Degree of protonation

In a later paper Knowles and Norman compared more fully nitrations of benzylic compounds in acetyl nitrate and in mixed acid (table 5.9), and interpreted the results in terms of three factors nitronium ion nitration in both media some degree of protonation of the oxygen... 

In addition we studied the complexation of ClC+ by the polyamines using microcalorimetry and potentiometry. The enthalpy changes measured are presented as function of the degree of protonation and the amount of CiT bound. 

Because a relates the sensitivity to structural changes that the proton-transfer process exhibits to that exhibited by dissociation of the acid, it is frequently assumed that the value of a can be used as an indicator of transition-state structure. The closer a approaches unity, the greater is the degree of proton transfer in the transition state. There are limits to the generality of this interpretaton, however. ... 

The Brpnsted coefficient a represents the sensitivity of the rate to the acid strength of the catalyst. It is a measure of the degree of proton transfer from catalyst to substrate in the transition state. For nearly all reactions where BH+ contains acidic N-H or O-H groups, a is in the range 0-1. 

For convenience these species are often represented by [p, q] and the overall formation constants denoted by /3pq. The degree of protonation of a particular species is given by the ratio q/p. The average degree of protonation of all species in solution, Z, is defined as... 

Changes in the composition of a mixture of solvent and sample may result in different degrees of protonation during ionisation, altering the quantitative results that might be obtained. 

The mechanism of hydrolysis of 2-carboxyphenylsulphamic acid (42) might be expected to follow that for the hydrolysis of salicyl sulphate, but actually it is thought to proceed by classic intramolecular acid catalysis rather than by hydrogen-bond catalysis. Evidence for a substantial degree of proton transfer from the carboxyl group in the transition state has been obtained (Hopkins and Williams, 1982). 

Tokiwa and Ohki (10) have shown that 1) the Ka value of the micellized LDAO is different from the molecular form, viz., 10 2) while the values of Ka are independent of the degree of protonation ( ) below the critical micelle concentration (CMC), they are dependent on 3 at concentrations above the CMC. These authors did not explain why the micellar pKa (5.9) at 3 = 0 is different from the molecular value (pKa = 4.9). 

---