formaldehyde - Sciencemadness Dot Org

MONOMERIC FORMALDEHYDE 23
Dieke and Kistiakowsky photographed the absorption spectrum under
high dispersion so that the rotational structure could be carefully analyzed.
From these photographs, they have calculated the molecular constants
for the normal state of the molecule and six vibrational states of the
excited electronic levels.
The infrared absorption spectrum of formaldehyde shows a weak maximum
at l—9ju and intense absorption at 3-5, 5-7, and 6-7jU with two bands
in the range 7.5 to approximately 10/x. Nielsen and co-workers 16 ' 17 - 1&
have made an exhaustive study in this field supplemented by the comparative
study of deutero-formaldehyde, CB2O. Other studies in the infrared
have been reported by Salant and West 24 , Titeica 34 and Xordsieck 20 .
The Raman spectrum of aldehydes 13 show carbonyl frequencies in the
neighborhood of Av 1720 and additional characteristic frequencies at Av
510 and 1390 in the simpler aldehydes. In the case of gaseous formaldehyde
10 , there is apparently a shift corresponding to Av 1768.
Properties of Liquid Formaldehyde
As previously stated, liquid formaldehyde polymerizes rapidly even at
temperatures of —S0°C unless extremely pure. However, even the purest
samples show signs of polymerization after about four hours. When
the liquid is heated in a sealed tube, polymerization attains almost explosive
violence. Satisfactory agents for inhibiting polymerization have not yet
been discovered, although Spence 27 states that quinol increases stability
to a slight extent.
According to Kekul^ 12 , the density of the pure liquid is 0.9151 at —80°C
and 0,8153 at —20°C. The mean coefficient of expansion calculated from
these values is 0.00283, a figure similar to the expansion coefficients of
liquid sulfur dioxide and liquid ammonia.
The partial pressure of the pure liquid has been recently measured over
a wide range of temperatures by Spence and Wild (Table 2) 29 . According
to these investigators, the partial pressure of liquid formaldehyde may
be accurately calculated with the following equation:
logioP* -1429/T + 1.75 log T -OMffiT + 3-010S
The heat of vaporization of liquid formaldehyde at its boiling point
(— 19.2°C), as calculated from the partial pressure, is 5.570 kcal per gram
moL Trouton's constant, AH/T, is 21.9 entropy units. The critical
temperature of the liquid has not been determined because of its rapid
polymerization on being warmed.
At low temperatures liquid formaldehyde is miscible in all proportions
with a wide variety of non-polar organic solvents, such as toluene, ether,
chloroform, and ethyl acetate. According to Sapgir 25 , solutions of liquid