The Composition and Structure of Ether

The preparation of alcohol (spirit of wine, vinic alcohol, ethanol, ethyl alcohol) by fermentation dates to antiquity. Closely related to alcohol -- both through history and chemistry -- is ether (ethyl ether, diethyl ether) a compound obtained from alcohol by the action of oil of vitriol (sulfuric acid). When we use the term ether today, we mean specifically diethyl ether or perhaps the generic class of compounds of which it is a member. In the early part of the 19th century, the term "ether" was applied to any volatile including muriatic ether (ethyl chloride) and acetic ether (ethyl acetate).

Raymond Lully (Raimondus Lullius), a Spaniard born in Majorca circa 1235, after a rather dissolute youth, wrote extensively on Christianity and strove to convert the Muslim Moors. The Moors, who brought alchemy to Spain, had an influence on Lully. He described the action of oil of vitriol on spirit of wine to produce oleum vitrioli dulce verum (true, sweet oil of vitriol; ether) in 1275. A description of the preparation of ether by Valerius Cordus, a German botanist/apothecary, was provided in 1561 by the editor of his works, Conrad Gesner.

Fourcroy suggested that ether was formed by the removal of water from ethanol, which Saussere (1807) and Gay-Lussac (1815) confirmed. Boullay discovered that a small amount of sulfuric acid could produce large amounts of ether and water via a continuous process. This observation indicated that sulfuric acid was not functioning as a [stoichiometric] dehydrating agent but rather, as suggested by Mitscherlich and Berzelius, as a catalyst. Neither of them recognized the formation of sulfovinic acid (ethyl hydrogen sulfate). Frobenius observed that the residue from the preparation could form more ether when charged with alcohol; he also gave ether its name, spiritus vini aethereus.

At this juncture it was clear that ether and water were formed by the action of sub-stoichiometric amounts of sulfuric acid and that sulfovinic acid was formed as an intermediate in the reaction. At the time there was no concept of valence or bonds, but rather one of affinity. Electrochemical Theory or Dualism, which held that atoms that were electronegative were attracted to atoms that were electropositive in analogy to the world of inorganic chemistry, had as its chief proponent, Berzelius. Bear in mind that the voltaic pile [Allesandro Volta, (1800)] had been invented and that electrolysis was well-known [Davy (1802), Gay-Lussac (1807)]. If an inorganic substance such as sodium chloride could be formed from electronegative chlorine and electropositive sodium, it followed, so the thinking went, that ethyl chloride, had as its electropositive unit, the radical ethyl.

In 1815, Gay-Lussac had determined that the weight of one volume of alcohol vapor was equal to the weight of one volume of water vapor and one volume of olefiant gas (ethylene), while the vapor density of one volume of ether was equal to the sum of the vapor densities of two volumes of olefiant gas and one volume of of water vapor. Schematically, and in modern terms,

1) EtOH = CH2=CH2 + H2O

2) Et2O = 2 CH2=CH2 + H2O

Using a little algebra, one can express alcohol as a function of ether and water

3) EtOH = 1/2 Et2O + 1/2 H2O

as Dumas and Bouillay did in 1828. Based on these findings, these authors demonstated the similarity between certain ammonium salts and related organic substances based upon the "radical" etherin, C2H2. Although Dumas employed vapor density measurements in this instance, he was not convinced of their general applicability.

A new radical view of ether was independently put forth by Kane (1833), Berzelius (1833) andLiebig (1834). Liebig's paper [Ann. Pharm., 1834, 9, 1] interpreted, and wrote, Dumas's formulas a different way (Fig. 1). In the formula for ether [Aether] and that for alcohol shown below it, the stoichiometry of Dumas is retained but the four volume formula for etherin, 4 C2H2, is written as a one volume formula, C4H8, using subscripts and C=12. The third line is the formula for ethyl acetate [Essigäether, the source of the term ester]. The term "A with a bar over it" is acetic acid written in its anhydrous form, C4H6O3, just as Dumas had done. Ethyl acetate is construed by this formulation as ether and acetic anhydride. Stoichiometrically, it is true; chemically it is not.

Fig. 1

Liebig noted the etherin radical with the symbol "Ae", and described a new radical theory based upon the radical, C4H10, which he called the ethyl radical and symbolized as "E". Ether is viewed as an oxide of ethyl while alcohol is a hydrate of ether (Fig. 2).

[Dumas and Liebig were influenced by the teachings of Lavoisier who contended that the role of an element was to combine with oxygen. Since the salt barium acetate was written BaO + C4H6O3, then Liebig wrote ethyl acetate as C4H10O + C4H6O3.]


Liebig's radical theory is in keeping with the empirical formulas of the two substances but it argues that the vapor density of alcohol should be greater than that of ether, which it is not. Solution --- ignore vapor density measurements. Since water and ether are formed from alcohol, Liebig was obliged to retain this relationship in defining the composition of alcohol. Note that Liebig's empirical formula is correct for sucrose (Fig. 3 & 4) but incorrect for glucose [halved formula = C6H14O7. Is this a monohydrate of glucose, C6H12O6?], just the reverse of Dumas's data.

Fig. 2

Fig. 3
Fig. 4
E = C4H10
Formula
German
English
E + O
C4H10O
Aether
ether
EO + H2O
C4H12O2
Alcohol
alcohol
E + Cl2
C4H10Cl2
Chlorowasserstoffäether
ethyl chloride
EO + Ox (C2O3)
C6H10O4
Oxaläether
ethyl oxalate
EO + A(bar)
C8H16O4
Essigëther
ethyl acetate
4CO2 + 2EO + 4H2O
C12H28O14
Traubenzucker
glucose
4CO2+ 2EO + H2O
C12H22O11
Rohrzucker
sucrose


Liebig was the influential editor of the Annalen der Chemie und Pharmacie at this time, a position that allowed him to attack those with whom he disagreed. He was quick to chastise others for the carelessness of their chemical ways, but like all humans, failed to see the errors of his own. One need only cite Liebig's inability (1831) to determine correctly the composition of chloroform formed by the action of alkali on chloral (trichloroacetaldehyde), a compound that Liebig had prepared by the action of chlorine on acetaldehyde. Appropriately, it was Dumas who corrected him. Liebig pressed Dumas to accept the ethyl radical theory in place of Dumas's own etherin theory. In 1837 Dumas and Liebig issued a joint paper before the Paris Academy that intimated that all that was needed to be known can be explained by radical theory and that in organic chemistry the radicals are complex while in inorganic chemistry they are simple. Dumas's interest in the restricted view of radical theory was undoubtedly short lived, for he was on the verge on a new approach to explaining the nature of organic compounds --- substitution and type theory. In brief, Dumas (1839) was able to demonstrate that chlorination of acetic acid formed trichloroacetic acid by substitution of 3/4 of the hydrogens in acetic acid with chlorine, and that the two acids were of the same type, owing to their similar chemical properties. Be reminded that the substitution of electronegative chlorine for electropositive hydrogen was anathema to the dualists, Berzelius in particular.

Berzelius died in 1848; his influence had diminished during the 1840's as he made valiant attempts to retrofit dualistic theory to the new facts of substitution theory. In the same year, Liebig moved from Giessen to Munich and, perhaps heeding Wöhler's admonition, divorced himself from the murky theories of organic chemistry. The Frenchmen, Auguste Laurent (1808-1853) and Charles Gerhardt (1816-1856), brought about new ways of thinking about organic structures. In particular, Gerhardt was able to demonstrate that the formulas used by Liebig should be halved, that organic compounds should be considered as a single entity (unitary theory), and through his theory of residues (the second type theory), formulated the double decomposition reaction. For all his insight, his personality was intransigent, which did not endear him to his more senior French chemists. Moreover, he died at the young age of 40 believing that one could never know the true structures of molecules.

[What does it mean to know the structure of a molecule? Today, we can say we know more than Gerhardt knew. We have NMR spectra and x-ray structures but they are manifestations of the molecule. Do we truly know reality? This is not a new problem? Plato's The Allegory of the Cave]

In the eventful, chemical year 1848, Wurtz prepared primary amines from ammonia by reaction with alkyl halides, and in the following year, Hofmann reported the formation of secondary and tertiary amines from ammonia by the same process. The similar properties of ammonia and the amines did not escape the attention of both Wurtz and Hofmann. With this work and subsequent efforts, Hofmann established the ammonia type.

The story now moves to England in 1850 and the contributions of Alexander Williamson (1824-1904). Williamson, blind in one eye and paralyzed in one arm, was to provide the critical experiment to confirm Gerhardt's ideas of type and double decomposition, and simultaneously define the water type. But this is not what Williamson had set out to do. He attempted to form a higher homologue (n-butyl alcohol) [term coined by Gerhardt] of ethanol by reacting the potassium salt of ethanol with ethyl iodide. Ethyl iodide was a compound first prepared by Liebig and employed by Frankland in the discovery of organometallic compounds, an investigation that led to the valence theory (1852). In Williamson's own words:

["The following experiments were made with the view of obtaining new alcohols, by substituting carburetted hydrogen for hydrogen in a known alcohol." Theory of Etherification, A. W. Williamson, J. Chem. Soc., 1852, 4, 106.]

[Iodide of potassium was readily formed on the application of a gentle heat, and the desired substitution was effected; but, contrary to expectation, the compound thus formed had none of the properties of an alcohol -- it was nothing else than common ether, C4H10O." loc. cit.]

It may be justifiably argued that the best experiment is the one that does not lead to the desired result but rather to the unexpected one. For the unexpected path leads to new avenues of discovery and understanding. Such was the case with Williamson because he did not isolate a homologous alcohol, but rather ethyl ether! Here was a totally new way of preparing ether under alkaline conditions as opposed to the centuries old reaction of sulfuric acid with ethanol. The formation of both potassium iodide and ether gave support to Gerhardt's ideas of double decomposition. Williamson formulated the reaction as follows (Fig. 5):

Fig. 5
[Some chemists may perhaps prefer doubling them [formulae], in order to avoid the use of atoms of hydrogen, potassium, &c.; but the author has not felt himself justified in doing so, because that would involve doubling the usual formula for water; for, as will presently be shown, water is formed in etherification by replacing the carburetted hydrogen of alcohol by hydrogen, which, of course, obliges us to assume the unity of oxygen in water. Alcohol is therefore water in which half the hydrogen is replaced by carburetted hydrogen, and ether is water in which both atoms of hydrogen are replaced by carburetted hydrogen, thus:", Fig. 6, loc. cit.]  
Fig. 6

Here we see Williamson define the water type. His chemical representations transcend the linear, condensed formulas of the dualists and radical theorists; they begin to look more like structural formulas without atom connectivity. By defining the water type, he demonstrated that water was H2O, and not OH, as had been maintained by many of his contemporaries, a tenet that was rooted in Dalton's "rule of greatest simplicity". Inherent in Williamson's proposal is the recognition that alcohol is not a hydrated ether from which water has been removed by the traditional action of acid. Moreover, it is not an oxide of ethyl, C4H10. Williamson was quick to anticipate the objections of those bound to the tradition of double formulas and the view of ether as a hydrate.

["This formation of ether might, however, be explained, after a fashion, by the other theory --- by supposing the potassium compound to contain ether and potash, which separate during the action of the iodide of ethyl; so that half the ether obtained would have been contained in that compound, and the other half formed by double decomposition between potash and iodide of ethyl, thus:" Fig. 7, loc. cit.]

Fig. 7

The equation above is merely a reformulation of the argument that alcohol is hydrated ether, and the formation of ether occurs under alkaline conditions. But Williamson was prepared to counter this argument.

["But although the insufficiency of this explanation becomes evident on a little reflection, a further and more tangible method of arriving at a conclusion was devised. It consisted in acting upon the potassium compound (i.e., C4H10.K2O) by iodide of methyl, in which case, if that compound C4H10.K2O) were ether and potash, the resulting mixture should consist of ether (C4H10O) and oxide of methyl (C2H6O); whereas, in the contrary case, (i.e., Williamson's formulation) a body of the composition C3H8O should be formed. Now this substance was actually obtained, and neither ether nor oxide of methyl." loc. cit.]

Thus, Williamson was able to dispel the notion that diethyl ether and dimethyl ether can each arise from their respective carbon sources, potassium ethylate and methyl iodide, but rather there is a union of the carbon residues of each reactant to form "this remarkable body", ethyl methyl ether. In yet another experiment, he demonstrated that this mixed ether was formed in the inverse reaction wherein potassium methylate reacts with ethyl iodide.

Williamson's next challenge was to explain the formation of ether from alcohol by the action of sulfuric acid. He did so by writing two double decomposition reaction equations that demonstrated the intermediacy of sulfovinic acid (ethyl sulfuric acid) and the catalytic role of sulfuric acid. In his words:

["...it consists in stating the fact, that sulphuric acid and alcohol are transformed into sulphovinic acid and water, by half the hydrogen of the former changing places with the carburetted hydrogen of the latter; thus: (Fig. 8)
 Fig. 8
Now from this point it is clear that the process is the is the same as in the decompositions above described; for by this sulphovinic acid coming in contact with an atom [molecule] of alcohol, it reacts exactly in the same manner as the iodide did, forming of course sulphuric acid and ether: (Fig. 9)
 Fig. 9
The sulphuric acid thus reproduced comes again in contact with alcohol, forming sulfovinic acid, which reacts as before; and so the process goes on continuously, as found in practice."]

In a subsequent paper, [On Etherification, A. W. Williamson, J. Chem. Soc., 1852, 4, 229.] Williamson presented a mechanistic view of how the double decomposition reaction might occur. This diagram may well have been one of the earliest attempts to define the mechanism of a chemical reaction.

["The reaction is easily understood by the following diagram, in which the atoms C2H5 and Na are supposed to change places by turning round upon the central point A." (Fig. 10)]

Fig. 10

This issue of the J. Chem. Soc. is not without further excitement in this matter. On pg. 111, Hermann Kolbe, an editor of a journal, as was Liebig, and a staunch, vituperative defender of the old ways, attacked Williamson's ideas in a paper entitled, Critical Observations on Williamson's Theory of Water, Ethers, ansd Acids. Unfortunately, Kolbe was beating a dead horse in his attempts to bring soon to be outdated ideas to challenge Williamson, and Gerhardt's, new vision. On pg. 122, a rebuttal by Williamson entitled, On Dr. Kolbe's Additive Formulae, he states:

["It thus becomes incumbent on me to offer a few further remarks on the subject; and in analysing his [Kolbe] arguments, I shall unavoidably be led to explain, more particularly than I wish to do, the characteristic defects and errors of Dr. Kolbe's theoretical notions, to which his original misconception was owing. As the discussion is of Dr. Kolbe's own seeking, he will of course not be offended at my freedom in criticising his views."]

Williamson's work on the water type inspired Gerhardt (1853) to explore the reactions of the salts of carboxylic acids with acid halides, thereby preparing the true anhydrides, which, in his view, were of the water type. These formative studies allowed Gerhardt to define the basic types: H2O, NH3, HCl, and H2. Through the efforts of the type theorists, organic chemistry was placed on a more rational basis than either dualism or radical theory could afford. While our approach to the subject is more sophisticated today, type theory through the concept of functional groups, is the predominate organizational motif of today's textbooks in the field.

Sources: Resources in addition to those cited.

1. Brock, W. H., Chemistry, 1994.
2. Crosland, M. P., Historical Studies in the Language of Chemistry, 1962.
3. Crosland, M., Gay-Lussac, Scientist and Bourgeois, 1964.
4. Partington, J. R., A History of Chemistry, Vol. 4, 1964.
5. N.B. After completing this document, the following excellent paper was uncovered: Priesner, Claus, Spiritus Aethereus - Formation of Ether and Theories on Etherification from Valerius Cordus to Alexander Williamson, Ambix, 1986, 33, 129.

copyright FEZiegler2001