The synthesis of the alkaloids _conine_, _atropine_, _cocaïne_, _piperine_, and _nicotine_ has been already referred to[5] as also that of _vanillin_, the aromatic principle of the dried fermented pods of certain orchids; _coumarin_, the odoriferous principle of woodruff and of the tonka bean; of _salicylic acid_, _oil of wintergreen_, _oil of mustard_, _bitter-almond oil_, and _camphor_. _Acetic_, _succinic_, _tartaric_, and _citric acids_ have also been artificially obtained, and may, indeed, be built up from their elements.
No synthesis of recent years created more widespread interest than that of _alizarin_, first effected by Graebe and Liebermann in 1868. Its successful commercial manufacture by Sir William Perkin in this country and by Caro in Germany created nothing less than a revolution in one of our leading industries, and completely destroyed a staple trade of France, Holland, Italy, and Turkey. To procure alizarin, anthraquinone is treated with sulphuric acid, and the product is fused with alkali and potassium chlorate.
The remarkable industrial results attending the synthetical formation of this madder-product naturally led to attempts to procure other important vegetable dye-stuffs artificially, notably _indigo_. The synthetical production of indigo has been accomplished by the joint labours of many chemists, notably Baeyer, Heumann, and Heymann, and the substance is now prepared on an industrial scale. The starting-point is _naphthalene_, obtained from coal-tar. This is converted into _phthalic acid_, which is then transformed into _phthalimide_. The last-named substance is converted into _anthranilic acid_, which, on treatment with monochloracetic acid, is changed into _phenylglycin_-ortho-_carbonic acid_. On melting this with caustic potash it yields _indoxyl acid_, which is transformed into _indoxyl_, and thence into _indigo_.
Another method is to treat the sodium salt of phenylglycin with sodamide, whereby _indoxyl_ is at once obtained, and this by condensation yields _indigo blue_:
C6H5.NH.CH2.COONa + Na.NH2 Sodium salt of Sodamide. phenylglycin.
CO CO / \ / \ →C6H4 C:C C6H4 \ / \ / NH NH Indigo blue.
Phenylglycin is obtained by the action of monochloracetic acid on aniline, which in its turn is obtained through nitrobenzene from benzene. Since benzene can be synthetically prepared by the condensation of acetylene, which can be obtained by the direct union of carbon and hydrogen at a high temperature, it is theoretically possible to build up indigo blue from inorganic materials.