Synthesis (Gr. synthesis, 'a putting together'), in Chemistry, is a term applied, in contrast to Analysis (q.v.), to the building up of compound substances from the elements they contain or from other compounds usually of less complexity than themselves. The term was formerly applied more particularly to the 'artificial' formation by the ordinary processes of the laboratory of substances which were at one time only obtainable as themselves products of the vital processes of plants or animals, or as derivatives from such products. Later the term was especially applied to the building up of complex carbon compounds; the greater part of modern organic chemistry consisting of such synthesis of new compounds of carbon. In the latest work on the synthetical methods for the preparation of organic compounds (that by Elbs, see below) the signification of the term has been still further restricted by the author (quite arbitrarily, however) to those operations only which give rise to compounds resulting from a new union of carbon with carbon.
Syntheses of new organic compounds are frequently the result of chance discoveries of new reactions, but a very large number of such reactions of more or less general applicability have been studied and classified, so that a new synthesis may be the result of a deliberate attempt to produce a new compound of a definite composition. Similarly the constitution of an important compound occurring in nature, such as, for instance, an alkaloid, is often known with tolerable certainty from the products yielded by it in a series of decompositions, and the information which has been gained from these decompositions indicates the lines upon which an attempt at synthesis may be made with a fair prospect of success. The mode of procedure indicated in the foregoing sentence has resulted in the synthetical or artificial production of numerous substances useful in the arts, in medicine, for domestic purposes, &c. As instances of substances naturally occurring in, or obtainable from, plants or animals, which have also been prepared synthetically, the following may be mentioned: indigo; alizarine, the colouring matter from madder; vanilline, the flavouring principle of the vanilla pods; citric acid, the acid of lemons; urea; uric acid, &c. Amongst the most familiar of synthetically prepared compounds not known as occurring naturally are a large number of the so-called coal-tar colours, saccharine, and many other coal-tar products, &c.
For further information, any large text-book of chemistry may be consulted; for instance, Roscoe and Schorlemmer's Treatise on Chemistry. Special works on organic synthesis are Lellmann's Principien der organischen Synthese (1887) and Elbs's Synthetische Darstellungsmethoden der Kohlenstoff-verbindungen (1889-91).