Solvent ionising power

T.W. Bentley and G. Llewellyn, Scales of Solvent Ionising Power, Prog. Phys. Org. Chem., 1990, 17, 121. 

T.W. Bentley and G. Carter, The Sn2-Sn1 Spectrum. 4. The Sn2 (Intermediate) Mechanisms for Solvolyses of rcrf-Butyl Chloride A Revised F Scale of Solvent Ionising Power Based on Solvolyses of 1-Adamantyl Chloride, J. Am. Chem. Soc., 1982,104, 5741. 

Table 6 Y values of solvent ionising power for some solvents and solvent mixtures (with water) derived from solvolyses of 1- or 2-adamantyl tosylates at 25 C... 

Rates of aquation of [Co(NH3)5Br], and of its chromium(III) analog, have been determined in methanol-, ethanol-, isopropanol-, and r-butyl-alco-hol-water mixtures at various temperatures. The well-known compensation effect is apparent for all these cosolvents. In the aqueous methanol and aqueous ethanol series of solvent mixtures there is correlation of rate constants with solvent ionising power. A correlation of rate constants with solvent polarity was established in a much more limited investigation of aquation of a-cis-[Co(edda)(OH2)Cl], studied in water and in 20% methanol, ethanol, and acetone. The variation of rate constants, and of the activation parameters and A5, with solvent composition was claimed to indicate an mechanism. 

Solutions of alkah metal and ammonium iodides in Hquid iodine are good conductors of electricity, comparable to fused salts and aqueous solutions of strong acids. The Hquid is therefore a polar solvent of considerable ionising power, whereas its own electrical conductivity suggests that it is appreciably ionized, probably into I" and I (triodide). Iodine resembles water in this respect. The metal iodides and polyiodides are bases, whereas the iodine haHdes are acids. 

These YA values are found not to run in parallel with the dielectric constant values for the solvents concerned. Obviously the dielectric constant value for the solvent must be involved in some way in YA, as separation of opposite charges is a crucial feature of the rate-limiting step in an SN1 reaction formation of the T.S. leading to the ion-pair intermediate (47). But specific solvation of the separating charges must also be involved and YA will reflect those, and quite possibly other properties of the solvent as well. It is common to describe YA as representing a measure of the ionising power of the solvent A. 

Standard halide, 2-chloro-2-methylpropane (46). Here k. and fco are the rate constants for solvolysis of any halide, in solvent A and in the standard solvent, respectively. Ya has already been defined as a solvent parameter representing the ionising power of solvent A, while m is a compound parameter characteristic of the particular halide it is given the value 1-00 for the standard halide, 2-chloro-2-methylpropane (46). The actual value of m can be taken as a measure of the susceptibility of the solvolysis of a particular halide towards the ionising power, Ya, of that solvent ... 

Intramolecular Friedel rafts vinylation reactions of aryl-tethered ( )-alkenyl-(aryl)iodanes 49 to afford 1,2-dihydronaphthalenes and 2/f-chromenes 50 have also been reported to occur via an addition-elimination process (Scheme 30) or alternatively an intramolecular Sat2 displacement at the vinylic centre. The intramolecular displacement was deemed energetically unstable on steric grounds but could not be ruled out. In these cases the presence of a vinylic cation was ruled out as the ionising power of the solvent has little effect on the cyclisation and the corresponding (Z)-isomers do not undergo ring closure. 

Stoichiometry (28) is followed under neutral or in alkaline aqueous conditions and (29) in concentrated mineral acids. In acid solution reaction (28) is powerfully inhibited and in the absence of general acids or bases the rate of hydrolysis is a function of pH. At pH >5.0 the reaction is first-order in OH but below this value there is a region where the rate of hydrolysis is largely independent of pH followed by a region where the rate falls as [H30+] increases. The kinetic data at various temperatures both with pure water and buffer solutions, the solvent isotope effect and the rate increase of the 4-chloro derivative ( 2-fold) are compatible with the interpretation of the hydrolysis in terms of two mechanisms. These are a dominant bimolecular reaction between hydroxide ion and acyl cyanide at pH >5.0 and a dominant water reaction at lower pH, the latter susceptible to general base catalysis and inhibition by acids. The data at pH <5.0 can be rationalised by a carbonyl addition intermediate and are compatible with a two-step, but not one-step, cyclic mechanism for hydration. Benzoyl cyanide is more reactive towards water than benzoyl fluoride, but less reactive than benzoyl chloride and anhydride, an unexpected result since HCN has a smaller dissociation constant than HF or RC02H. There are no grounds, however, to suspect that an ionisation mechanism is involved. 

It should be noted that self-ionisation is not an essential prerequisite for a satisfactory polar solvent. Liquids such as acetonitrile CH3CN or dimethylsulphoxide SO(CH3)2 appear not to ionise but they make very useful solvents for electrolytes as well as for polar molecular substances. As with H20, NH3, H2S04 etc., they owe their solvent powers to their polarity, leading to dipole-dipole interaction in the case of polar molecules as solutes and ion-dipole attraction in the case of electrolytes. There may in addition be considerable covalent bonding, via coordinate bond formation, in the case of cations. In solvents which do undergo appreciable self-ionisation, coordination often needs to be considered explicitly in discussing acid/base and other reactions and equilibria. 

Dorfman and collaborators have recently developped a very promising technique for the production of carbenium ions as transient species in halocarbon sdvents based on the dissociative ionisation of suitable precursors induced by pulse radiolysis of the solvent. While the extremely interesting kinetic results vdiich this group is obtaining will be discussed in Sect. II-G4, it is emphasised here that the fast time response of the apparatus used allows the characterisation of carbenium ions hitherto unobservable because of their excessive reactivity. The ultraviolet absorption spectrum and some reactions of the benzylium ion have been studied for the first time wdth this powerful tool. From the point of view of cationic pdymerisation, the information obtained in this type of work is particularly relevant, since it deals vrith the identification and reactivity of carbenium icais formed in very low concentration in the nght kind of medium. Cation radicals had already been prepared by pulse radiolysis involving nondissociative ionization (electron ejection or transfer), as will be discussed in Sect. II-K. 

Before leaving the subject of 3,5-cyclosteroids, comment must be made about unusual situations where inverted 3 a-substituted products are formed from cholesteryl tosyiate and similar steroids. Several authors have recorded the occurrence of Sjt2 substitutions, with Walden inversion at C 3>, by powerful nucleophiles. These reactions occur particularly in solvents of low dielectric constant, where ionisation processes become relatively unfavourable. Ammonia, methylamine, and benzyl-amine are sufficiently reactive as nucleophiles to give moderate yields of the corresponding AS-3a-amines [33,36], and the more powerfully nucleophilic dimethylamine is reported to give virtually complete conversion into the ga-dimethyl-amino-A -derivative [36]. Other reactive nucleophiles which... 

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