Neutral solvents, acid-base reactions

Scheme 2.19 depicts a typical example of the coupling of acid-base reactions, here protonations, with electron transfer. In a dry aprotic solvent [e.g., /V./V-dimethylformamide (DMF)], an aromatic hydrocarbon such as anthracene exhibits two successive reversible cyclic voltammetric waves (suspensions of neutral alumina may be used efficiently to dry the solvent... 

All these electrolytes are neutral in Bronsted acid-base properties. Although rather exceptional, an acid, a base, or a pH buffer may be added to the supporting electrolyte of neutral salts. The acid-base system to be selected depends on the purpose of the measurement. We often use trifluoromethanesulfonic acid (CF3S03F1) as a strong acid acetic acid, benzoic acid, or phenol as a weak acid an amine or pyridine as a weak base and tetraalkylammonium hydroxide (ILtNOH) as a strong base. Examples of buffer systems are the mixtures of picric acid and its R4N-salt and amines and their PlCl04-salts. Here, we should note that the acid-base reactions in aprotic solvents considerably differ from those in water, as discussed in Chapter 3. 

Acid-base reactions. According to the solvent concept, the acidic species characteristic of liquid ammonia is NH4+ and the basic species is NH2. Neutralization reactions in liquid ammonia thus become equivalent to the reaction of these ions ... 

On the other hand, if HA is an uncharged acid z = — V, e.g. CH3—CO2H), the right-hand side of Eq. (4-10) involves the sum of two reciprocal radii (zha = 0) and a strong influence of the relative permittivity on the ionization equilibrium is expected. Because in acid/base reactions of this charge type, neutral molecules are converted into anions and cations, which attract each other, reaction (4-5) will shift to the right with an increase in relative permittivity of the solvent in which HA is dissolved. Ionization increases when increases. This rule is qualitatively verifiable for water and alcohols as... 

An interesting example of a Lewis acid/base reaction between neutral reactants is the formation of tris(n-butyl)phosphonium-dithiocarboxylate, ( -Bu)3P" — 82 , from tris(n-butyl)phosphane and carbon disulfide in solution. As expected, this equilibrium is strongly shifted in favour of the dipolar zwitterion with increasing solvent polarity (diethyl ether dimethyl sulfoxide) [272, 273]. 

This concept serves well in protonic solvents like water, ammonia, acetic acid etc. but fails in case of some obvious acid-base reactions e.g. it can not explain how acidic oxides such as an hydrous carbon dioxide, sulpher dioxide, sulphur trioxide etc. neutralize basic-oxides like calcium oxide and barrium oxide even in the absence of solvent. 

In molecular solvents, process (1.1.5) is superimposed on other acid-base reactions, namely the reaction with neutral solvent molecules or with other dissolved substances of acidic or basic character. The complete acid-base process is represented by the equation of type ... 

The acid and base of a half-reaction are called conjugate pairs. Free protons do not exist in solution, and there must be a proton acceptor (base) before a proton donor (acid) will release its proton. That is, there must be a combination of two half-reactions. Some acid-base reactions in different solvents are illustrated in Table 7.1. In the first example, acetate ion is the conjugate base of acetic acid and ammonium ion is the conjugate acid of ammonia. The first four examples represent ionization of an acid or a base in a solvent, while the others represent a neutralization reaction between an acid and a base in the solvent. 

It is apparent from the above definition that a substance cannot act as an acid unless a base is present to accept the protons. Thus, acids will undergo complete or partial ionization in basic solvents such as water, liquid ammonia, or ethanol, depending on the basicity of the solvent and the strength of the acid. But in neutral or inert solvents, ionization is insignificant. However, ionization in the solvent is not a prerequisite for an acid-base reaction, as in the last example in the table, where picric acid reacts with aniline. 

The adsorption of neutral molecules on smectites is driven by various chemical interactions hydrogen bonds, ion-dipole interactions, coordination bonds, acid-base reactions, charge transfer, and van der Waals forces. Several polar molecules, such as alcohols, amines, and acids, form intercalation complexes with montmorillonites. The intercalation can be performed from the vapor, liquid, or even solid state. In intercalation from solution, solvent molecules are generally coadsorbed in the interlayer space. Guest molecules may be intercalated in dried clay minerals or may displace the water molecules of hydrated montmorillonite. 

These reactions involve water as reactant or product, in addition to its common role as solvent. Of course, an acid-base reaction (also called a neutralization reaction) occurs when an acid reacts with a base, but the definitions of these terms and the scope of this reaction class have changed over the years. For our purposes at this point, we U use defiifitions that apply to substances found commonly in the lab ... 

The effect of the solvent upon neutralization will be treated in Chapter 8 since a previous discussion of displacement is necessary. The type equation of the Brpnsted theory eliminates neutralization from all acid-base reactions covered by the theory. This is because the equation represents displacement reactions of secondary acids and bases. Since the Lewis theory includes the Br0nsted... 

When the solvent takes part in acid-base phenomena, the reactions are usually displacement reactions. Considering only acid-base reactions, solvents may be divided into three classes (1) those that are ordinarily inert toward acids and bases, e.g., benzene, carbon tetrachloride, and chlorobenzene (2) those that are ionizable, e.g., water, ammonia, sulfur dioxide, phosgene, and selenium oxychloride (3) those that do not ionize but do react with acids and bases, e.g., ether and pyridine. If we consider the neutralization of boron trichloride by triethylamine in the three types of solvents, we find that the net result may be the same as when the neutralization occurs in the absence of a solvent ... 

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

The acid dissociation of neutral molecules is such a highly endothermic reaction that the acid dissociation of nitromethane can hardly take place. The results of the calculations presented here provide a theoretical support for nitromethane as an ideal model of aprotic solvent in the acid-base theory of organic molecules. 

A system of parallel reactions as shown in Fig. 5.3-9 was studied by Paul et at. (1992). The reactions are an acid-base neutralization and a base-catalysed hydrolysis of product (C). The labile compound (Q is in solution in an organic solvent, and aqueous NaOH is added to raise the pH from 2 to 7. Enolization occurs under basic conditions and is accompanied by irreversible decomposition (ring opening), which is not shown in the figure. The system was studied in the laboratory using the 6-Iitre reactor shown in Fig. 5.3-10. 

Oae found that for both base- and acid-catalyzed hydrolysis of phenyl benzenesul-fonate, there was no incorporation of 0 from solvent into the sulfonate ester after partial hydrolysis. This was interpreted as ruling out a stepwise mechanism, but in fact it could be stepwise with slow pseudorotation. In fact this nonexchange can be explained by Westheimer s rules for pseudorotation, assuming the same rules apply to pentacoordinate sulfur. For the acid-catalyzed reaction, the likely intermediate would be 8 for which pseudorotation would be disfavored because it would put a carbon at an apical position. Further protonation to the cationic intermediate is unlikely even in lOM HCl (the medium for Oae s experiments) because of the high acidity of this species a Branch and Calvin calculation (See Appendix), supplemented by allowance for the effect of the phenyl groups (taken as the difference in between sulfuric acid and benzenesulfonic acid ), leads to a pA, of -7 for the first pisTa of this cation about -2 for the second p/sTa. and about 3 for the third Thus, protonation by aqueous HCl to give the neutral intermediate is likely but further protonation to give cation 9 would be very unlikely. 

If an amphiprotic solvent contains an acid and base that are neither mutually conjugate nor are conjugated with the solvent, a protolytic reaction occurs between these dissolved components. Four possible situations can arise. If both the acid and base are strong, then neutralization occurs between the lyonium ions and the lyate ions. If the acid is weak and the base strong, the acid reacts with the lyate ions produced by the strong base. The opposite case is analogous. A weak acid and a weak base exchange protons ... 

The catalysts are best prepared in situ by mixing a half-equivalent of the di-chloro-metal aromatic dimer with an equivalent of the ligand in a suitable solvent such as acetonitrile, dichloromethane or isopropanol. A base is used to remove the hydrochloric acid formed (Fig. 35.3). If 1 equiv. of base is used, the inactive pre-catalyst is prepared, and further addition of base activates the catalyst to the 16-electron species. In the IPA system the base is conveniently aqueous sodium hydroxide or sodium isopropoxide in isopropanol, whereas in the TEAF system, triethylamine activates the catalyst. In practice, since the amount of catalyst is tiny, any residual acid in the solvent can neutralize the added base, so a small excess is often used. To prevent the active 16-electron species sitting around, the catalyst is often activated in the presence of the hydrogen donor. The amount of catalyst required for a transformation depends on the desired reaction rate. Typically, it is desirable to achieve complete conversion of the substrate within several hours, and to this extent the catalyst is often used at 0.1 mol.% (with SCR 1000 1). Some substrate-catalyst combinations are less active, requiring more catalyst (e.g., up to 1 mol.% SCR 100 1), in other reactions catalyst TONs of 10000 (SCR 10000 1) have been realized. 

---