Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

ALDEHYDES 211
The aromatic aldehydes have a pleasant odour. (Oil of bitter
almonds, vanillin, piperonal.)
The most general method for the preparation of aldehydes consists
in removing two hydrogen atoms from a primary alcohol (afcohol dehydrogenatus)
; from secondary alcohols, in the same way, ketones are
formed. Since the hydrogen is generally removed by an oxidising
agent the process is regarded as an oxidation. Alcohols can, however,
also be decomposed catalytically into aldehyde and hydrogen. Such
decomposition is brought about by palladium black in the cold or by
copper at higher temperatures. The part played by the copper in the
preparation of formaldehyde (according to 0. Loew) consists in the
separation of hydrogen (dehydrogenation), and the purpose of the air
which is mixed with the methyl alcohol is to burn the hydrogen and
upset the equilibrium:
CH30H ^~~ > H2.C:0 + 2 H .
Substances having triple linkages when dissolved in sulphuric acid
take up the elements of water, particularly if mercuric salts are present.
The simplest example of this reaction is the preparation of acetaldehyde
from acetylene itself in the manner described above.
H,C—CH
HC=CH —> 0 —> I —> H3C.CH + HgSO4.
HgO-SO2
This process has great technical importance for the synthesis of
acetic acid.
The aldehydes are readily oxidised and therefore behave towards
ammoniacal silver solution and towards Fehling's solution as reducing
agents.
Experiment 1 .•—Dilute a few drops of formaldehyde or acetaldehyde
with a few c.c. of water, add a small amount of ammoniacal
silver solution, and divide the mixture between two test tubes. Into
one test tube run a few drops of sodium hydroxide solution; an
immediate separation of metallic silver takes place. From the
other solution after standing for some time in the cold, or more
quickly on warming, the silver separates. Thus the oxidising action
of ammoniacal silver solution is very considerably increased by
sodium hydroxide (Tollens). Also test the reducing action of the
aldehydes on Fehling's solution.
By oxidation aldehydes are converted into carbozylic acids. This
process is indeed a direct continuation of the dehydrogenation of the