Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

214 COLOUK KBACTIONS
Another possibility, which has already been of great use in many
syntheses, consists in the energetic reduction of hot esters with a little
alcohol and much metallic sodium to the corresponding alcohols
(Bouveault); these can then be oxidised to aldehydes in the usual way.
Experiment 4.—Colour reaction with fuchsine and sulphurous
acid. A fragment of fuchsine is dissolved in a large volume of hot
water to form an approximately 0-2 per cent solution, and after this
has been cooled, concentrated aqueous sulphurous acid is added
gradually until, after standing for some time, decolorisation has
taken place. The solution can be kept for a long time in a wellclosed
vessel. The sensitivity of the reaction should be tested with
progressively diluted solutions of formaldehyde and acetaldehyde.
When testing aldehydes which, like benzaldehyde, are sparingly
soluble in water, some alcohol is added. The alcohol must first be
tested since, on long standing, particularly when exposed to light, it
may contain detectable amounts of acetaldehyde. In the case of
formaldehyde the colour developed with the fuchsine solution becomes
pure blue by the action of concentrated hydrochloric acid
whilst with other aldehydes, under the same conditions, the colour
almost completely disappears. (Distinction between formaldehyde
and acetaldehyde.)
The colour reaction with fuchsine-sulphurous acid provides a
means of distinguishing sharply between aldehydes and ketones. In
dilute aqueous solution dextrose reacts negatively. Concerning the
mechanism of the colour reaction see Ber., 1921, 54, 2527.
Experiment 5. Angeli-Rimini Reaction.—A few drops of an
aldehyde (any of those prepared) are dissolved in aldehyde-free 1
alcohol and about the same amount of benzene sulphohydroxamic
acid (for the preparation of which see p. 192) is added ; in the case
of aliphatic substances, twice as much of the acid is used. To this
mixture, kept cool and shaken, 2 iV-sodium hydroxide is added, in
an amount judged to be about two molecular proportions. After
standing for fifteen minutes the alkaline mixture is made just acid
to Congo red and finally a drop of ferric chloride solution is added.
An intense red colour is produced.
It was mentioned on p. 193 that benzene sulphohydroxamic acid is
decomposed by alkali into benzene sulphinic acid and the very unstable
compound nitroxyl, 0=NH. If nitroxyl is produced in the presence
1 This ia naturally only important when, in actual practice, an unknown substance
is to be tested as to its aldehyde character.