Proton-addition constant

The species distribution diagrams (0-pH) for flotation reagents can be calculated from proton addition constants given in Appendices A and B. Some examples are given below. 

Appendix B Proton addition constants of some flotation reagents... 

Reagents Proton addition constants Reagents Proton addition constants ... 

When a Br nsted plot includes acids or bases with different numbers of acidic or basic sites, statistical corrections are sometimes applied in effect, the rate and equilibrium constants are corrected to a per functional group basis. If an acid has p equivalent dissociable protons and its conjugate base has q equivalent sites for proton addition, the statistically corrected forms of the Br insted relationships are... 

X = 0, CH2, CHCOOH, C(COOH)2, NH, NCH3 N(CH2CH=CH2), N(CHs)2 Cl Bobrowski and Das published a series of papers on the transients in the pulse radiolysis of retinyl polyenes31-37, due to their importance in a variety of biomolecular processes. They studied32 the kinetics and mechanisms of protonation reaction. The protons were released by pulse radiolysis, on a nanosecond time scale, of 2-propanol air-saturated solutions containing, in addition to the retinyl polyenes, also 0.5 M acetone and 0.2 M CCI4. Within less than 300 ns, the electron beam pulse results in formation of HC1. The protonated products of retinyl polyenes were found to absorb optically with Xmax at the range of 475-585 nm and were measured by this absorption. They found that the protonation rate constants of polyene s Schiff bases depend on the polyene chain... 

More recent quantum-based MD simulations were performed at temperatures up to 2000 K and pressures up to 30 GPa.73,74 Under these conditions, it was found that the molecular ions H30+ and OH are the major charge carriers in a fluid phase, in contrast to the bcc crystal predicted for the superionic phase. The fluid high-pressure phase has been confirmed by X-ray diffraction results of water melting at ca. 1000 K and up to 40 GPa of pressure.66,75,76 In addition, extrapolations of the proton diffusion constant of ice into the superionic region were found to be far lower than a commonly used criterion for superionic phases of 10 4cm2/s.77 A great need exists for additional work to resolve the apparently conflicting data. 

The use of thermodynamic relationships to determine the basicity constants and their temperature dependence presupposes that the existence and structure of a proton addition complex of this type has been proved. 

The formation of a proton addition complex can take place either in a binary system (equation (6)) or in a ternary system (equation (6)). Correspondingly, one obtains the basicity constants and K respectively, according to (7) and (8), and these are related to one another through the equilibrium (9) and the relationship (10). It is in the nature of these strongly acid systems that conventional methods for analysing the equilibrium composition can only be used with difficulty. 

If an aromatic hydrocarbon (A) is dissolved in a strong acid, and a proton addition complex is formed according to equation (5), the equilibrium constant of this reaction is given by equation (7). 

The preceding section summarized basicity results obtained from the stability constants of 7r-complexes and EDA-complexes. These results can only reflect a qualitative gradation of the basicity. If one moves from these complexes to u-complexes, then exact values for the basicity of unsaturated compounds can be obtained by measuring the formation equilibria of the proton addition complexes in strongly acid solutions. The experimental methods and the calculations have been described in Sections III, A-C. 

Equation (50) uses the fact that the a-bond energies and the entropy components may be assumed constant. However, this formula does not allow for the fact that several isomeric proton addition complexes may be present in the solution. In that case one obtains the more general relation ... 

Here Z denotes the number of isomeric equilibria, which have to be summed from i = l to i = n. fj, denotes a proportionality constant relating the w-electron energy and free energy differences. The results of these calculations are summarized in Table 27. In contrast to the tables given by Ebrenson (1961), no account is taken of proton addition complexes in which the addition of the proton takes place at a C-atom which carries a methyl group. 

The NMR spectra of heterocyclic compounds with seven or more ring members are as diverse as the shape, size and degree of unsaturation of the compounds. Proton-proton coupling constants provide a wealth of data on the shape of the molecules, while chemical shift data, heteroatom-proton coupling constants and heteronuclear spectra give information of the electronic structure. Some data on seven-membered rings are included in Table 7, Several additional examples of NMR spectroscopy for large heterocycles are discussed below. 

In the next section we will show that for most compounds, the pH-independent terms k c = kf + k A [Equation (8)] determined in aqueous solution can be attributed to water reacting as a general base, path (b), that corresponds to the second term, where k is the rate constant for proton addition to Ee. 

The rate constants for protonation of the excited singlet states of several compounds were determined by Weller (1961). Although the measurement of excited state equilibrium constants has become more common, there have been relatively few determinations of the rate constants involved. Trieff and Sundheim (1965) investigated the effects of solvent changes on the rates of protonation and deprotonation of 2-naphthol in the S) state. The dissociation rate constant decreased progressively with the addition of methanol or glycerol to the aqueous solution but the protonation rate constant varied in a more complex manner. As mentioned above, Stryer (1966) found both rate constants smaller in D20 than in H20. 

The relative mobilities of the axial and equatorial hydrogen atoms from C(2) and C(6) positions of rigid cyclohexanones have been determined by several authors from the relative rate constants of hydrogen-deuterium exchange. Indeed, according to the principle of microscopic reversibility, the elementary rate constants for axial and equatorial proton detachment correspond to those of proton addition on the two faces of the enol or enolate (26). 

It is of great interest to compare this last value with the keto-enol equilibrium constant obtained similarly for acetone = 0.35 x 10-8). Indeed, in many enzyme-catalysed reactions, aldolisation for example, enamine formation is not rate-limiting, and the rate is usually controlled by subsequent electrophilic additions. Consequently, the rate depends on enamine reactivity and on the enamine concentration at equilibrium. Therefore, if one wants to compare the two processes, via enol and via enamine, in order to explain why the enamine route is usually preferred, the difference in equilibrium constants for enol and enamine formation must be taken into account. Data on ketone to enol and ketone to enamine equilibrium constants show that the enamine and enol concentrations are of similar magnitude even for relatively small concentrations of primary amine. Thereafter, since the enamine is much more reactive than the enol for reactions with electrophilic reagents (in a ratio of 4-6 powers of ten for proton addition), it can be easily understood why the amine-catalysed pathway is energetically more favourable. 

In addition, three g-factors (unitless) and two proton hyperfine constants (MHz) were determined ... 

There is an extra R.E. of 6 kcal but the basicity of aniline is also very much smaller than that of an aliphatic primary amine, such as cyclohexylamine. This special resonance does not exist any longer in the anilinium ion because the free pair has now become a bonding pair by proton addition. The base constant Kb is related to the (free) energy AF of the proton addition according to the expression AF = —RTlnKb. 

---