formaldehyde - Sciencemadness Dot Org

STATE OF DISSOLVED FORMALDEHYDE 37
-of various concentrations, Delepine made use of the fact that an equilibrium
is obtained instantaneously when formaldehyde is added to dilute caustic
(page 118). Making use of this phenomenon, he found that so long as the
final concentrations of caustic and formaldehyde in the diluted solutions
were identical in each experiment, the quantities of heat evolved on diluting
formaldehyde solutions containing one gram mol of formaldehyde with
dilute caustic were equivalent, even though the concentrations of the solutions
varied from 1.5 to 30 per cent. Heat evolution due to changes in
caustic concentration was shown to be inappreciable under the conditions
of experiment, and it was also demonstrated that the Cannizzaro reaction
did not take place to any measurable extent. As a result of these findings,
Delepine concluded that the final value for the heat evolved when one gram
•of formaldehyde is dissolved in a formaldehyde solution is independent
of concentration up to concentrations of 30 per cent.
Solutions of Formaldehyde in Alcohols
Formaldehyde is readily soluble in the lower alcohols, giving solutions
which do not readily give up formaldehyde on heating. Delepine 2 observed
that a concentrated solution of formaldehyde gas in methanol boiled at
'92°C, 26°C higher than the normal boiling point of the pure solvent.
When pure, dry methanol is added carefully to liquid formaldehyde at
— 80 C C, a clear, mobile solution is first obtained without noticeable heat
evolution. On standing, a violent reaction takes place; heat is evolved,
.and the mixture solidifies. On warming to room temperature the solid
melts and a clear solution is obtained 24 . This solution probably contains
methanoi-formaldehyde addition products analogous to the hydration
products in aqueous solutions. These may be envisaged as hemi-acetals
having the structural formulas: CH3OCH2OH, CH3OCH2OCH2OH3
CH3OCH2OCH2OCH2OH, etc.
Approximate heats of solution for gaseous formaldehyde in methanol,
w-propanol, and ^-butanol at 23 C C 24 are shown below:
A H (Kcal per
Solvent Mol CH20)
Methanol 15.0
n-Propanol 14.2
n-Eutanol 14.9
The heat of solution of formaldehyde gas in water obtained in the same
^apparatus under similar conditions was 14.8 kcal per mol.
It will be noted from these data that approximately the same quantity
•of heat is liberated when formaldehyde is dissolved either in alcohols or in
water. The heat of polymerization for formaldehyde, although somewhat
lower than the 15 kcal value originally reported by Delepine 6 , is also of the
same order of magnitude. Since the carbonyl linkage in the formaldehyde