Solvent ionizing ability

Values of Solvent Ionizing Ability as Defined by LFERs Eqs. 8.38,8.39 and 8.41 ... 

If the reaction is performed on two molecules that differ only in the leaving group (for example, /-BuCl and /-BuSMe ), the rates should obviously be different, since they depend on the ionizing ability of the molecule. However, once the carbocation is formed, if the solvent and the temperature are the same, it should suffer the same fate in both cases, since the nature of the leaving group does not affect the second step. This means that the ratio of elimination to substitution should be the same. The compounds mentioned in the example were solvolyzed at 65.3°C in 80% aqueous ethanol with the following results 11... 

Actually, two factors are relevant in regard to the ionizing ability of solvents. First, a high dielectric constant increases ionizing power by making it easier to separate ions. This is because the force between charged particles varies inversely with the dielectric constant of the medium.7 Thus water, with a dielectric constant of 80, is 40 times moreveffective than a hydrocarbon with a dielectric constant of 2. Second, and usually more important, is the ability of the solvent to solvate the separated ions. Cations are solvated most effectively... 

Ionization of an alkyl halide requires formation and separation of positive and negative charges, similar to what happens when sodium chloride dissolves in water. Therefore, SN1 reactions require highly polar solvents that strongly solvate ions. One measure of a solvent s ability to solvate ions is its dielectric constant (e), a measure of the solvent s polarity. Table 6-6 lists the dielectric constants of some common solvents and the relative ionization rates for fm-butyl chloride in these solvents. Note that ionization occurs much faster in highly polar solvents such as water and alcohols. Although most alkyl halides are not soluble in water, they often dissolve in highly polar mixtures of acetone and alcohols with water. 

Some ionizing solvents are of major importance in analytical chemistry whilst others are of peripheral interest. A useful subdivision is into protonic solvents such as water and the common acids, or non-protonic solvents which do not have protons available. Typical of the latter subgroup would be sulphur dioxide and bromine trifluoride. Non-protonic ionizing solvents have little application in chemical analysis and subsequent discussions will be restricted to protonic solvents. Ionizing solvents have one property in common, self-ionization, which reflects their ability to produce ionization of a solute some typical examples are given in table 3.2. Equilibrium constants for these reactions are known as self-ionization constants. 

Loosely bound aggregates (chemical effects) are formed with the hydrocarbons acting as electron donors (Lewis base) and the solvents acting as electron acceptors (Lewis acid). The hydrocarbon that forms the most stable complex with the solvent experiences a decrease in volatility. Electron donors are rated by ionization potential, and electron acceptors are rated by their electron affinities. The selectivity will be higher, the larger the difference in ionization potential between the hydrocarbons and the larger the electron affinity of the solvent (9). While data on ionization potentials of hydrocarbons can be found (15, 16), electron affinities data are rare because of difficulties in their experimental determination. Prausnitz and Anderson (8) recommend that the sigma scale, proposed by Hammett (17), be used to determine approximately the solvents relative ability to form complexes with the two hydrocarbons. Attempts by this author, however, to use this scale were not conclusive. Prausnitz and Anderson (8) should be consulted to understand better the physical and chemical effects. 

Many synthetic reactions, that proceed via carbocations, produce these intermediates from mixtures of alkyl halides and Lewis acidic metal halides. The concentration of carbocations produced under these conditions depends on the tendency of the alkyl halides to ionize ( carbocation stability ) and the strengths of the Lewis acids in a certain solvent. Because the ionizing abilities of alkyl halides can be derived from the schemes and correlations given in Sections B through F, we shall now concentrate on the relative halide affinities of the Lewis acidic metal halides. For this purpose, we consider Eq. (13) which describes the exchange of a chloride ion between the Lewis acids R+ and MCI,. 

It is furthermore remarkable that an approximately linear relationship between (fir — l)/(2er + 1) and Ig k values for reaction (5-102), measured in 19 solvents, is found only for non-HBD solvents [cf. Eq. (5-87) in Section 5.4.3), whereas protic solvents are much better ionizing media than their relative permittivity would suggest [265]. For example, acetic acid and tetrahydrofuran have very similar relative permittivities (6.2 and 7.6, respectively), and yet ionization in acetic acid exceeds that in tetrahydrofuran by a factor of 2 10 The reason for this extraordinary rate acceleration is again that the departing tosylate is better solvated due to hydrogen bonding in the protic solvent. The ability of the protic solvent to form hydrogen bonds is not reflected in its relative permittivity or in the dipole moment [265]. 

To conclude this section, it should be emphasized that the solvosystem concept is restricted to self-ionizing solvents it is, therefore, not suitable for completely describing ionic solvents and those which have no ionizing ability. 

The insensitivity of allylic azide isomerization rates to substituent effects and solvent ionizing power, and the relatively small value of the negative volume of activation make it very unlikely that ion pair intermediates are involved in these reactions. The negative entropy and volume of activation are compatible with a concerted process involving a cyclic transition state resembling a 1,2,3-triazine. However, the known ability of alkyl and aryl azides to add to olefins - suggests the possibility of an intramolecular addition-elimination mechanism,viz. 

This chapter will show that the potent ionizing ability of water is a key to predicting bimolecular versus unimolecular reactions. Aprotic solvents favor bimo-lecular reactions. It is assumed that water is required for ionization. Primary alkyl halides rmdergo Sn2 reactions under most conditions, whereas tertiary alkyl halides never undergo Sn2. If a nucleophile is classified as the conjugate base of a strong acid, it is a weak base and will probably not induce an E2 reaction. 

Fig. 9. Effect of solvent on rate as measured by the response of log k versus a for a series of primary substrates in solvents of a wide range of ionizing abilities and nucleophilicities. Reproduced by permission of John Wiley and Sons.

Since the Y parameter was based upon a reaction that has little nucleophilic assistance, those reactions that have m values near 1 reflect nearly full ionization in the rate-determining step. For those reactions that have an m value less than 1, the reaction is not as sensitive to the ionizing ability of the solvent as is f-BuCl. This means less charge has been created in the transition state, which is most often accomplished by some degree of nucleophilic assistance (Chapter 11 discusses the shades of grey between pure SnI and pure Sn2 reaction mechanisms). Hence, a reaction with some Sn2 character will have a reduced extent of charge development in the transition state and therefore an m value less than unity. More-... 

D Solvent Effects on SnI Reactions The Ionizing Ability of the Solvent... 

The ionization of RX occurs more rapidly in more polar solvents. The ionizing ability of the solvent is unambiguously related to e only for aptotic solvents in which dependence of the following type (see Chapter 5) is observed ... 

For protic solvents forming hydrogen bonds, e is not a characteristics of the ionizing ability of the solvent. Therefore, E. Grunwald and S. Winstein proposed to characterize this ability by the rate constant of solvolysis of (CH3)3CC1, choosing a solution of 20% H2O and 80% C2H5OH as a standard... 

High solubility of indifferent electrolytes (not less than mole/liter) and sufficiently high ionizing ability in the solvent ... 

The ionization eonstant should be a function of the intrinsic heterolytic ability (e.g., intrinsic acidity if the solute is an acid HX) and the ionizing power of the solvents, whereas the dissoeiation constant should be primarily determined by the dissociating power of the solvent. Therefore, Ad is expeeted to be under the eontrol of e, the dieleetrie eonstant. As a consequenee, ion pairs are not deteetable in high-e solvents like water, which is why the terms ionization constant and dissociation constant are often used interchangeably. In low-e solvents, however, dissociation constants are very small and ion pairs (and higher aggregates) become important species. For example, in ethylene chloride (e = 10.23), the dissociation constants of substituted phenyltrimethylammonium perchlorate salts are of the order 10 . Overall dissociation constants, expressed as pArx = — log Arx, for some substanees in aeetie acid (e = 6.19) are perchloric acid, 4.87 sulfuric acid, 7.24 sodium acetate, 6.68 sodium perchlorate, 5.48. Aeid-base equilibria in aeetie acid have been earefully studied beeause of the analytical importance of this solvent in titrimetry. 

Ho, the acidity function introduced by Hammett, is a measure of the ability of the solvent to transfer a proton to a base of neutral charge. In dilute aqueous solution ho becomes equal to t d Hq is equal to pH, but in strongly acid solutions Hq will differ from both pH and — log ch+. The determination of Ho is accomplished with the aid of Eq. (8-89) and a series of neutral indicator bases (the nitroanilines in Table 8-18) whose pA bh+ values have been measured by the overlap method. Table 8-19 lists Ho values for some aqueous solutions of common mineral acids. Analogous acidity functions have been defined for bases of other structural and charge types, such as // for amides and Hf for bases that ionize with the production of a carbocation ... 

---