THE WORK OF CHEVREUL

The fame of Chevreul is mainly based on his discovery of several fatty acids, most of them currently named as Chevreul decided. Thus, he discovered and named stearic acid, butyric acid, capric acid and caproic acid. We also owe names like glycerin, cholesterol, stearin, olein, and cetyl, to Chevreul. He also discovered palmitic acid which he called margaric acid because of the aspect of its salt, and isovaleric acid, which he called phocenic acid after its source, dolphin oil. He concluded that the alkali used to saponify the fat has no influence on the types of fatty acids that are formed. He also observed that the sum of the weights of the fatty acids and the glycerol formed during saponification exceeded the weight of the fat by a few percent corresponding to water participating to the fat hydrolysis. He introduced the melting point as a way to characterise a mixture of fatty acids as obtained by the saponification of an animal fat. He purified lipids using recrystallization and was the first to control their purity by measuring their melting point, which he also introduced as a way to characterise lipids. Chevreul also proposed a theory according to which oils and fats consisted of mixtures of various compounds comprising a single fatty acid and glycerol. Thus, a liquid oil would contain relatively more of the oleic acid compound and a solid fat more of the stearic acid or the palmitic acid. This concept may be considered actually valuable. In the field of applied chemistry it must be recalled that with Gay-Lussac he patented the preparation of candles from stearic acid, which was a new way to light without smoke.

1 – FIRST ANALYTICAL PERIOD (1807-1811)

When Chevreul began his scientific career in the Vauquelin’s laboratory, he was introduced to the study of coloring principles in natural products.
In 1807, Chevreul began a series of research of real importance which occupied him until 1811, the year he shifted towards investigations of animal fats. Vauquelin asked Chevreul to separate natural dyes from the complex mixtures of oils, resins, gums, etc.. One of the first compounds he isolated was the red dye in Brazil wood.
The main works published at that time were :

– Expériences sur l’indigo. Ann Chim 1808, 66, 5-53
– Expériences chimiques sur le bois de Brésil et de Campèche; Extrait d’un travail sur les matières colorantes. Ann Chim 1808, 66, 225-265
– Analyse chimique de l’Isatis tinctoria et de l’Indigofère. Ann Chim 1808, 68, 284-312
– Mémoires sur les substances amères, formées par la réaction de l’acide nitrique sur l’indigo. Ann Chim 1809, 72, 113-142
– Mémoires sur les substances tannantes (précipitant la gélatine), formées par la réaction de l’acide nitrique sur plusieurs matières végétales. Ann Chim 1810, 73, 36-66
– Recherches chimiques sur le bois de Campèche et sur la nature de son principe colorant. Ann Chim 1812, 81, 128-169; 82, 53-85; 82, 126-147

From 1806 and during all that period, he published on a variety of topics, such as the effect of nitric acid on cork, the effect of nitric and nitrous acids on oxides, uric acid and others. He also separated sugar from the urine of diabetic showing that it corresponded in every way with grape sugar (glucose). While the presence of a sweet substance was ascertained earlier, its characterization as a substance similar to grape sugar was made for the first time by Chevreul.

– Examen chimique des os fossiles trouvés dans le département de Maine et Loire. Ann Chim 1806, 57, 45-50
– De l’action de l’acide nitrique sur le liège. Ann Chim 1807, 62, 323-333
– Note sur les urines de chameau, de cheval et sur l’acide urique des excréments d’oiseaux. Ann Chim 1808, 67, 294-308
– Mémoire sur différents composés, formés par la réaction de l’acide sulfurique sur le camphre. Ann Chim 1810, 73, 167-196
– Expériences chimiques sur le cartilage de Squalus peregrinus. Ann Museum Hist Nat 1811, 18, 136-153
– Sur la liqueur contenue dans les cavités intervertébrales de Squalus peregrinus. Ann Museum Hist Nat 1811, 18, 154-155
– Examen chimique des feuilles de Pastel et du principe extractif qu’elles contiennent. Ann Museum Hist Nat 1811, 18, 257-291
– Recherche chimiques sur le bois de Campèche et sur la nature de son principe colorant. Ann Chim 1812, 81, 128-169; 82, 53-85; 82, 126-147
– Note sur le tournesol. Ann Museum Hist Nat 1831, 20, 337-338
– Mémoire sur le moyen d’analyser plusieurs matières végétales, et le liège en particulier. Ann Chim 1815, 96, 141-189
– Note sur le sucre de diabétique. Ann Chim 1815, 95, 319-320

2 – CHEVREUL AND FAT CHEMISTRY

During the time Chevreul was the “preparator” of Vauquelin, he investigated the chemical nature of a sample of soap. From 1813 and during the ten years that he was Professor of Chemistry at the Lycée Charlemagne, in Paris, he published a long series of papers which dealt with saponification and fatty acids. Since the first report on soap chemistry by Tachenius in the 17th century and the work on oils and fats by Braconnot in 1815, no one had published so many scientific results on lipid chemistry.
In 1823, he summarized his research on fats publishing them in his famous memoir which can be considered as the first book of lipochemistry :

Recherches chimiques sur les corps gras d’origine animale
 Paris, FG Levrault, 1823. In-8°, XVI-484 p. et pl.
(Re-printed in 1889 by the National printing-house)

A PDF copy of this book has been numerized by Google and may be freely downloaded : Chevreul

From the beginning of this period, the first contributions were exclusively devoted to the study of soap and saponification.
In the first one (read at the Académie on July 5, 1813), Chevreul reported that, by diluting soap from pig fat in water, a portion dissolved and another deposited in small and pearly crystals. After decomposing this substance with an acid he prepared a solid fatty substance with acidic properties, in contrast to the original neutral fat. He named it “margarin” and later “margaric acid”. This was the origin of the concept of “fatty acids” which was developed when later he isolated  several other fatty acids from various fats. Moreover, this paper may be considered as the first devoted to lipochemistry.

“Recherches chimiques sur plusieurs corps gras, et particulièrement sur leurs combinaisons avec les alcalis. Mémoire 1 – Sur une substance nouvelle obtenue du savon de graisse et de potasse”, Ann Chim 1813, 88, 225-261

In the second one (read at the Académie on November 2, 1813), Chevreul reported that the portion of the soap dissolved in water contained a second acidic product, a liquid, which he named “fluid fat”, later named “oleic acid”. Thus, he has shown that the potash soaps consisted of two different soaps, formed by the union of potash with two different fatty acids. He announced also that he was able, after Sheele, to isolate the “sweet principle” in the water phase. He named it “glycerine” (Greek : glukeros, sweet, now glycerol) and concluded that fat was a combination of an anhydride of glycerol and an organic acid. Thus, during saponification, the alkali displaced the anhydride which combined with water to form glycerol, and the acid part of the fat combined with the alkali to form soap.

“Examen chimique du savon des graisses de porc et de potasse”, Ann Chim 1815, 94, 80-107

The third contribution (read at the Académie on April 4, 1814) was devoted to the saponification process. He first considered whether oxygen was necessary in running saponification in the absence of air. Thus, obtaining successful saponification, he disproved Fourcroy’s theory  that oxygen took part in the reaction. Furthermore, he confirmed his own theory by determining the carbon, hydrogen, and oxygen in the original fat and in the glycerol and acid after saponification. These results convinced him that water entered into the glycerol.

“De la saponification de la graisse de porc, et de sa composition”, Ann Chim 1815, 94, 113-144

In the fourth contribution (read at the Académie on May 8, 1815), Chevreul reported that the saponification products from pig fat were the same whatever the base he employed, the amount of oxygen being the same in the soap samples. Thus, he found that the fatty acids were typical acids which formed a distinct class of salts.

“De l’action de quelques bases salifiables sur la graisse de porc, et des capacités de saturation de la margarine et de la graisse fluide”, Ann Chim 1815, 94, 225-280

In the fifth contribution (read at the Académie on September 19, 1815), he reported the properties of “adipocire” (fat of corpses), spermaceti, and the crystalline substance present in biliary calculi. He showed that these calculi contained an unsaponifiable substance (thus at that time not a fat) but which was a pure compound (“an immediate principle”). Spermaceti was able to saponify but did not form a sweet principle. Chevreul was unable to analyze correctly that substance. Later, he described the presence of a fatty alcohol esterifying “margaric acid” which he named “ethal” (now cetyl alcohol). Finally, he found that adipocire was a mixture of fats.

“Des corps qu’on a appelés adipocire, c’est à dire, de la substance cristallisée des calculs biliaires humains, du spermaceti et de la substance grasse des cadavres”, Ann Chim 1815, 95, 5-50

In the sixth memoir (read at the Académie on August 26, 1816), Chevreul examined the composition of fats other than pig fats. He found that all fats could be resolved into two principles a solid one, “stearine”, and a liquid one, “elaine”, containing oleic acid. Furthermore, he decided that the difference in properties of the examined fats depended on the different proportions of these principles.
In this memoir, Chevreul coined the term “cholesterine” for the crystalline substance he purified from biliary calculi and the term “cetine” for the spermaceti.

“Examen des graisses d’homme, de mouton, de boeuf, de jaguar et d’oie” Ann Chim Phys 1816, 2, 339-372

In 1817, Chevreul published a seventh memoir in three parts on the composition of spermaceti and dolphin oil. After saponification, cetine gives “cetic acid” which he recognized as merely a mixture of margaric acid and oleic acid. From dolphin oil, Chevreul isolated a volatile fatty acid (“delphinic acid”) which he named later “phocenic acid”, recognised far later to be valeric acid (a 5 carbon acid) .

– “De la cétine (spermaceti)” Ann Chim Phys 1817, 7, 155-181
– “De l’huile de Delphinus globiceps”, Ann Chim Phys 1817, 7, 264-275
– “De l’acide delphinique”, Ann Chim Phys 1817, 7, 367-382

In 1823, Chevreul discovered by repeated crystallization of fatty acid baryum salts that in addition to the usual non-volatile fatty acids, cow and goat butter contained volatile acids. He referred to these compounds as butyric, caproic and capric acids. In another paper, the analytical results and formulas for these acids were reported. Far later, in 1861, he reported the presence of butyric acid in fruit of Ginkgo biloba originated from Montpellier which explained their smell of rancid butter (Comp Rendus 1861, 53, 1225-1226).

In the same year, Chevreul gave in his first famous book, “Recherches chimiques sur les corps gras d’origine animale“, a systematic presentation of his work on fats with comprehensive details on the experimental procedures he had used for 10 years and theoretical conclusions on the nature of fats and of saponification. 

He devised his book into six chapters.
– In the first, he gave definitions and a description of his method of elementary analysis based on solvent solubility, crystallization, saponification, melting point, distillation, and salt fractionation.
– The second chapter contains a description of all the species of fats and their saponification products. In this part, he described how he resolved the soap of sheep fat into two distinct components leading to the discovery of stearic acid, different from the early described “margaric acid” by its melting point.
– The third chapter contains a description of the preparation and isolation of the saponification products.
– The fourth chapter contains a comparison of the different animal fats.
– The fifth chapter contains a comparative examination of his analyses with regard to atomic relations and a discussion of saponification.
– The sixth chapter is a summary and a series of conjectures on the immediate components of fats. It must be noticed that these conclusions remained unchanged until the syntheses of fat principles by Berthelot in 1854 and the understanding that glycerin possessed a triple alcoholic function in 1850 (Berthelot and Wurtz).

In this original work, Chevreul presented for the first time a model of a complete set of research in organic chemistry while, in 1823, methods of chemical investigation were poorly developed. Thus, Chevreul may be considered as the father of lipochemistry but also of the whole organic chemistry.

In 1815, Braconnot H published data on the constitution of fats (Sur la nature des corps gras, Ann Chim 1815, 93, 225-278) similar to the first data obtained by Chevreul as soon as 1813. He showed in his memoir that oils and fats are not pure substances. He classified them in tallow, fats, and oils according to their fluidity and their relative content in solid or liquid compound. He used a mechanical process (compression between sheets of filter paper in the cold) to separate a fluid part from a solid one. By saponifying fats, Braconnot showed that soaps contained a solid and waxy compound and an oily compound which was soluble in alcohol, thus differing from a natural oil. He did not discovered the acidic property of the fluid component nor the presence of glycerol.

As Chevreul announced in 1813 and published in 1815 these results, he sent a letter to the journal “Annales de Chimie” claiming his priority and contesting the originality of Braconnot’s work (Lettre sur le mémoire de Braconnot, relatif aux graisses et à la saponification, Ann Chim 1815, 94, 73-79).

Following the achievement of his research on fats, Chevreul wrote in 1824 a general treatise on organic analysis, “Considérations générales sur l’analyse organique“, Paris 1824. In this work, he presented for the first time a clear and accurate account of the methods of immediate analysis which must necessarily precede an ultimate analysis.

The great Swedish chemist Berzelius JJ (1779-1848) wrote that Chevreul’s investigation was the most complete and best executed series of experiments yet found in all of chemistry. He asserted that Chevreul’s work ought to serve as a model for young scientists who wished to do research in any field of chemistry.

The German chemist Kopp HF (1817-1892) declared in 1844 that Chevreul’s work was a masterpiece of complete investigation in organic chemistry.

The German chemist Liebig claimed in 1841 that this work was the basis of all prevailing methods of analysis and investigations of organic materials.

As Chevreul accepted a new challenge in being appointed in 1824 director of dyeing at the Gobelins, the royal tapestry work in Paris, he abandoned near completely fat chemistry.
In 1850, he was officially in charge of a report on oil paint (Mem Acad Sci 1850, 22, 685-732) where he showed, nearly 50 years after De Saussure (1767-1845), that paint drying is accompanied by oxygen fixation which he has detected by a weight increase.

Another field of lipid chemistry was explored by Chevreul during his work on chemistry applied to dyeing, the composition of wool wax. He published several contributions on that complex mixture during 26 years without definitive results.

– Note sur les matières grasses de la laine. C R Acad Sci 1842, 14, 783-785
– Note sur la nature du suint de mouton. C R Acad Sci 1856, 43, 130-131
– Nouvelle communication sur la nature du suint de mouton. C R Acad Sci 1866, 62, 1015-1017

The contribution of Chevreul to lipid chemistry has been reviewed (Leray C, OCL 2023, 30, 9).