Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

DETEKMINATION OF THE ACETYL GKOUP 83
adjust the rate of flow of air through the apparatus to 30 bubbles
per minute by regulating the precision screw clip and make the
ground joint A gas-tight with a little moist phosphorous pentoxide.
Make the joint of the funnel tube B gas-tight in the same way, run
2-3 c.c. of the 50 per cent sulphuric acid into the flask, insert the
glass rod S, and pour 1 c.c. of water into the funnel. Then heat
the contents of the flask under reflux to boiling and continue to
boil at a moderate rate.
The hydrolysis of O-acetyl compounds is usually complete in
thirty minutes, that of O-benzoyl compounds in sixty minutes. Up
to three hours are required for the complete hydrolysis of N-acetyl
and N-benzoyl compounds. The hydrolysis may be allowed to proceed
over night, using some concentrated sulphuric acid.
When the hydrolysis is complete carefully wash out the condenser
with 10-12 c.c. of water. Then distil up to 5 c.c. of liquid
into a small conical quartz flask. Use the condenser in the downward
position and if necessary add a few " boiling capillaries " to
the liquid in the flask. Eepeat the distillation three more times,
each time adding 7 c.c. of water. Test the distillate (volume about
20 c.c.) with some barium chloride for sulphuric acid (none should
be present), boil for seven to eight seconds and titrate at once with
0-033 N sodium hydroxide solution 1 from a micro-burette having
0-02 c.c. scale-divisions. Use phenolphthalein as indicator and continue
the titration until the colour becomes just pink and remams so
for a few seconds. For the second titration distil 2 x 7 or 3 x 7 c.c. and
for the third and fourth titrations only about 7 c.c. on each occasion.
Example
1st titration (about 20 c.c. of distillate) : 5-885 c.c. of alkali
2nd „ ( „ 2x7 c.c. „ ): 0-680 c.c.
3rd „ (,,2x7 c.c. „ ) : 0-040 c.c.
4th „ (,,1x7 c.c. „ ) : 0-040 c.c.
In the final titration not more than 0-05 c.c. of 0-033^ sodium
hydroxide solution should be required.
1 c.c. of 0-033 N sodium hydroxide solution corresponds to
1-434 mg. of COCH3 or 3-5033 mg. of COC6H5. Limits of error of
the method + 0-5 per cent.
In the apparatus described above C-methyl groups can also be
1 Determine the factor of the alkali by titration with oxalic acid of approximately
the same normality.