Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

AUTOXIDATION 213
Thus the autoxidation of the aldehydes leads finally to acids. That
a per-acid is first formed can be very easily shown in the case of acetaldehyde
by the immediate liberation of iodine from potassium iodide
solution which is caused by this strong oxidising agent. In the case of
benzaldehyde, which combines exceptionally rapidly with oxygen, it
has been possible to trap the per-acid with acetic anhydride as benzoylacetyl
peroxide (Nef) :
C6H5.C:O C6H5.C=O
I +(CH3CO)2O—> | +CH3.COOH.
O—OH O—O--CO.CH3
Experiment 2.-—One c.c. of freshly prepared acetaldehyde is
shaken for a few minutes in a long glass cylinder provided with a
tightly fitting rubber stopper. Half of the liquid is then poured into
a little dilute potassium iodide solution and half into two or three
volumes of water which is then tested with litmus paper to show the
presence of the acetic acid formed. It will be found that the aldehyde
which has been dissolved in water hardly liberates any iodine
from potassium iodide solution after standing for some time.
Experiment 3/—Two drops of benzaldehyde are exposed to the
air on a watch-glass for one hour.
For preparative purposes the method of obtaining aldehydes from
the primary alcohols is preferable by far, at least in the aliphatic series.
The simple aromatic aldehydes can be obtained by alkaline hydrolysis
of the arybdene chlorides, R.CHC12, which are produced from the
hydrocarbons by substitution with chlorine (technical method for the
preparation of benzaldehyde). In addition to these methods the elegant
synthesis of Gattermann and Koch should be mentioned here. This
synthesis, which proceeds like that of Friedel-Crafts, consists in acting
on the aromatic hydrocarbon with carbon monoxide and hydrogen
chloride in the presence of aluminium chloride and cuprous chloride.
H
>+CO + HCl
The carbon monoxide may be replaced by hydrogen cyanide (Gattermann)
or by fulminic acid (in the form of mercury fulminate, Scholl),
the first product then being respectively an imine or an oxime.
There is no absolutely general method for converting carboxylic
acids into aldehydes ; in many cases, however, the chlorine of acid
chlorides can be replaced by hydrogen activated by means of palladium
(Rosenmund).
C6H5.C=O 2H C6H5.C=O
Cl -> H