Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

82 THE ACBTYL AND BENZOYL GEOUP
round. If the operation is correctly carried out the colour of the
bromine disappears in a few seconds. Shake vigorously so as to
bring about absorption of the bromine in the gaseous phase. If
the colour of the bromine does not disappear within a few minutes,
the indication is that insufficient sodium acetate has been used.
To the decolorised solution add some dilute sulphuric acid and
about 1 g. of potassium iodide, cover the flask, and leave to stand for
five minutes. Then titrate the iodine liberated with 0-1N thiosulphate
solution from a micro-burette having 0-02 c.c. scale-divisions,
adding starch solution when the colour has diminished to
yellow and then continuing the titration until the colour disappears.
1 c.c. of 0-1N thiosulphate solution corresponds to 0-51706 mg.
of OCH3 and to 0-75067 mg. of OC2H5.
2. DETERMINATION OF THE ACETYL AND BENZOYL GROUP X
The substance is hydrolysed by boiling under reflux with 50
per cent sulphuric acid and the acetic acid or benzoic acid produced
is distilled in steam and titrated with sodium hydroxide solution,
using phenolphthalein as indicator.
The apparatus (Fig. 44) is supplied by the laboratory. The
following are required for the
determination:
50 per cent sulphuric acid.
Sodium hydroxide solution
0-033 N.
Carrying out the Determination.—
Fill the bubblecounter
with 50 per cent
potassium hydroxide solution
(non-frothing) and the
U-tube and drying tube at-
44 tached to it with calcium
chloride.
For the determination of acetyl or benzoyl weigh 20-30 mg. of
substance in the small weighing tube with long stem (see p. 70)
and transfer to the reaction flask. Set the condenser in the reflux
position, making the ground j oint C gas-tight with a drop of water,
1 R. Kuhn and H. Roth, Ber., 1933, 66, 1274.