ANALYST, SEPTEMBER 1986, VOL. 111 1103 Titrations in Non-aqueous Media Part Ill.* Basicity Order of Aniline, N-Alkyl- and N-Aryl-substituted Anilines and Pyridine in Nitrobenzene Solvent Turgut Gunduz,t Necli Gunduz, Esma Kilq and Adnan Kenar Department of Chemistry, Faculty of Science, University of Ankara, Ankara, Turkey The relative basicity order of ammonia, pyridine, aniline and N-methyl-, N,N-dimethyl-, N-ethyl-, N,N-diethyl-, N-aryl- and N,N-diaryl-substituted anilines have been determined potentiometrically with perchloric acid in nitrobenzene solvent and found to be NH3 > Py > PhNEt2 > PhNMe2 > PhNHEt > PhNHMe > PhNH2 > Ph2NH > Ph3N. This order in general conflicts with the results observed by other workers in either the gas phase or in the condensed phase.N-Alkyl substitution increases the basicity of the aniline, and N-aryl substitution decreases its basicity. Moreover, the number and size of the substituent influence the basicity of aniline. N-Ethyl-substituted anilines are more basic than the corresponding N-methyl-substituted anilines. The position of pyridine in the order is surprising and difficult to interpret. Keywords: Non-aqueous titration; potentiometric titration; anilines; nitrobenzene solvent; basicity order It is well known that two main effects influence the acidity or basicity of molecules, namely, structural and solvent (medium) effects.1.2 Unfortunately, in most molecules, there are two or more structural effects and it is usually difficult to decide how much each contributes to the acidity or basicity of the molecules.Small differences in acidity or basicity between similar molecules are also extremely difficult to interpret , and one must be very careful in deciding which structural effect is the main influence on the acidity or basicity. Solvent or medium effects are also very important, and the acidity or basicity of a molecule usually varies when its solvent is changed. For example, the basicity orders of n-butylamines against the Bronsted acids are Bu3N > BuzNH > BuNH2 in chlorobenzene solvent,3 Bu2NH > Bu3N > BuNH2 in benzene solvent3 and BUN& > Bu2NH > Bu3N in cyclohex- ane solvent.4 Hence a change of solvent may completely reverse the basicity order. The relative basicity orders of N-alkylated and N-arylated anilines , including pyridine, have been determined in differ- ent solvents or media,5-7 but have not as yet been determined in nitrobenzene solvent.In this work, the relative basicity orders of ammonia and eight amines have been determined potentiometrically with perchloric acid in nitrobenzene sol- vent. The amines titrated were pyridine (Py), aniline (PhNH2) , N-methylaniline (PhNHMe) , N, N-dimethylaniline (PhNMe2) , N-ethylaniline (PhNHEt), N, N-diethylaniline (PhNEt,), diphenylamine (Ph2NH) and triphenylamine (Ph3N). Titration graphs of these are given in Fig. 1. As is evident from the titration graphs, no homoconjugation reaction took place during the titrations and all but diphenyl- and triphenylamine gave good S-shaped potential vs. mequiv. acid or mequiv.base graphs. Half-neutralisation potentials of the amines have been determined by means of the graphs. Half-neutralisation potentials of the diphenyl- and triphenyl- amines, which did not give proper titration graphs, have been determined stoicheiometrically, i. e., the volumes of the titrant acid needed to reach the equivalence points have been calculated from the milliequivalent numbers of the titrated samples, and then the potentials at the half-volumes of the acid used have been taken as half-neutralisation potentials. As the half-neutralisation potentials are slightly concentra- tion dependent, titrations have been carried out on solutions with identical concentration, and also on dilute solutions (0.001 ~ ) . 8 , 9 In order to minimise the errors that might arise from the dilution of the solutions during the titrations, 0.034 M perchloric acid was used.* For Part 11 of this series, see p. 1099. t TO whom correspondence should be addressed. -100 0 100 200 > 300 E 400 500 600 700 800 0.0 1 .o Mequiv. acid/mequiv. base Fig. 1. Potentials versus mequiv. aciddmequiv. base. Base: A, ammonia; B, pyridine; C, N,N-diethyianiline; D, N,N-dimethylani- line; E, N-ethylaniline; F, N-methylaniline; G, aniline; H, diphenyl- amine; and I, triphenylamine Experimental Apparatus All apparatus used was as under Potentiometer and accessory in Part 11, and the electrode was washed and filled in an identical manner. Chemicals Nitrobenzene and perchloric acid were prepared and stored as described in Part 11. Ammonia. Ammonia was generated from NH&l (Merck, Pure).1104 ANALYST, SEPTEMBER 1986, VOL.111 Pyridine. Pyridine was purchased from Merck (Pure) and was used without further purification. Aniline and N-rnethyluniline. Aniline and N-meth ylaniline were purchased from Riedel de Haen (Pure) and were used without further purification. N,N-DiethyZuniline. N, N-Diethylaniline was obtained from Riedel de Haen (fur Analyse) and was distilled twice under vacuum before use. N , N- Dirnethy luniline. N, N-Dime t h ylaniline was purchased from Fluka (Puriss) and was used without further purification. Diphenylamine. Diphenylamine was purchased from BDH Chemicals (Indicator) and used without further purification. Triphenylurnine. Triphenylamine was purchased from BDH Chemicals (GPR) and used without further purification. All amines and ammonia solutions were titrated immedi- ately after preparation. Results and Discussion It is generally held that the relative basicity order of amines in a series does not change with a change of Bronsted acid titrant in the same solvent because the proton itself is too small to cause any appreciable steric hindrance.10 Therefore, the differences observed in the relative basicity order of the same series, titrated with different Bronsted acids in different solvents, can be attributed to the change of the solvent. For this reason, when comparing the relative order of the amines in a series, only the change of solvent is taken into account. As can be seen from the titration graphs obtained in nitrobenzene solvent given in Fig.1, ammonia is more basic than the eight amines titrated in this work, and is followed by pyridine. Including ammonia, the over-all order of the amines is NH3 > Py > PhNEt, > PhNMe2 > PhNHEt > PhNHMe > PhNH2 > Ph2NH > Ph3N (I). All N-alkyl-substituted anilines are more basic and all N-aryl-substituted anilines are less basic than aniline. More useful relative basicities can be extracted from sequence (I) above: PhNMe2 > PhNHMe > PhNH2 (11); (IV); Py > PhNMe2 > PhNHMe (V); and NH3 > Py > PhNH2 WI). As is evident from Fig. 1, diphenylamine and triphenyl- amine do not give clear end-points. The order (I) shows that N,N-diethylaniline is a stronger base than N-ethylaniline and N, N-dimethylaniline is a stronger base than N-methylaniline in nitrobenzene solvent. Moreover, N-ethyl-substituted anilines are more basic than their N-methyl-substituted aniline analogues.This means that an increase in the number or in the size of an alkyl group increases the basicity of the aniline. This is in good agreement with the results found by Brauman and co-workers in the gas phase with other compounds.5,6Jl The order (11) disagrees with the order PhNH2 > PhNHMe > PhNMez found by Benoit et ul. in dimethyl sulphoxide solvent.’ In addition, the order (V) also disagrees with the order PhNH2 > Py > PhNHMe > PhNMe2 found by the same workers.’ Moreover, sequence (V) is also at variance with the order PhNMe2 > Py > PhNHMe > PhNH2 observed by other workers in the gas phase,12J3 and the order (IV) does not fit the order Ph3N > PhNH2 obtained by others in the gas phase.SJ4 According to Dzidic,s the phenyl group is electron- donating relative to the proton in the gas phase, whereas it is electron-attracting relative to a saturated system.On the other hand, the order (V) is consistent with the order Py > PhNH2 observed in the gas phase by other investigators.5J2.13 As is evident from order (I), pyridine is more basic than even N,N-dialkyl-substituted anilines, which are expected to be highly basic.15-17 Hence, pyridine acts in nitrobenzene as if it was in the gas phase. This is a surprising observation, because the general belief is that the sp3 hybrid orbital is more basic than the sp2 hybrid orbital. As the nitrogen atom in PhNEt2 > PhNHEt > PhNH2 (111); PhNH2 > Ph2NH > Ph3N \o PhNHEt 1 I t PhNEt, o Y 0 2 4 6 8 1 0 1 2 No.of H atoms Fig. 2. Half-neutralisation potentials of amines versus number of hydrogen atoms pyridine is sp2 hybridised, it should show a lower basicity than aniline, which is sp3 hybridised. The higher basicity of pyridine relative to aniline in the gas phase can be interpreted as intrinsic basicity (in the absence of solvent effects), but the same property observed in nitroben- zene is very difficult to interpret. This is because nitrobenzene is not an inert solvent, and yet it has no similarity with the gas phase. It has a dielectric constant of 36. Hence, the gas-phase basicities of the amines may not be diagnostically useful in studying the basicities of the com- pounds.The order (VI) is the reverse of the result Py > NH3 found by Dzidic in the gas phase.5 The order between pyridine and aniline in dimethyl sulphoxide is in the opposite sense (PhNH2 > Py) to that in nitrobenzene solvent. This is another unexpected result, as dimethyl sulphoxide and nitrobenzene solvents are not at first sight so different in character as to reverse the order of basicity. Finally, another surprising result is the regular increase in basicity of the anilines with the number and the size of N-alkyl substitutions, e.g., PhNEt, > PhNMe2 > PhNHEt > PhNMe > PhNH2. In this series, if the number of hydrogens in aniline is arbitrarily taken to be zero, then the number of hydrogens in N-methyl- , N-ethyl- , N, N-dimethyl- and N, N-diethylanilines becomes 3, 5, 6 and 10, respectively.When the number of hydrogen atoms is plotted versus the half-neutralisation potentials of the amines, a nearly straight line is obtained (Fig. This relationship is very difficult to explain. It is as if each carbon-hydrogen bond gives a small part of its electron density to the nitrogen atoms of the amine. An increase in basicity implies a greater polarisability of the lone pair of electrons on the nitrogen atom, and a greater polarisability means that the lone pair of electrons on nitrogen is more easily donated. This results in a stronger attraction of the proton, and hence a greater basicity. 2) * We express our sincere thanks for the valuable assistance of Dr. Ziya K i l i ~ who helped us in writing the manuscript and in drawing the graphs.References 1. 2. 3. 4. 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SOC., 1978, 100,7328. Ikuta, S., and Kebarle, P., Can. J . Chem., 1983, 61, 97. Munson, M. S . , J . Am. Chem. SOC., 1965, 87, 2332. Brauman, J . I., and Blair, L. K., J . Am. Chem. SOC., 1968,90, 5636. Brauman, J. I., and Blair, L. K., J. Am. Chem. SOC., 1971, 93, 3911 and 3914. Paper A6155 Received February 19th, I986 Accepted March 3rd, I986