DISSOCIATION CONSTANTS OF TRIAZO-COMPOUNDS. 925

LXXX VI. - The Dissociation Constants o f Friazoacetio and a- Triaxopropionic Acids.

By JAMES CHARLES PHILIP.

IT has been generally recognised that the behaviour of azoimide is in many respects similar to that of the hydrogen halides. The similarity, however, does not extend to all their properties, for, whilst hydrogen chloride, hydrogen bromide, and hydrogen iodide in aqueous solution are among the strongest acids known, azoimide is only feebly acidic, approximately equal in strength to acetic acid (Curtius and Raden- hausen, J.pr. Chern., 1891, [ii], 43, 207 ; West, Trans., 1900, 77, 705). Since the preparation of triazoacetic and a-triazopropionic acids by Forster and Fierz (this vol., pp. 72 and 669), a little more material is available for the study of the way in which the strength of an acid is affected by the substitution of the triazo-group for a halogen atom. The dissociation constants of triazoacetic and a-triazopropionic acids have therefore been determined, specimens of the two compounds being very kindly supplied by Dr. Forster.

The conductivity of various solutions of the acids and of their sodium salts was determined at 25' by the usual Kohlrausch method. The pure redistilled water employed in making up and diluting the solutions had a specific conductivity of 2 x 10-6 at 25'. A correction was made for this in the case of the sodium salt solutions only. These last were prepared by exactly neutralising a known quantity of standard sodium hydroxide with the acid under examination.

The equivalent conductivities of the sodium salts at infinite dilution were found by extrapolation from the observed values in the way suggested by Bredig (Zeitach. physikal. Chem., 1894, 13, 191). For sodium triazoacetate, A, = 91, the same as the corresponding figure for sodium monobromoacetate ; for sodium a-triazopropionate, A, = 87, which is also the value for sodium a-bromopropionate. If the ionic conductivity of hydrogen at 35' is taken as 350, and that of sodium

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926 DISSOCIATION CONSTANTS OF TRIAZO-COMPOUNDS.

as 51, the values of A, at this temperature for triazoacetic and a-triazopropionic acids are respectively 390 and 386.

The results obtained for the conductivity of the acids are recorded in the following tables.

Wazocccetic Acid .

K= 2 A a= _-. Mols.

0.0698 4 3 7 0.112 0*000R9 0.0349 59.5 0.153 0.00096 0.01 745 80 ‘1 0-205 0.00093 0’00698 11i-1 0.300 0*00090 0.0034 9 151% 0.389 0.00086

per lit,re. A. A, (1 -a) Y ’

a- Trinzopropionic Acid. Mols.

per litte. A. 0,0614 43‘4 0.112- 0.00088 0.0307 59 ‘9 0.155 0*00088 0.01535 81.2 0.210 0 *00086 0.00614 119.6 0,310 0*00086 0.00307 155.5 0’403 0 00084

The values of the expression ___ a2 instead of being independent

of the dilution as might be expected, show a distinct decrease in the case of triazoacetic acid and 8 slight decrease in the case of a-triazo- propionic acid. The cause of this is probably a slight trace of sulphuric acid in the specimens of the acids. The apparatus and method of work were tested by determining the conductivity of potassium chloride solutions; values were obtained in close agreement a t all dilutions with the figures recorded by Ostwald, Bredig, and Walden, so tha t the diminution in the value of K for triazoacetic and a-triazopropionic acids cannot be attributed to errors in the determination of the conductivity. I t was thought tha t it might be due to some slight decomposition pro- moted by the finely-divided platinum on the electrodes, but this explanation was discounted by the observation that the resistance of a solution of triazoacetic acid did not change during several hours’ con- tact with the platinised platinum electrodes of the conductivity cell. It appears therefore most probable tha t the specimens of the acids which were used contained a slight trace of sulphuric acid, which would be sufficient to account for the diminution in the value of K with dilution. Sulphuric acid was used in the preparation of the speci- mens, and, as it was not possible to purify the acids by distillation or crystallisation (see Forster and Fierz, Zoc. c i t . ) , the presence of such an impurity is not improbable.

In spite of the fact that the value of R decreases on dilution, it will

(1 - a)V’

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SPONTANEOUS CRYSTALLISATION, ETC. 927

be sufficient for the purpose of this investigation to take for the dis- sociation constant the mean of the figures given in the foregoing tables. Hence, for triazoacetic acid, loOK= 0-093 ; for a-triazopro- pionic acid, 10011=0*086. It is instructive to compare with these the values of lOOK for the corresponding halogen derivatives of acetic and propionic acids :

100 K. Monochloroacetic acid.. ................ 0.155 Monobromoacetic acid ................ 0.138 Monoiodoacetic acid .................. 0 075 a-Bromopropinnic acid ............... 0.108

From these figures, it will be seen that the introduction of a triazo- group into the molecule of acetic acid has an effect on the strength of the acid which is rather less than t h a t due to a bromine atom and rather greater than that due to an iodine atom. This result is further evidence of similarity in the functions of the triazo-group and a halogen atom.

ROYAL COLLEGE OF SCIENCE, LONDON, S. W.

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