1837)

a question well designed to excite the highest degree of competition among chemists; for once resolved the most beautiful triumphs were promised to science. The mysteries of plants, the mysteries of animal life would be unveiled before our eyes; we would seize the key to all the changes of matter, so sudden, so swift, so singular, that occur in animals and plants; more importantly we would find the means of duplicating them in our laboratories.

In fact to produce with three or four elements such varied combinations, more varied perhaps than those which make up the whole inorganic kingdom, nature has chosen a path as simple as it was unexpected; for with elements she has made compounds which behave in all their properties like elements themselves.

Note on the Present State of Organic Chemistry by MM. Dumas and Liebig (1837)

Well, we are not afraid to say it, and it is not an asser-tion which we make lightly: this great and beautiful ques-tion is today answered; it only remains to follow through on all the consequences which its solution entails...

And this, we are convinced, is the entire secret of organic chemistry.

Note on the Present State of Organic Chemistry by MM. Dumas and Liebig (1837)

Thus organic chemistry possesses its own elements, which sometimes play the role of chlorine or oxygen in inorganic chemistry and sometimes, on the contrary, play the role of metals. Cyanogen, amide, benzoyl, the radicals of ammonia, of aliphatics, of alcohol, and analogous substances, these are the true elements with which organic chemistry operates…

+

To discover these radicals, to study them, to characterize them, this has been our daily study for ten years.

We then understood that united we could undertake a task before which either of us in isolation would have recoiled…

We will analyze every organic substance…to establish reliably what sort of radical it refers to…

Note on the Present State of Organic Chemistry by MM. Dumas and Liebig (1837)

Sometimes, none the less, our opinions have appeared to differ, and then, with each of us drawn on by the heat of our battle with nature, there arose between usdiscussions whose liveliness we both regret.

Actually when we were able to discuss questions which separated us in several friendly meetings, we soon realized that we were in agreement on the principles...

Note on the Present State of Organic Chemistry by MM. Dumas and Liebig (1837)

Each of us has, in fact, opened his laboratory to all young men who were motivated by true love of science; they have seen all, understood all. We have worked under their eyes, and have had them work under ours, in such a way that we are surrounded by young rivals, who are the hope of science, and whose work will be added to ours

and mingle with ours, for it will have been conceived in the same spirit and carried out by the same methods…

(unimaginative megalomania)

This is not an effort conceived for personal gain or in the interest of narrow vanity. No, and in a collaboration which is perhaps unheard of in the history of science, this is an undertaking in which we hope to interest every chemist in Europe.

Trouble in Paradise

C7H5O • H C7H5O • ClCl H • Cl+ = +C7H5O • Cl+ -

H • Cl+ -

The electronic character of radicals wastroublesome for Coulombic dualism.

++C7H5O • H

?

e.g. preparation of benzoyl chloride

late 1830s - 1850s

“Substitution”or “Type”

or “Unitary”Theory

Tuileries

Bal costumé au Palais des Tuileries. 1867

organic compounds can

"fix" chlorine gas!

Violently irritating fumes from wax

candles spoil soiréeat the Tuileries Palace (~1830)

Dumas identifies the culprit as HCl from wax that had

been bleached by Cl2.

Mus

ée d

'Ors

ay (

ww

w.h

isto

ire-

imag

e.or

g)

cough!choke!

Two ways wax could "fix" chlorine:

Cl Cl*

C

CH2

H2

+*+

Addition of Cl2 to an Alkene (HOMO/LUMO)

ClC

CCl

H2

H2

C

C

H2

H2Cl

Cl

(actually both steps at once)

••

••

••

HOMOLUMO LUMOHOMO

HOMO / LUMO

“olefiant gas” “oil of Dutch chemists” (1795)Cl

n

Free-Radical Substitution of Cl for H (SOMO)• •

Two ways wax could "fix" chlorine:

H CH3Cl Clweak bond

(58 kcal/mole)

••

SOMO

H Cl

•

CH3 Cl Cl

CH3Cl

•

Cl

single-electrons

single-barbedarrows

"free-radical chain"

“make-as-you-break”

“make-as-you-break”

But stillhard to break!

Use light to promote an electron to *Cl-Cl

lots of product from one initial photolysis

No high HOMO or low LUMO!

Cl ClC

CH2

H2

+

Addition of Cl2 to an Alkene (HOMO/LUMO)

ClC

CCl

H2

H2

C

C

H2

H2Cl

Cl

C2H3O • OH C2H2ClO • OH

Photochlorination of acetic acid

Cl H • Cl+ = +C2H3O • OH C2H2ClO • OH

More Trouble for Radicals - Dumas (1839)

Hydrogen may be substituted by anequivalent amount of halogen, oxygen, etc.

Similar Acids! C2HCl2O • OH

C2Cl3O • OH

without changing molecular type.

transmuted the acetyl "element".

Four Types Recognized by 1853

H

H

H

Cl

H

HO

H

HN H

C2H5

HO

C2H5

HN H

KC2H5

C2H5

C2H5

I+ =

C2H5

C2H5

O + KI

Williamson Ether Synthesis (1850)

Butyl_BromideTwo-Word Relic

of Radical Dualism

BromobutaneOne-Word Relic

of "Unitary" Theory

Unitary (not Dualistic) Theory [Molecules are like] planetary systems held together by a force resembling gravitation, but acting in accord with much more complicated laws. Dumas (1840)

Formulae…may be used as an actual image of what we rationally suppose to be the arrangement of constituent atoms in a compound, as a orrery is an image of what we conclude to be the arrangement of our planetary system. A. W. Williamson (age 27, 1851)

A neutral anhydrous tartrate loses an atom of water at +190°; it has ceased to be a tartrate and has become another salt

Berzelius (1838) “By reacting chlorine with ordinary ether [Dumas] pro-duced a very interesting compound which he reckoned, according to the theory of substitutions, to be an ether in which 4 atoms of chlorine replace 4 atoms of hydrogen. An element as eminently electronegative as chlorine would never be able to enter into an organic radical : this idea is contrary to the first principles of chemistry…”

first use of “R” to denote a generalized radical

“Liebig’s Annalen”

I am a far cry from sharing the ideas that M. Dumas

has linked to the so-called laws of

the substitution theory.

1840

On the Reaction of Chlorine with the Chlorides of Ethanol

and Methanol and Several Points of the Ether Theory.

On the Substitution Lawof M. Dumas.

Remarks on the Previous Paper

1. Can one in any compound sub-stitute the elements, equivalent

for equivalent, by simple or com-pound bodies that play their role?

On the Law of Substitutions andthe Theory of Types.

Justus Liebig

*) Letter to J.L.

On the Law of Substitutions andthe Theory of Types.*

YES!

On the Substitution Law and the Theory of Types

(letter to Justus Liebig) Paris, 1 March 1840

Monsieur! I am eager to communicate to you one of the most striking facts of organic chemistry. I have confirmed the substitution theory in an extremely remarkable and completely unexpected manner. Only now can one appreciate the great value of this theory and foresee the immense discoveries that it promises to reveal.

manganese acetate [MnO + C4H6O3]

[Cl2Cl2 + Cl8Cl6Cl6]

[MnCl2 + C4Cl6O3]

[Cl2Cl2 + C4Cl6O3]

[MnO + C4Cl6O3]

On the Substitution Law and the Theory of Types

(letter to Justus Liebig)

For all I know, in the decolorizing action of chlorine, hydrogen is replaced by chlorine, and the cloth, which is now being bleached in England, preserves its type accor-ding to the substitution laws.* I believe, however, that atom-for-atom substitution of carbon by chlorine is my own discovery. I hope you will take note of this in your journal and be assured of my sincerest regards, etc.

S. C. H. Windler

* I have just learned that there is already in the London shops a cloth made from chlorine thread, which is very much sought after and preferred above all others for night caps, underwear, etc.

*

In 1849 Kolbe Prepared Free Methyl Radical

(electrolysis)

CH3 • CO2H CH3 CO2 H+ +

but a decade later molecular weight would show he had its dimer H3C-CH3

(Cannizzaro, 1860)

CO

HC

O

OH

O2

CO

OHC

O

OH

Cl2H3C CH2Cl

CO

OCH3

-e-

H3C CH3-CH3•

Curiously the key reactions of both radical and type theories did involve free radicals

(SOMO not HOMO/LUMO reactions)

CO

•

CO

OHH2C

•

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

End of Lecture 22Oct. 25, 2010

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