A Manual of the Chemistry of the Carbon Compounds

360 TBS CHEMISTRY OP
(4.) When the sodium salts of formic acid and benzenesulphonie
acid are fused together, sodium benzoate is formed:—
CjH^OjKa + CHOaNa - HNaSO, + C6HB.COaNa
Benzoic acid is also produced, together with phthalio acid
C-H^COgH)^, formic acid, and other products, by treating benzene
with manganese dioxide and dilute sulphuric acid. If some formic
acid be previously added to the benzene, the yield of beuzoic acid is
increased; and from this it appears that benzoio acid is produced by
the simultaneous oxidation of benzene and formic acid;—
C6H8 + CHaO2 + 0 = C6H6.COgH + HgO
Benzoic acid is obtained from gum benzoin by carefully heating
the coarsely-powdered resin in a shallow iron pan, over which a sheet
of paper pierced with a number of pin-holes is stretched, and the
whole is then covered with a cap of stiff paper. The vapour of the
acid condenses in the cap in light, feathery crystals, possessing the
fragrant odour of the gum, which is due to a small quantity of a
volatile oil A more productive method consists in boiling the resin
with milk of lime, concentrating the filtrate, and precipitating the acid
by hydrochloric acid.
Benzoic ncid, being used in the manufacture of aniline colours, is
now prepared in quantity from naphthalene C10tL, a hydrocarbon
occurring g in the heavy y coal-tar oil. It is first oxidized to phthalio p
acid id CJH^COJH) CH^COH) and d the th calcium li salt lt of f this thi acid id is i mixed id with ih
slaked lime, and heated for several hours to 300°—350°:—
2C6H4(CO2)aCa + Ca(OH)9 = (C6H6.COa)aCa + 2CaCo8
Phthalio acid may also be converted into benzoio acid by distilling
its ammonium salt, which is thus converted into pUhalimide
C6H.(COa)NH, On distilling this body with lime, it yields benzonitrue;—
C0H4(C0)8NH + CaO = C0HyCN + CaCO8
The nitrile is boiled with caustic soda, and the sodium salt decomposed
by hydrochloric ncid,
Benzoio aoid is also obtained from urine of cows and howea, as will
be described under Hippuric Acid,
Benzoic acid crystallizes from a hot aqueous solution in large, thin,
flexible, and brilliant needles or plates, melting at 120°. It boils at
250°, but sublimes readily at a much lower temperature, and vola«
tilizes by boiling its solution, It is only sparingly soluble in cold
water, but readily in boiling water and in alcohol. It has a peculiar
aromatic odour, winch, on heating the acid, becomes irritating, and
produces coughing.