Protonic Species in Water

The above reaction can also be written as H20(aq) H+(aq) + OH (aq), depending on how one views the proton species in water. More aqueous reactions can be found in Table 1-la. 

Reactions in water often involve a ubitiquous species known as hydronium (H3O+). Is hydronium properly described as an isolated ion, or at the other extreme, as a proton dissolved in water ... 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent ... 

Compounds with an acidity constant, pK, in the range of 4 to 10, i.e. weak organic acids or bases, are present in two species forms at ambient pH. This pA a.i. range includes aromatic alcohols and thiols, carboxylic acids, aromatic amines and heterocyclic amines [15]. Conversely, alkyl-H and saturated alcohols do not undergo protonation/deprotonation in water (pA iw 14). 

Protonation to the conjugate acid (iminium cation) increases the potential of the itnine to act as an electrophile (compare carbonyl see Section 7.1), and this is followed by nucleophilic attack of water. The protonated product is in equilibrium with the other mono-protonated species in which the nitrogen carries the charge. We shall meet this mechanistic feature from time to time, and it is usually represented in a mechanism simply by putting H+, +H+ over the equilibrium arrows. Do not interpret this as an internal transfer of a proton such transfer would not be possible, and it is necessary to have solvent to supply and remove protons. 

Note that A is called the conjugate base of HA and BH+ the conjugate acid of B. Proton transfer reactions as described by Eq. 8-1 are usually very fast and reversible. It makes sense then that we treat such reactions as equilibrium processes, and that we are interested in the equilibrium distribution of the species involved in the reaction. In this chapter we confine our discussion to proton transfer reactions in aqueous solution, although in some cases, such reactions may also be important in nonaqueous media. Our major concern will be the speciation of an organic acid or base (neutral versus ionic species) in water under given conditions. Before we get to that, however, we have to recall some basic thermodynamic aspects that we need to describe acid-base reactions in aqueous solution. 

Partial molar volumes of electrolytes may present an additional difficulty if the partial molar volumes of individual constituent ions are needed. The subject has been dealt with as part of a valuable discourse on metal ion solvation.158 For species in water and some other solvents, the reference point for calculation of other partial molar volumes of single ions is the partial molar volume of the solvated proton, although there have been different views on what this value should be.159 However, the variation in values is not large and the consequence of choice of value does not have a huge impact particularly for large ionic species. Theoretical models of partial molar volumes of hydrated ions have been developed in order that estimates of them can be calculated. 160-162... 

Of the three isomers of l-methyl-3-cyanodihydropyridines only the 1,2-isomer underwent rapid and quantitative reduction by NaBH4 in the presence of trimethoxyborane116 (Scheme 87). The role of BH3 in generating the protonating species, in contrast to the direct protonation by water suggested by other authors117,118, was clearly evident. 

A SCF CNDO calculation [108] for the HsOJ species similarly suggests that proton transfer in water occurs by tunnelling, the estimated rate coefficient being of the order of 1014 sec-1. 

This scale, in fact, would allow comparing the basicities of the different solvents in a general way. It is based on the question of the chemical potential of the proton (as a single-ion species) in water, W, and the organic solvent, S. Taking the hypothetical IM solution as the standard state, the chemical potential is given, according to Eq. (1), as... 

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