Page 15 - A HANDBOOK OF ORGANIC ANALYSIS QUALITATIVE AND QUANTITATIVE

14 ORGANIC ANALYSIS
In addition, most a-chloro ethers evolve hydrochloric acid
on treatment with water.
The nature of the vapours evolved on strongly heating
the substance, as well as the character of the flame formed
on ignition, constitute fairly definite indications of the general
class to which it belongs. Thus the odour of phenols, aromatic
nitro-compounds and aldehydes, and amines, as well as the
• decomposition-products of carbohydrates and certain hy-
droxy-acids, are all more or less characteristic ; while the rule
that unsaturated substances and saturated compounds con-
taining more than four or five atoms in the molecule burn
with a smoky flame is generally applicable.
The action of hot and cold concentrated sulphuric acid
should be tried. Although it is difficult to draw up a com-
plete table of inferences, it should be observed whether the
substance dissolves and whether change of colour, charring,
or effervescence is produced. The nature of any gases evolved
should also be examined.
The effect of heating in a tube with soda-lime and with
zinc dust should also be observed, any well-defined distillate
or sublimate being isolated and examined.
Unsaturated linkages may be detected by the action of
bromine water and of neutral permanganate solution. Un-
saturated compounds decolorise these reagents, while in
general saturated compounds do not. These reactions, how-
ever, do not afford a very certain test, as most polyhydroxylic
compounds, many aldehydes, certain acids and esters, most
phenols and ketones, and several other types of compound
are thus attacked.
Unsaturated compounds in which an ethenoid linkage is
conjugated with an aromatic nucleus may absorb bromine
but slowly. Hence this test must not be considered to have
failed until, after gentle warming, the mixture has stood for
at least five minutes without appreciable diminution in the
intensity of the colour of the bromine. Thus cinnamic acid
scarcely decolorises bromine water until the solution is
•