In the Classroom
Amyl: A Misunderstood Word1 Richard A. Kjonaas Department of Chemistry, Indiana State University, Terre Haute, IN 47809 “Amyl” is not necessarily just another word for “pentyl”. Unfortunately, many chemists are not aware of this, so there is often a lot of confusion about it. This confusion comes about when reading textbooks, catalogs, handbooks, and bottle labels. Before giving some examples, however, it would be best to look at the history of this often misunderstood word. In 1492, Christopher Columbus crossed the Atlantic Ocean. One of the five “seeds of change” that altered the way of life on one side of the ocean or the other was the potato (1). (The other four were disease, corn, sugar, and horses.) Although Europeans were amused with the potato plant, they weren’t interested in eating this New World oddity—at least not at first. But potatoes produce more calories per acre than any other crop; so, as the population grew, Europeans not only began to eat potatoes, they also became dependent on them (2, 3). This was abundantly demonstrated, especially by the Irish potato famine. Of course this incredibly prolific source of food was an equally prolific source of ethyl alcohol (4). But there is something unique about alcohol that comes from the fermentation of potatoes as opposed to wheat, barley, rye, rice, and even corn. Alcohol obtained by the fermentation of potatoes is especially rich in fusel oil—up to 1.1% (5). Not surprisingly then, fusel oil has long been referred to as potato oil. It was also called amylic alcohol, and it turned out to be a very interesting and important material. The following excerpt is from an 1858 textbook (6). Amylic Alcohol. In the process of distilling whiskey from potatoes, there is generated, in connection with the crude spirit, a volatile, oil body, possessing a pungent, disagreeable odor. This substance, the complete separation of which from the crude spirit is a matter of difficulty appears to be analogous, in its composition and chemical reactions, to alcohol, and is regarded as the hydrated oxyd of a radical, termed amyle,—the hydrated oxide itself being called amylic alcohol, or more generally fusel oil (from the German fuseloel) or oil of potato spirits.* ...It exists in almost all ordinary alcohol in small quantity, and is the occasion of the persistent and somewhat faintly-disagreeable odor which alcohol leaves upon a surface after evaporation. The extraordinary character of the compounds and derivatives of amylic alcohol (fusel oil), render it one of the most interesting products of organic chemistry. *
Amyle derives its etymology from the Latin amylum, starch, the substance being a product of the fermentation of starch.
A slightly newer textbook (7) devoted about one half of an entire chapter (or “lecture”) to what it called the “potato oil group”, which included potato oil itself (amylic alcohol) along with some of its derivatives such as amyle (a hydrocarbon), amylic ether, chloride of amyle, and a few others. But what is this material called fusel oil or potato oil or amylic alcohol? The composition of fusel oil varies with the source, but in each case, the major component
is isopentyl alcohol (8). In fact, to this day, some chemical catalogs still list fusel oil, and this listing is usually accompanied with a structural formula of isopentyl alcohol or a note to “see isopentyl alcohol” (9). The rather foul odor of amylic alcohol is due to the presence of trace amounts of some nonalcoholic components such as aldehydes, acids, pyridine, and certain alkaloids. These components, along with some low molecular weight alcohols (especially isobutyl alcohol), can be removed by a combination of washing, chemical treatment, and distillation. The resulting product is called refined amyl alcohol; it is 85% isopentyl alcohol and 15% (S)-(–)-2-methyl-1-butanol. Refined amyl alcohol and its derivatives, especially amyl acetate, became important articles of commerce. For example, as the U.S. automotive industry began to grow, there was an increasing demand for amyl acetate as a lacquer solvent (5).2 So, the original amyl did not mean n-pentyl, but rather a mixture of isomeric pentyl groups; or since isopentyl alcohol was by far the main component of this mixture, it could be said that the original amyl really meant isoamyl! Does amyl still mean isoamyl? Perhaps not to the extent that it did just a couple of decades ago, but one still needs to be on the lookout. For example, in one recent chemical catalog “amyl benzoate” refers not to npentyl benzoate but to isopentyl benzoate [94-46-2] (10). In another catalog, amyl nitrite refers to isopentyl nitrite [110-46-3] (11). The current edition of the Merck Index (12) lists “amylacetic ester” as an alternate name for isoamyl acetate, and as recently as the 9th edition (13) it also listed “amyl acetate” as an alternate name for isoamyl acetate. This n-amyl/isoamyl confusion is evident even in some chemistry textbooks. At least one organic chemistry textbook (14) and several general/organic/biological chemistry textbooks (15) associate bananas with n-pentyl acetate rather than with isopentyl acetate (16). Perhaps some of these authors knew that bananas contain amyl acetate, and then incorrectly assumed that this meant n-pentyl. This is a natural assumption, since IUPAC rules tell us to use the words propyl, butyl, and pentyl in place of n-propyl, n-butyl, and n-pentyl. However, no such rule applies to amyl. Does the word amyl still refer to a mixture of isomeric pentyl groups? Yes, of course, but what leads to so much confusion is the fact that there still seems to be a tendency to use amyl with a word that is in the singular form even when referring to a mixture. For example, the Aldrich catalog uses the word “hexanes” (plural form) for a mixture of isomeric C6H14 compounds, but uses “amyl alcohol” and “amyl acetate” (singular forms) for those corresponding isomeric mixtures (17). When amyl refers to a mixture, it is often not a random mixture, but one that is available by current technology. Originally, this was the mixture available from fusel oil. When the supply of fusel oil could no longer meet the demand, it became necessary to make synthetic amyl alcohol. Beginning in 1926, this was done by the chlorination–hydrolysis of n-pentane and isopentane.
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In the Classroom
This gave a mixture of all of the amyl alcohols except neopentyl. Since the 1950s the oxo process (18) has been the main source of amyl alcohols. By this method, 1- and 2-butenes are converted to a mixture of n-pentyl alcohol and 2-methylbutyl alcohol, and isobutylene is converted to isoamyl alcohol. This accessibility is reflected in the composition of currently available fine chemicals such as amyl alcohol and amyl acetate (19, 20). In conclusion, it is perhaps best to avoid using the word amyl unless one uses n- or iso or some other prefix with it. If one wants to use the word amyl when referring to a mixture of isomers, then it is perhaps best to make the word plural, such as “amyl acetates” or “amyl alcohols”. The word amyl will probably not go away for quite some time, so chemists should certainly be prepared to exercise caution when encountering it. At the very least, when making up unknowns for an organic chemistry lab, never use anything from a bottle labeled “amyl alcohol”. One of my students and I learned this the hard way. Notes 1. Presented in part before the Division of Chemical Education at the 205th meeting of the American Chemical Society, March 28–April 2, 1993. 2. Although the heavy demand for fusel oil was met by using potatoes in Europe, corn was used in the United States.
Literature Cited 1. “Seeds of Change,” a special exhibition at the Smithsonian Institution in 1992 commemorating the quincentenary of the voyages of Christopher Columbus. 2. Meltzer, M. The Amazing Potato; Harper Collins: New York, 1992; Chapter 3. 3. Salaman, R. N. The History and Social Influence of the Potato; Cambridge University: Cambridge, 1949; reprinted 1970. 4. Gilbert, A. W. The Potato; MacMillan: New York, 1917; Chapter 14. 5. Hunt, K. C. In Encyclopedia of Chemical Technology; Kirk, R. E.; Othmer, D. F., Eds.; Interscience: New York, 1947; Vol.1, pp 844–849. 6. Wells, D. A. Principles and Applications of Chemistry; Iverson and Phinney: New York, 1858; p 448. 7. Draper, H. Textbook on Chemistry; Harper and Brothers: New York, 1872; pp 430–433. 8. Aries, R. S. In Encyclopedia of Chemical Technology; Kirk, R. E., Othmer, D. F., Eds.; Interscience: New York, 1947; Vol. 1, p 263. 9. For example, The Fisher Catalog; Fisher Scientific: Pittsburgh, 1995–1996; p 127C. 10. Rare and Fine Organic Chemicals; ICN Pharmaceuticals: Costa Mesa, CA, 1995–1996; p 29. 11. Pfaltz & Bauer Chemicals Catalog, 12th ed.; Pfaltz & Bauer Div. of Aceto Corporation: Waterbury, CT, 1991–1992; p 34. 12. Budavari, S., Ed., The Merck Index, 12th ed.; Merck: Rahway, NJ, 1996; p 876, item 5125. 13. Windholz, M. Ed., The Merck Index, 9th ed.; Merck: Rahway, NJ, 1976; p 672, item 4958. 14. Schumm, M. K. Understand Organic Chemistry; Macmillan: New York, 1987; p 369. 15. For example (a) Holum, J. R. Fundamentals of General, Organic, and Biological Chemistry, 5th ed.; Wiley: New York, 1994; p 485; (b) Ouellette, R. J. Introduction to General, Organic, and Biological Chemistry, 3rd ed.; Macmillan: New York, 1992; p 426; (c) Kroshwitz, J. I., Winokur, M. Chemistry: General, Organic, Biological, 2nd ed.; McGraw-Hill: New York, 1990; p 543; (d) Timberlake, K. Chemistry, 4th ed.; Harper and Row: New York, 1988; p 397; (d) Hill, J. W.; Feigl, D. M.; Baum, S. J. Chemistry and Life, An Introduction to General, Organic, and Biological Chemistry, 4th ed.; Macmillan: New York, 1993; p 399. 16. Issenberg, P.; Wick, W. L. Agric. Food Chem. 1963, 11, 2–7. 17. Aldrich Catalog Handbook of Fine Chemicals; Aldrich Chemical: Milwaukee, 1996–1997, pp 108, 791. 18. Papa, A. J. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J. I., Exec. Ed.; Howe-Grant, M., Ed.; Wiley: New York, 1992; Vol. 2, pp 709–728. 19. Ref 17, p 104. 20. SP Chemicals Catalog; Baxter Diagnostics, Scientific Products Division: McGraw Park, IL, 1994–1995; p 31.
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