From Resinous Tar to Molecular Baskets

"A man is wise with the wisdom of his time only and ignorant with its ignorance. Observe how the greatest minds yield in some degree to the superstitions of their age"

Henry David Thoreau, Journal, 1951

1.1 The Resinous Tar

The path of scientific research is seldom straight, often taking twists and turns quite unexpected at the outset of an odyssey. Such is the case with phenol-formaldehyde chemistry, which began over a century ago in the laboratories of Adolph von Baeyer. It has developed in ways that could not have been foreseen by this eminent scientist but which would certainly provide him with amusement and delight were he still alive to enjoy the passing scene of twenty-first-century chemistry.

Johann Friedrich Wilhelm Adolph von Baeyer was one of the great organic chemists of the nineteenth century and received the Nobel Prize in Chemistry in 1905. Born in Berlin in 1835, he received his training in chemistry from Bunsen and Kekulé in Heidelberg. His first professional appointment was at the University of Ghent with Kekulé, but his career as an independent investigator really began in 1860 when, at the age of 25, he joined the staff of the Technical Institute at Berlin. Twelve years later he moved to Strasbourg as director of the chemical laboratories of the university where his talents as a teacher and researcher blossomed, leading in 1875 to an appointment as successor to the great Justin Liebig at the University of Munich. He held this position until his retirement many years later. Although Baeyer was best known for his elucidation of the structure and synthesis of the naturally occurring dye indigo as well as his work on carbocyclic ring compounds, he also delved into many other areas of organic chemistry. Among these was a brief foray into the reaction of phenols with formaldehyde. Two short papers' were published from Berlin in 1872 followed by a somewhat longer one later that year after he had moved to Strasbourg. These appeared in Chemische Berichte and describe the results of mixing aldehydes and phenol in the presence of strong acids.

In the first of the three papers Baeyer simply reported that a thickening of the reaction mixture of aldehydes and phenols occurs with the formation of a "kittartige Substanz" ("a cement-like substance"). Specific examples of these reactions were discussed in more detail in the second paper, including the reaction of benzaldehyde and pyrogallol (benzene-1,2,3-triol), which gave a red-brown, resin-like product. Not until the third paper, however, was the reaction with formaldehyde introduced, heralding the beginning of the field of phenol-formaldehyde chemistry. This delay can probably be attributed to the fact that in 1872 formaldehyde was a rare chemical not easily available. Baeyer had to prepare it by reducing iodoform (CHI3) with HI and red phosphorus to methylene diiodide (CH2I2) and then replacing the iodine atoms with oxygen moieties by treatment with silver acetate in acetic acid. This yielded the liquid which Baeyer formulated as

(i.e. the adduct of HCHO and CH3CO2H), recognizing with great insight the proclivity of the C=O group of formaldehyde to add proteated nucleophiles. Thus, acquiring formaldehyde in sufficient quantity for use in reactions was a major undertaking, in marked contrast to today where it is one of the world's cheapest and most readily available chemicals (current production exceeds 2.5 million tons per year).

Using formaldehyde synthesized as described above, Baeyer showed that it reacts with phenol in a fashion similar to that of the larger aldehydes to produce a resinous material. However, he was unable to isolate pure substances from any of these reactions and, therefore, could not obtain elemental analyses which might have allowed him to propose possible structures. In retrospect we can appreciate how formidable a problem he faced, for even today with our impressive array of analytical techniques the structure of the phenol-formaldehyde product is not known in complete detail. However, Baeyer did manage to provide some structural insights by proposing a dimesitylmethane structure for the more tractable product obtained from mesitylene and formaldehyde. So, even though he was unsuccessful in characterizing the phenol- formaldehyde product, Baeyer had, nevertheless, given birth to phenol-formaldehyde chemistry.

The next chapter in this nascent field occurred in 1894 when two German chemists, L. Lederer and O. Manasse, independently studied the base-induced reaction of formaldehyde with phenol and succeeded in isolating o-hydroxy-methylphenol and p -hydroxymethylphenol as well-defined crystalline solids, viewing the reaction as a dehydration process in which formaldehyde reacts in its hydrated form, viz.

o-Hydroxymethylphenol was of particular interest, because it had already been isolated a half century earlier as a naturally occurring compound present in nature as a glucoside. The success of the Lederer-Manasse reaction, as it came to be called, depended on the use of mild and well-controlled conditions. Under more strenuous conditions the base-induced reaction, like its acid-catalyzed counterpart, yields a resinous tar of ill-defined structure.

At the dawn of the twentieth century phenol-formaldehyde chemistry, introduced a quarter of a century earlier, remained a rather intransigent and largely unattended area of investigation that seemed to hold little reward for explorers intrepid enough to set foot within its confines. But, explorers there were, such as Blumer, Storey, Luft and others, all of whom tried to tame these resinous tars and find practical applications for them. For one reason or another, however, all failed to produce materials with marketable qualities.

Success was to go to another explorer, Leo Hendrik Baekeland, who was born in 1863 in Ghent, Belgium. From early childhood Baekeland's brilliance as a scholar was evident, and by 1884 at the youthful age of 21 he had already acquired a Doctor of Science degree. After several years of restless associations with chemistry departments at Ghent and Bruges in Belgium, Cambridge and Oxford in England, and Edinburgh in Scotland, Baekeland and his young wife sailed to America in 1889, there to continue his research on photographic papers.

This eventually led to a commercially successful material that he called "Velox", a name still known to an older generation of pre-digital photographers. So successful was this product that the Velox process was purchased by George Eastman in 1900 for one million dollars, making Baekeland a wealthy man at the age of 37. Constrained by the terms of the sale from continuing to carry on his photographic...