A Manual of the Chemistry of the Carbon Compounds

254 THE CHEMISTRY OF
Glycerin was first obtained by heating olive oil, water, aitd lead
oxide; insoluble lead salts are thereby formed (lead plaster), while
glycerin remains in solution. The latter is freed from lead by
hydrogen sulphide, and the solution evaporated. Glycerin is now
obtained in large quantities and in a state of great purity in the
manufacture of stearin candles. The fats, as tallow, &c, are distilled
by means of overheated steam; the distillate separates into two
layers, the lower one consisting of an aqueous solution of glycerin,
while the upper one contains the acids, The solution of glycerin is
concentrated by evaporation, and the residue again distilled with
steam in an apparatus in which glycerin oondenses, whilst the much
more volatile steam escapes.
Glycerin can also be produced artificially from propaiw. By tho
action of chlorine on this compound a large quantity of propeno
dichloride is formed, which, when heated with iodine chloride, is converted
into propenyl trichloride CoILClj. This chloride ia decomposed
by heating with water to 170, the products being hydrochloric
acid and glycerin.
Pure glycerin is a colourless, very viscid liquid, having a specific
gravity of 1'27. It can be mixed with water and alcohol m all
proportions, and possesses a very sweet taste. When exposed to
strong winter-cold it sometimes solidifies to transparent and strongly
refractive hard crystals; but commonly it remains liquid, and
solidifies only at —40° to an amorphous gum-like mass. When quickly
heated it distils at 280°, with partial decomposition; but under a
diminished pressure it may be distilled without alteration, boiling at
210° under a pressure of 50 mm.
When glycerin is added to an alcoholic solution of sodium ethylate,
(OH
the compound CaH6-< OH + CgH^OH separates out in small crystals
(ONa
grouped in stars, which at 100° lose alcohol, a white deliquescent
powder of mnosodium gli/cerate being left behind,
When yeast is added to an aqueous solution of glycerin, and tho
mixture is allowed to stand at a temperature of 20° to 30° for some
months, propionic acid is produced.
Glycerin is easily reduced to secondary propyl iodide by distilling
it with an excess of concentrated hydriodio acids—
C8H8O3 + 5HI « C3H7l + 3H,0 + 2I2
Secondary propyl-iodide is generally obtained by this method; in
preparing it amorphous phosphorus is added to the mixture, which
converts the free iodine again into hydriodic acid:—
+ H2O + P + I = C3H7I +
Mmwchlorhydrin or Propenyl Monochlorhydrate CjH6Cl(OH)2 is