Although these directions have been used many times with results
exactly as described, in a few cases the yields have dropped to 60-65
per cent without any apparent reason. At present no explanation
can be given for this.
In the Skraup synthesis of quinoline the principal difficulty has
always been the violence with which the reaction generally takes place;
it occasionally proceeds relatively smoothly, but in the majority
of cases gets beyond control, with consequent loss of material
through the condenser. By the addition of ferrous sulfate,
which undoubtedly functions as an oxygen carrier, the reaction
is extended over a longer period of time. It is thus possible
to work with much larger quantities of material when ferrous
sulfate is employed.
It is important that the materials should be added in the correct order;
should the sulfuric acid be added before the ferrous sulfate,
the reaction may start at once. It is also important to mix
the materials well before applying heat; the aniline sulfate should
have dissolved almost completely and the ferrous sulfate should be
distributed throughout the solution. To avoid danger of overheating,
it is well to apply the flame away from the center of the flask
where any solids would be liable to congregate.
In the apparatus for steam distillation, the greater portion
of the condensation is effected by the stream of water passing
over the receiver. It is, therefore, necessary that the stream
passing through the condenser should be sufficiently rapid to cause
it to form a uniform film over the receiving flask. A 12-l. flask
is even more efficient as a condenser than the 5-l. flask. It is
important that the tube through which the vapors leave the distillation
flask should be neither too short nor, especially, too narrow.
Where the external diameter of the steam inlet tube is 5-8 mm.,
the internal diameter of this steam head should be not less than 28 mm.
Were it less, the current of steam passing through it would be so rapid
as to prevent small quantities of liquid from returning to the flask,
and these would be driven over into the receiver.
Much time can be saved by the use of the steam distillation apparatus
described, especially when large quantities have to be handled.
The above directions avoid the use of extraction methods, which not
only consume more time but may lead to appreciable losses of material.
If the downward condenser is of iron, the apparatus is even more
efficient and the time for the steam distillation is halved.
The percentage yields have been based on the amount of aniline taken.
It would probably be more legitimate to base the calculation on the amounts
of aniline taken and of nitrobenzene not recovered, since undoubtedly
the latter is reduced to aniline during the course of the reaction.
If this be done, the yield is found to be only 55 to 60 per cent
of the calculated amount.
In a number of experiments, the glycerol used contained an appreciable
amount of water. Under these conditions, the yield of product
is much lower. "Dynamite" glycerol containing less than half a per
cent of water is best employed; U. S. P. glycerol contains 5 per
cent of water and usually gives lower yields.