Chemistry 125: Lecture 22 October 25, 2010 Radical and Type Theories ( ) 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 Wöhler: The oil of bitter almonds has come with 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 Wöhler: Here I am again in my gloomy lonesomeness 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 of Bitter Almonds C7H6O2C7H6O2 O2O2 C 7 H 5 OCl Cl 2 Liebig & Wöhler (1832) C7H6OC7H6O C 7 H 5 OBr Br 2 C 7 H 5 OI KI C 7 H 7 ON NH 3 C 14 H 10 O 2 S PbS So? Persistence of C 7 H 5 O Benzoyl Radical Bz H Bz OH Bz Cl Bz Br Bz I Bz NH 2 Bz 2 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 (Me 2 As ) 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) CH 3 methyl from methylene "Daughter of wood spirits" hence meth yl ene (Dumas, 1835) CH 3 OH = CH 2 + H 2 O (1852) ethylene C 2 H 4 : ethyl :: methylene : methyl : C 2 H 5 :: CH 2 : CH 3

C 3 C 4 C 3 H 7 Propyl C 3 H 6 Prop (yl) ene C 4 H 9 Butyl C 4 H 8 But (yl) ene From C 3 Propionic Acid (1847)  (protos first)  (pion fat) Derivatives of acids with >C 2 were like fats, unlike C 1 (formic) and C 2 (acetic), which mixed freely with water. From C 4 Butyric Acid (1826) Lat. butyrum (butter) from rancid butter

Jean-Baptiste André Dumas (7/14/ ) Post-Napoleonic Guardian of French Chemistry Persistent Opponent of Liebig and Berzelius Chaired Professor: Sorbonne École Polytechnique École de Médicine

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, printartist 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 us discussions 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 C 7 H 5 O HC 7 H 5 O ClClH Cl +=+ C 7 H 5 O Cl + - H Cl + - The electronic character of radicals was troublesome for Coulombic dualism. ++ C 7 H 5 O H ? e.g. preparation of benzoyl chloride

late 1830s s “Substitution” or “Type” or “Unitary” Theory

Tuileries Bal costumé au Palais des Tuileries  organic compounds can "fix" chlorine gas! Violently irritating fumes from wax candles spoil soirée at the Tuileries Palace (~1830) Dumas identifies the culprit as HCl from wax that had been bleached by Cl 2. Musée d'Orsay ( cough!choke!

Two ways wax could "fix" chlorine: Cl **  C CH2H2 H2H2 + ** + Addition of Cl 2 to an Alkene (HOMO/LUMO) Cl C C H2H2 H2H2 C C H2H2 H2H2 (actually both steps at once) HOMOLUMO HOMO 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 CH 3 Cl weak bond (58 kcal/mole) SOMO H Cl CH 3 Cl CH 3 Cl Cl single-electrons single-barbed arrows  "free-radical chain" “make-as- you-break” But still hard 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 C CH2H2 H2H2 + Addition of Cl 2 to an Alkene (HOMO/LUMO) Cl C C H2H2 H2H2 C C H2H2 H2H2

C 2 H 3 O OHC 2 H 2 ClO OH Photochlorination of acetic acid ClH Cl +=+ C 2 H 3 O OHC 2 H 2 ClO OH More Trouble for Radicals - Dumas (1839) Hydrogen may be substituted by an equivalent amount of halogen, oxygen, etc. Similar Acids! C 2 HCl 2 O OH C 2 Cl 3 O OH without changing molecular type. transmuted the acetyl "element".

Four Types Recognized by 1853 H H H Cl H H O H H NH C2H5C2H5 H O C2H5C2H5 H NH K C2H5C2H5 C2H5C2H5 C2H5C2H5 I + = C2H5C2H5 C2H5C2H5 O + KI Williamson Ether Synthesis (1850)

Butyl_Bromide Two-Word Relic of Radical Dualism Bromobutane One-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 On the Reaction of Chlorine with the Chlorides of Ethanol and Methanol and Several Points of the Ether Theory. On the Substitution Law of 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 and the Theory of Types. Justus Liebig *) Letter to J.L. On the Law of Substitutions and the 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 + C 4 H 6 O 3 ] [Cl 2 Cl 2 + Cl 8 Cl 6 Cl 6 ] [MnCl 2 + C 4 Cl 6 O 3 ] [Cl 2 Cl 2 + C 4 Cl 6 O 3 ][MnO + C 4 Cl 6 O 3 ]

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 ) CH 3 CO 2 HCH 3 CO 2 H + + but a decade later molecular weight would show he had its dimer H 3 C-CH 3 (Cannizzaro, 1860)

Curiously the key reactions of both radical and type theories did involve free radicals (SOMO not HOMO/LUMO reactions)

Genealogy Top

Archibald Scott Couper ( ) Dilettante Edinburgh/Berlin Classics, Metaphysics Logic, Moral Philosophy, Concerts 1855 Chemist Berlin 1856 Chemist Paris (Wurtz) - Salicylic Acid Fired Mental Invalid 1858

Kirkintilloch (near Glasgow)

Endrick Water where 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 22 Oct. 25, 2010 Copyright © J. M. McBride 2009,2010. 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).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