Proton in aqueous solution

The behavior of protons in aqueous solutions differs widely from that of other ions and from that of protons themselves in organic solvents. The proton s hydration energy (about llOOkJ/mol) and its mobility in aqueous solutions are two to four times higher than the corresponding parameters of other ions. 

Amines are Brdnsted-Lowry bases due to the presence of a lone pair of electrons on N to accommodate incoming protons. In aqueous solution, an amine will hydrolyze in an equilibrium to produce hydroxide ions. 

But a word of caution species other than metal hydroxides can act as bases. Ammonia is such an example, since it can abstract protons in aqueous solution according to... 

The total concentrations of all components are equal to the sum of all the relevant species, multiplied by the appropriate stoichiometric factors. With the exception of the -[OH] term in the third equation, the equations below are self-explanatory. It is most convenient to allow the [ff+]tot to become negative if ]OH]>[H+], i.e. if pH>7. Otherwise [ff+]tot would need to include all water protons. In aqueous solution, the addition of x moles of OH is equivalent to removing x moles of H. ... 

We consider dehydration-adsorption of hydrated protons (cathodic proton transfer) and desorption-hydration of adsorbed protons (anodic proton transfer) on the interface of semiconductor electrodes. Since these adsorption and desorption of protons are ion transfer processes across the compact layer at the interface of semiconductor electrodes, the adsorption-desorption equilibrium is expressed as a function of the potential of the compact layer in the same way as Eqns. 9-60 and 9-61. In contrast to metal electrodes where changes with the electrode potential, semiconductor electrodes in the state of band edge level pinning maintain the potential d(hi of the compact layer constant and independent of the electrode potential. The concentration of adsorbed protons, Ch , is then determined not by the electrode potential but by the concentration of h3 ated protons in aqueous solutions. 

The solvent isotope effect on the dissociation constant of an acidic proton in aqueous solution has been used to deduce a value for the fractionation... 

Under special conditions, C(PPh3)2 can be protonated to form the cations (HC (PPh3 2) and (H2C(PPh3 2), respectively by deprotonation of solvents like halogenated hydrocarbons, THF, DMSO, etc. ((3) and (4)). No reports are known about protonation in aqueous solution [83] ... 

A particularly important concept in chemistry is that associated with proton loss and gain, i.e. acidity and basicity. Acids produce positively charged hydrogen ions H+ (protons) in aqueous solution the more acidic a compound is, the greater the concentration of protons it produces. In water, protons do not have an independent existence, but become strongly attached to a water molecule to give the stable hydronium ion H3O+. In the Brpnsted-Lowry definition ... 

Weak acids and bases are not restricted to amino adds many other compounds also behave similar in aqueous solutions. The dissociation into anion A and proton (in aqueous solution the dissociated protein is captured by a water molecule to form a hydronium ion HsO ) in the case of acids, and the addition of a proton to a base B to form the cation BH", is an equilibrium... 

The notation H+ (aq) represents the hydrated proton in aqueous solution without specifying the hydration sphere. It means that the species being oxidized is always the H2 molecule and is always related to a reduction. This is the reason why we speak of reduction potentials. In the opposite case, the numerical value of E would be the same but the sign would differ. It should be mentioned that in old books, for example, in Latimer s book [1], the other sign convention was used however, the International Union of Pure and Applied Chemistry (lUPAC) has introduced the unambiguous and authoritative usage in 1974 [2, 3]. 

The next development in direct detection of nitrenium ions came from McClelland et al. ° who applied the azide method to LFP measurements. This permitted the direct detection of those arylnitrenium ions implicated in carcinogenic DNA damage. McClelland s approach proved to be particularly useful in the study of 4-aryl and 4-alkoxy substituted phenylnitrenium ions. Apparently, the corresponding singlet nitrenes are sufficiently long lived to allow for protonation in aqueous solution. Several arylnitrenium ions studied by this route are described in Table 13.6. 

A whole series of known anomalies in. the static and mnetie properties of the proton in aqueous solution may he explained by the assumption that it is present as the hydrated hydroninm ion. Earlier considerations [1 j have already resulted in the following most probable model for the hydration shell of the proton (Pig. I). H3(F as centre is strongly... 

Note that this is equivalent to the definition that the free energy of formation of the proton in aqueous solution is equal to zero [i.e., AfG , (aq) = 0 at any temperature (for more details see, e.g., Stumm and Morgan, 1996)]. Hence, we can rewrite Eq. 8-2 as ... 

In the literature (e.g., Thauer et al. 1977 Hanselmann, 1991) you find the AfG°(aq) values at 25°C for all the species involved in reaction Eq. 1. Note that by convention, the free energies of formation of the elements in their naturally occurring most stable form, as well as of the proton in aqueous solution, are set to zero. From these values, calculate the standard free energy of reaction Eq. 1 ... 

The complexones are weak acids and therefore dissociate one or more protons in aqueous solution, depending on the pH of the solution. For them the general symbol HpL is used, where p represents the number of dissociable protons which is identical with the number of carboxylic groups of the acid. It can be obtained experimentally from the titration of a solution of the neutral acid H,L with a solution of strong base and measuring the pH value. The curve obtained in the case... 

The term general base is used to describe any substance that is capable of binding a proton in aqueous solution. Enzymes use a variety of functional groups in this role. 

The further anomaly in all measurements is that the electron appears to have an extra mobility over that normally associated with anionic or cationic species. This behavior resembles that associated with the solvated proton in aqueous solutions. 

Distribution ratios between other solvents than 1-octanol and water, or rather their logarithms, are linearly related to the log P values, so when these have not been determined directly, they can be obtained from those for the other solvents. In the cases of acidic or basic solvents that dissociate or associate with a proton in aqueous solutions a dilute buffer is used to keep the solvent molecules in their neutral form, and extrapolation to zero ionic strength should be applied in order to obtain accurate results. The log P data for the 1 -octanol/water partition obtained either directly or indirectly by means of correlations with data for other... 

L14 incorporates a further aminoethyl pendant arm as a potential donor group, but its Cu11 complex displays a somewhat lower stability than with L11, but at the same time, a much higher tendency to form a monoprotonated [Cu(HL14)]3+ complex (Table 1). This behaviour can be ascribed to the presence in [Cu(L14)]2+ of an uncoordinated aminoethyl pendant arm which can be readily protonated in aqueous solution. 

One can draw a useful analogy between acid-base and oxidation-reduction reactions. Both involve the transfer of a species from a source, the donor, to a sink, the acceptor. The source and sink nomenclature implies that the tendency of the proton (in the case of acids) or of the electron (for reducing agents) to undergo transfer is proportional to the fall in free energy. From the relation AG° = RTIn Ka. you can see that the acid dissociation constant is a measure of the fall in free energy of the proton when it is transferred from a donor HA to the solvent H2O, which represents the reference (zero) free energy level of the proton in aqueous solution. 

It has been recognized for many years that in a general way the basicity of the ligands has a great influence on the stability of complexes. After all, the formation of the coordinate bond is an acid-base reaction in the Lewis sense. However, as usually measured, basicity is toward the proton in aqueous solution. It sometimes provides a measure of the availability of electrons that might be expected when the ligands form coordinate bonds to metal ions. 

The protonated diaziridine (138) is more easily reducible than the diazirine which is too weakly basic to be protonated in aqueous solution. In alkali, where both are unprotonated, the diazirine is the... 

Generally, hydride is an electron donor or reductant. Accordingly, (H to Ir ) LMCT excitation could initiate the observed photolysis. However, how can this photoredox reaction be related to water or proton reduction While various hydride complexes undergo such a reductive elimination of H2 their s5mthesis is generally based on complicated procedures and cannot be formed by a simple reaction with water or protons in aqueous solution (8). 

Reviews by Gorte and coworkers [35, 36] deal with the adsorption complexes formed by strong and weak bases with acid sites in zeolites. They examine the adsorption enthalpies of a series of strongly basic molecules such as alkylamines, pyridines and imines. These workers also performed studies of the adsorption properties of weak bases, including water, alcohols, thiols, olefins, aldehydes, ketones and nitriles. They report a poor correlation between the differential heats of adsorption on H-MFl zeolites and the enthalpies of protonation in aqueous solutions, but a much better correlation with gas-phase proton affinities [37]. 

The -COOH and -NH3 groups of an amino acid are ionizable, because they can lose a proton in aqueous solution. As a result, they have different pATg values. The pATa of the -COOH group is typically 2, whereas that of the -NH3 group is 9, as shown in Table 28.1. 

The protonated diaziridine is more easily reducible than the diazirine, which has too weakly basic properties to be protonated in aqueous solution. In alkaline medium in which both are unprotonated, the diazirine is the more easily reducible. This explains why it has been difficult to obtain good yields of diaziridine by chemical and catalytic reduction of a diazirine if such a reduction is to result in a high yield of diaziridine, it must be performed in an alkaline medium. 

Since neither protons nor OH are present in this reduction half-reaction, and since T is not protonated in aqueous solution (because HI is a very strong acid), this reaction has the same potential in acidic or basic solution, +0.535 V (this can be confirmed by consulting Resource Section 3). 

---