Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

216 BISULPHITE COMPOUNDS
mentally difEerent for each of the three aldehydes, but in each case the
reaction begins with an addition :
H H .OH
—C : 0 + NH3 —> —C<
The reaction with acetaldehyde does not proceed further, but with the
other two water is eliminated and further changes take place.
Experiment 7.—A few drops of benzaldehyde are vigorously
shaken in a test tube with three parts of commercial bisulphite
solution. The crystals which separate are the sodium bisulphite
compound of the benzaldehyde.
The bisulphite compounds are formed according to the following
equation:
/OH
E—C:O + H8O3Na —> E—C<
H H\8O3Na
The problem of the constitution of the bisulphite compounds of the
aldehydes and ketones seemed to have been solved in accordance with
the above formula by the investigations of Easchig and Prahl. 1 It
follows from their work, that these compounds are salts of a-hydroxysulphonic
acids, the sulphonic groups of which are mobilised by the
adjacent hydroxyl group. G. Schroeter, 2 however, has obtained from
esters of dimethylmethanedisulphonic acid by hydrolytic elimination
of one sulphonic group in the manner indicated below the salt of the
a-hydrozyisopropylsidphonic acid and this salt is not identical with
acetone sodium bisulphite.
H3CX /8O3E H3CX /OH
3 X /
>C< +3NaOH—> >C< + Na2SO3 + 2 EOH .
\8O3Na
This contradiction has yet to be explained.
Since the bisulphite compounds are decomposed into their constituents
by warming with sodium carbonate solution or with dilute
acids, they are particularly useful for enabling aldehydes (and ketones)
to be separated from mixtures with other substances.
Polymerisation.—The simple aldehydes polymerise very easily. Anhydrous
formaldehyde, indeed, cannot be kept for long, but changes
very rapidly into an amorphous solid of high molecular weight (CH2O)n,
paraformaldehyde. At ordinary temperature this substance breaks
down again slowly to the simple molecule, a change which takes place
1 Annalen, 1926, 448, 265; Ber., 1928, 61, 179.
2 Ber., 1926, 59, 2341 ; 1928, 61, 1616.