Johann von Baeyer | Robert Bunsen | August Wilhelm von Hofmann
Most students of chemistry will have heard of Friedrich Wöhler, as being the first person to create an organic compound (urea) from an inorganic source (ammonium cyanate). This was a real breakthrough as, in those days, chemical compounds were divided into two classes depending on their origin. Inorganic compounds had a mineral origin whilst organic compounds were derived from living things which were believed to possess a 'life force'. Thus it was not thought possible to create an organic substance from an inorganic one.
Friedrich Wöhler was born at Eschersheim, near Frankfurt, on 31 July, 1800. In 1823 he graduated from the University of Heidelberg as a Doctor of Medicine. However, on the advice of Leopold Gmelin, Professor of Medicine and Chemistry, rather than pursuing a medical career, he pursued his study of chemistry. He thus spent a year in the laboratory of Berzelius in Stockholm, whose methods of analysis he learned.
In 1825, Wöhler became a teacher of chemistry in the Municipal Technical School, Berlin. In 1831 he was appointed Professor at the Technical School at Kassel and in 1826 he became Professor of Chemistry at the University of Göttingen, where he remained until his death on 23 September, 1882.
Bridging the Gulf - The Birth of Organic Chemistry
During the 18th Century a considerable number of compounds, derived mainly from vegetable (but also some from animal sources were known), had been isolated and their properties studied. These included such substances as starch, sugar, certain oils, and some dyestuffs. There had, however, been little attempt to study these substances in a systematic manner. Their compositions were not known and they were classified according to origin and general characteristics rather than any chemical relationships. Due to their associations with the living world, that is 'organised' matter1, such substances were said to be 'organic'. This distinguished them from the inorganic substances which make up, or which can be formed from, inanimate mineral matter2.
Although Lavoisier had shown that organic compounds are, for the most part, composed of carbon, hydrogen and oxygen together with, sometimes, smaller amounts of nitrogen, phosphorus and sulphur - which are all substances belonging to the mineral kingdom, chemists held the view that some 'vital force' was necessary to assemble these elements into the complex compounds which make up the living world.
Meanwhile, Berzelius (1814) had shown that, like inorganic substances, the composition of organic substances followed the laws of fixed and multiple proportions and can be represented by formulae. Nevertheless, he still held the view that organic compounds were produced from their elements by laws different to those governing the formation of inorganic compounds. Thus, they could only be produced under the influence of a 'vital force' and could not be produced artificially.
This all changed in 1828 when Friedrich Wöhler, who had studied and worked with Berzelius, discovered that urea [H2N.CO.NH2], which occurs in the urine of mammals, birds and some reptiles, as well as in milk and blood, could be produced by heating ammonium cyanate (NH4CNO). Indeed, they had the same chemical composition. This was instrumental in stimulating Berzelius' work that led to the concept of isomers (from the Greek 'isos', equal and 'meros', a part). Isomers are compounds having the same chemical composition (molecular formula) but which have different chemical and/or physical properties. In a letter to Berzelius, Wöhler wrote:
I must tell you that I can make urea without the need of kidneys or of any animal whatever.
These words heralded the birth of modern organic chemistry, although for a number of years, this production of an organic compound from inorganic material remained the only one of its kind.
There was, however, still a small 'fly in the ointment'. Although this synthesis certainly weakened the distinction between organic and inorganic chemistry it did not sever it completely because critics pointed out that Wöhler had prepared his ammonium cyanate from ammonia and cyanic acid; both of which were of animal origin.
In contradiction to this, Partington (1960) has subsequently pointed out that Priestley (1781) had produced ammonia by reduction of nitric acid, which was later synthesised from its elements by Cavendish (1785). Also, potassium cyanide had been obtained by Scheele (1783) by passing nitrogen over a strongly-heated mixture of potassium carbonate and carbon. Since one form of carbon was graphite, this reaction also was carried out using only inorganic materials. Since potassium cyanide is readily converted to potassium cyanate, Wöhler's synthesis is one which can start from the elements.
Nevertheless, the distinction between organic and inorganic compounds had to wait until Kolbe's synthesis of ethanoic acid from its elements in 1845, before it was finally resolved.
The Quest for Organic Radicals
In 1815 Gay-Lussac had shown that 'prussic acid' is the hydrogen compound of a 'radical' or group of atoms, CN, which can pass unchanged from compound to compound. It thus behaves as an element, giving rise to compounds which are analogous to the chlorides.
In 1832, Liebig and Wöhler reported a more striking example of this with the radical of benzoic acid. They showed that the benzoyl radical, C7H5O - was present in 'oil of bitter almonds' (C7H5O.H; benzaldehyde), which itself oxidised to benzoic acid (C7H5O.OH). The chloride and cyanide of this could be represented as C7H5O.Cl and C7H5O.CN, respectively. As a result of these discoveries, organic chemistry was re-defined as being the chemistry of the compound radicals.
In 1845, Wöhler established the specific gravity (density) of aluminium, prepared using a refinement of Oested's method, thus discovering one of its unique properties - lightness. Aluminium has about one-third the density of other common metals.