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Application des relations de Taft à la complexation

1967; Pergamon Press; Volume: 23; Issue: 6 Linguagem: Inglês

10.1016/0584-8539(67)80045-x

1873-3824

D. Clotman, Th. Zeegers‐Huyskens,

Electrochemical Analysis and Applications

Abstract By infra-red spectrometric measurements, the constants for complex formation (K) of phenol with several primary (I), secondary (II) and tertiary aliphatic amines (III) have been determined in carbon tetrachloride solution at 27°C. The plot of log K in function of the sum of the polar substituent constants of Taft, Σ σ*, give approximately a straight line, but no linear relation was found for secondary and tertiary amines. Taking into account, the sum of the Taft steric factor, Σ Es, the following equations were found: for system I: log K = −1·07 Σ σ* + 0·13 Σ Es + 2·56 for system II: log K = −0·80 Σ σ* + 0·25 Σ Es + 1·97 for system III: log K = −0·38 Σ σ* + 0·44 Σ Es + 1·84. As found previously by Hall, the pKa of aliphatic amines in aqueous solution depends only on the sum of the polar constants, but the coefficients are different for the three classes of amines. In this optic, the theory of solvatation of Trotman—Dickenson and the calculations of Folkers and Runquist, and Condon are discussed. The difference between the complexation and the ionisation can be explained by two factors: 1. (a) The molecule of phenol has a greater steric hindrance than the molecule of water and the approach of this first molecule to the nitrogen atom is more difficult; 2. (b) The distances O…N in the complex and the distance N+H…O− in the ion are different. The average N+H…O− distance given by Pimentel is about 2·80 A. It was shown previously that, in the complex formed by phenol with propylamine, the O…N distance is about 2·53 A. These facts can explain the sensitivity of complexation to steric factors. In this optic, we may remark that Bayles and Chetwyn have determined the complexation constants of 2–4 dinitrophenol with mono-, di- and tributylamine in an aprotic solvent. The complex is in the ionic form (formation of a NH+…O− bond) and the order of the complexation constants is: amine 3 > amine 2 > amine 1.