166 ODOROGRAPHIA. 
The synthetic formation of the odorous principle of vanilla can 
be effected from the starting-point of coal, the stages of the 
process being as follows :— 
Tar. 
Carbolic acid. 
Phenol. 
Potassium phenate. 
Paraoxybenzoic acid,—produced by the reaction of carbonic 
acid on potassium phenate (the analogous reaction with 
sodium phenate giving salicylic acid *). 
mixture at a low temperature. The most convenient method being to sift the 
tine powder over the surface of the hydrate, which is kept fused at a gentle heat. 
When the mixture has acquired a bright orange-brown colour, it is poured on to 
a stone or an iron plate, and allowed to cool. The cake is then broken jup and 
dissolved in about 20 parts of hot water, to which dilute sulphuric acid is added 
from time to time, so as to render the solution slightly acid. The dark brown 
solution is then allowed to stand for 24 hours, when it deposits a large quantity 
of sodic sulphate in crystals; the mother liquors which contain the proto- 
catechuic acid, after being filtered to remove a small quantity of tarry matter, 
are agitated with ether, the ethereal solution is then separated, and the ether 
recovered by distillation in the water-bath: the dark syrupy residue on standing 
deposits the protocatechuic acid in crystals. Or,—the mother liquors may be 
readily separated from the crystals by means of the vacuum-pump, and the acid 
is then easily purified by one or two crystallizations from a small quantity of 
hot water, with the addition of some freshly precipitated lead sulphide, which 
removes the brown colouring matter.” It crystallizes in monoclinic needles, 
containing one molecule of water which is lost at 106°, and melts at 194°; 
it dissolves in 53-55 parts of water at 14° and is very soluble in alcohol. 
* Salicylic acid is manufactured according to Kolbe’s process as follows :— 
The calculated quantity of pure phenol is dissolved in strong caustic soda 
solution, the whole evaporated to dryness and the residue rubbed into a dry 
powder; this is then gradually heated up to 180° C. in a metal retort, in a 
current of carbon dioxide which has been previously warmed. After some 
time phenol commences to distil over and is subsequently given off in larger 
quantity ; the temperature is then raised to 200° and the operation continued 
until no more phenol comes over. The residue is dissolved in water and 
fractionally precipitated with hydrochloric acid ; resinous and colouring matters 
are first thrown down, followed by tolerably pure acid, which is re-erystallized 
from water and purified by distillation with superheated steam (Compt. Rend. 
vill. p. 537). According to another patented process, carbonyl chloride, which 
is now manufactured on a large scale, is passed into a mixture of sodium 
carbonate and phenate heated to 140°, the temperature being finally raised 
to 200°. 
