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9088814 
Journal Article 
Potentiometric determination of the base strengths of amines in non-protolytic solvents 
Hall Jr, HK 
1956 
1 
Journal of Physical Chemistry
ISSN: 0022-3654 
60 
1 
63-70 
English 
A reproducible titration method was devised and successful potentiometric titrations were performed on a large number of monoamines and diamines in five solvents of varying nature: ethyl acetate, acetonitrile, nitrobenzene, nitromethane and ethylene dichloride. The titrant acids were perchloric, p-toluenesulfonic and perfluorobutyric, all in dioxane solution. Methanesulfonic acid was used as a solution in the given solvent. The mid-point of the titration curve was taken as a measure of the dissociation constant of the amine. The existence of quantitative linear relationships between the midpoint readings (E1/2) and the Hammett σ constants in water for m- and p-substituted anilines was established. For the alkylamines the observed order of base strength was: NH3 < RNH2, R2NH > R3N, an order similar to that observed in water or alcohol. As the length of the alkyl chain increased to four or five carbon atoms, the base strength was lowered. Diamines, such as ethylenediamine, hexamethylenediamine and the piperazines behaved in acetonitrile as rather strong bases. Polar substituents in the piperidine ring as in morpholine, monoacylpiperazines, etc., decreased the base strength of the parent amine as expected. The order and even the quantitative strength of the bases were independent of solvent or of acid for the five organic solvents. When data for organic solvents were plotted against those for water, acceptable linear plots were obtained (with one exception - see below). Even this extreme change of solvent does not affect relative base strength. The exception noted above referred to amines containing a strongly polar group near the N atom, such as morpholines and monoacylpiperazines. These bases are much stronger in organic solvents than their pKa values in water would predict. A discrepancy between the predicted and observed slopes of the plots of E1/2 against pKa is explained in terms of variations in the activity coefficients of the ammonium ions relative to the corresponding amines.