Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

PAKALDBHYDB 217
more rapidly on heating. From the aqueous solution (formalin), in
which form it is usually prepared, the anhydrous aldehyde cannot be obtained,
since it only begins to distil with water vapour when the solution
is boiled, and even then distils very slowly. This is due to the fact that,
ATT
for the most part, it is present in solution as a hydrate, H 2 C < Q T T •
Acetaldehyde also polymerises gradually on keeping, yielding the
liquid paraldehyde (CH 3.CHO) 3, which boils without decomposition at
124°. If a little hydrogen chloride is passed, with cooling, into acetal-
dehyde the likewise polymeric metaldehyde crystallises out.
Experiment 8.-—To 5 c.c. of freshly distilled acetaldehyde in a
moderate-sized conical flask one drop of concentrated sulphuric acid
is added with cooling. W h e n the vigorous reaction is over, the
paraldehyde produced is shaken in a small separating funnel with
water in order to r e m o v e the sulphuric acid, and the polymeride,
w h i c h is insoluble in water, is separated if necessary b y extraction
with ether. After being dried with a little calcium chloride the
substance is distilled from a small distilling flask. Boiling point
124°.
Conversely paraldehyde can be reconverted into acetaldehyde b y
adding a few drops of concentrated sulphuric acid a n d distilling from
the water bath through a column.
B y this m e a n s fresh acetaldehyde can be prepared w h e n e v e r
required.
Experiment 9.-—Pure paraldehyde is tested b y the previously
described aldehyde reactions: with a m m o n i a c a l silver nitrate,
fuchsine-sulphurous acid, a n d bisulphite solutions. All are negative.
Vapour density determinations show that paraldehyde is formed b y
the union of three molecules of acetaldehyde. Since the substance has
no aldehydic properties the following structure, that of a cyclic triacetal,
is properly given to it:
CH 3
C H
0 0
CHg.CH CH.CHg
v
0
(Compare with this the polymerisation of acetylene to benzene.)
As was seen above, a small amount of concentrated sulphuric acid