The Vapor Phase Dissociation of Some Carboxylic Acids. II. Formic and Propionic Acids^1,2

Abstract

The vapor phase dissociation of formic and acetic acids has been studied in the pressure range below one-tenth atmosphere and the temperature range of 50-150°. The data are summarized by the equations logic K --3083¡T + 10.743 (formic acid); logw K = -3316/T + 10.834 (propionic acid). The data for formic acid agree with the results obtained by Coolidge.The data for propionic acid yield a heat of dissociation much lower than that reported by MacDougall. These data combined with those previously published for acetic acid suggest that the hydrogen bond strength in the acids considered increases in the order HCOOH < < CH3COOH < C2H5COOH. It is suggested that the strength levels off and remains essentially constant beyond propionic acid. Other data are summarized which support this order.

Study of resonances in formic acid by means of vibrational excitation by slow electrons

“…The equilibrium constant for dimer formation and its temperature dependence have been measured by Taylor and Bruton [19]. Their formula indicates the value K = 5.4 Torr at 35…”

A selected ion flow tube study of the reactions of various nitrogen containing ions with formic acid, acetic acid, and methyl formate

“…At this pressure, which is less than the saturated vapor pressure of all the liquids, the compound will be predominantly in the gas phase. However, there will still be a degree of dimerization within the vapors of HCOOH and CH 3 COOH and this degree of dimerization can be calculated from the measured literature equilibrium constants [19,20] (no such dimerization occurs for CH 3 OCHO so this complication is not present with this higher vapor pressure liquid). When the vapors are introduced into the flow tube, the equilibrium is such that a negligible amount of dimer will be present if there is sufficient time for equilibrium to be approached.…”

Ideal gas thermodynamic properties of methanoic and ethanoic acids

“…On the . assumption that the formic acid vapor contained only monomers and dimers, he derived· the equilibrium constants for the dimerization reaction at different temperatures and calculated the enthalpy change of this dimerization reaction to be -14.12 kcal mol-I, as shown in table 5, The P-V-T measurements of Coolidge were confirmed by later experimental determinations of Dawson and Haydon [2] and also Taylor and Bruton [3], and were selected as the most reliable data.…”

“…3 COOHb, was also included in this study. Furthermore, since these acid vapors contain both monomers and dimers at room tempelatule, the ideal ga:s thelmodynamic properties fen the monomer-dimer equilibrium mixtures of formic and acetic acids were also derived and compared with availa.ble experimental elata.…”