Electron-Transfer in Aqueous Solution

Very little has been reported regarding 2,4 -bipyridine. As expected, it is quaternized preferentially on the y-pyridyl ring " and with excess methyl iodide the diquaternary salt l,r-dimethyl-2,4 -bipyridinium diiodide is ob-tained. ° The latter salt, like diquat, is reduced to a radical cation by one-electron reducing agents. The potential E of the one-electron transfer in aqueous solution is —0.64 lV-alkyl-2,4 -bipyridinones have been... 

There is little experimental information on possible solvent dynamical effects for electron transfer in aqueous solution. However, water is a dynamically "fast solvent, vos being determined by "solvent inertial effects so that the usual transition-state formula [eqn. (22)] should be applicable for determining vn (Sect. 3.2.1). Consequently, solvent dynamical effects in this and other "low friction media (e.g. acetonitrile) should be controlled by the rotational frequency of individual solvent molecules and limited to reactions involving only very small inner-shell barriers (Sect. 3.3.1). 

Curves for Electron Transfer in Aqueous Solutions Theory and Simulations. 

When this probability is equal to 1 (uniform concentration), the reaction is of pseudo-first order. This is the case, for example, in photoinduced proton transfer in aqueous solutions from an excited acid M (=AH ) (see Section 4.5) M is always within the encounter distance with a water molecule acting as a proton acceptor, and thus proton transfer occurs effectively according to a unimolecular process. This is also the case of photoinduced electron transfer in aniline or its derivatives as solvents an excited acceptor is always in the vicinity of an aniline molecule as an electron donor. In both cases, the excited-state reaction occurs under non-diffusive conditions and is of pseudo-first order. 

Willson RL (1970) Pulse radiolysis study of electron transfer in aqueous disulphides solutions. Chem Commun 1425-1426... 

Triorganyl-sulfonium, -selenonium and -telluronium salts are reduced by carbon dioxide radical anions/solvated electrons produced in aqueous solution by radiolysis. The radical expulsion accompanying reduction occurred with the expected leaving group propensities, i.e. benzyl > secondary alkyl > primary alkyl > methyl > phenyl. Much higher product yields in the reduction of selenonium and telluronium compounds have been accounted for in terms of a chain reaction with carbon-centred radicals, with formate serving as the chain transfer agent.282... 

Electrochemical irreversibility caused by slow heterogeneous electron-transfer kinetics at the electrode surface can limit the ability of the measurement to yield thermodynamically meaningful potentials. While proton transfer in aqueous solutions is generally very fast, heterogeneous... 

The recommended value of the reduction potential for SO," indicates that it is a moderate one-electron oxidant. This is supported by the rate constants measured for its reactions with one-electron reductants in aqueous solution, a few of which are listed in Table 3. The reactivity of SO," towards these compounds increases with decreasing reduction potential in some cases, for example for the aromatic amines, this leads to a reversal in reaction direction. Therefore, while N,N,A N -tetramethyl-/ -phenylenediamine and p-phenylenediamine are oxidized by SO,", the radical cations of A, A -dimethylaniline and aniline oxidize to SO," [65]. The oxidation of phenol and hydroxyphenols depends strongly upon the degree of deprotonation, reflecting both the lower reduction potentials of the anions and the requirement that the electron transfer from the neutral phenol also be accompanied by either deprotonation or proton transfer [66]. 

The standard electrode potentials , or the standard chemical potentials /X , may be used to calculate the free energy decrease —AG and the equilibrium constant /T of a corrosion reaction (see Appendix 20.2). Any corrosion reaction in aqueous solution must involve oxidation of the metal and reduction of a species in solution (an electron acceptor) with consequent electron transfer between the two reactants. Thus the corrosion of zinc ( In +zzn = —0-76 V) in a reducing acid of pH = 4 (a = 10 ) may be represented by the reaction ... 

The complex cyanides of transition metals, especially the iron group, are very stable in aqueous solution. Their high co-ordination numbers mean the metal core of the complex is effectively shielded, and the metal-cyanide bonds, which share electrons with unfilled inner orbitals of the metal, may have a much more covalent character. Single electron transfer to the ferri-cyanide ion as a whole is easy (reducing it to ferrocyanide, with no alteration of co-ordination), but further reduction does not occur. 

When interpreting proton transfers in Chapter 7, we found that the experimental data showed that for most solute species in aqueous solution the values of J lay between 0.25 and 1.0 electron-volts. We shall now be interested in the values of L that are necessary to account for the observed solubilities of solids in water. We may expect the range of values of L to be rather similar the main difference is that in the solution of a crystal the value of Aq in (8G) is never less than 2, whereas in most of the proton transfers discussed in Chapter 7 the value of Aq was either unity or zero. 

Two Cells Placed Back to Back. In Sec. 57 of Chapter 6 we discussed the e.m.f. of two cells placed back to back. Both cells contained the same solute in aqueous solution, but at different concentrations. We saw that, when a current flows, the net result is simply to transfer an amount of solute from one solution to the other. Hence the observed resultant e.m.f. of the pair of cells is a measure of the change in free energy on transferring a pair of ions from one solution to the other in fact, this change of free energy expressed in electron-volts is numerically equal to the e.m.f. expressed in volts. 

The Sulfate Ion. In Fig. 36 we see that the vacant level of the (SO ) ion in aqueous solution lies only 0.13 electron-volt above the occupied level of HCOOH. If the interval has a comparable value when sulfate ions are present in formic acid as solvent, the thermal agitation should transfer a large number of protons from solvent HCOOH molecules to the (SO4)" ions. This was found to be the case when Na2SC>4 was dissolved in pure formic acid. Such a transfer of protons from molecules of a solvent to the anions of a salt is analogous to the hydrolysis of the salt in aqueous solution and is known as solvolysis, as mentioned in Sec. 76. In a 0.101-molal solution of Na2SC>4 in formic acid the degree of the solvolysis was found to be 35 per cent.1... 

Furthermore, since in Sec. 121 we found the value J = 0.36 electron-volt for the proton transfer (211), this gives the occupied proton level of the (HCOOII2)+ ion a position at (0.52 — 0.36) = 0.16 electron-volt above that of the (H30)+ ion in formic acid as solvent. This is shown in Fig. 65, where, for comparison, a diagram for proton levels in aqueous solution has been included, the level of the (H30)+ ion in aqueous solution being drawn opposite to the level of the same ion in formic acid solution. This choice is quite arbitrary, but was made in order to show more clearly that we may expect that one or more acids that are strong... 

Another common type of reaction in aqueous solution involves a transfer of electrons between two species. Such a reaction is called an oxidation-reduction or redox reaction. Many familiar reactions fit into this category, including the reaction of metals with acid. 

Mixed second-order kinetics. Consider an electron transfer reaction between the triva-lent ions of neptunium and iron in aqueous solution 23... 

One of the attractions of aprotic solvents is that the electron transfer behaviour of many compounds is much simpler than in protonic media. However, this is not always so for example, the quinone/hydroquinone couple is very simple in aqueous solution but it is complicated in aprotic solvents by the number of protonation equilibria which no longer lie well to one side as they do in aqueous solution (Bessard et al., 1970). 

A hydrogen cation is a hydrogen atom that has lost its single electron, leaving a bare hydrogen nucleus. A bare hydrogen nucleus is a proton. Thus, any reaction in which H moves from one species to another is called a proton-transfer reaction. Protons readily form chemical bonds. In aqueous solution, they associate with water molecules to form hydronium ions. 

At electrode potentials more negative than approximately - 2.8 V (SHE), free solvated electrons appear in the solution as a result of (dark) emission from the metal. At this potential the electrochemical potential of the electrons according to Eq. (29.6) is about —1.6 eV, which is at once the energy of electron hydration in electron transfer from vacuum into an aqueous phase. 

Cyanide and thiocyanate anions in aqueous solution can be determined as cyanogen bromide after reaction with bromine [686]. The thiocyanate anion can be quantitatively determined in the presence of cyanide by adding an excess of formaldehyde solution to the sample, which converts the cyanide ion to the unreactive cyanohydrin. The detection limits for the cyanide and thiocyanate anions were less than 0.01 ppm with an electron-capture detector. Iodine in acid solution reacts with acetone to form monoiodoacetone, which can be detected at high sensitivity with an electron-capture detector [687]. The reaction is specific for iodine, iodide being determined after oxidation with iodate. The nitrate anion can be determined in aqueous solution after conversion to nitrobenzene by reaction with benzene in the presence of sulfuric acid [688,689]. The detection limit for the nitrate anion was less than 0.1 ppm. The nitrite anion can be determined after oxidation to nitrate with potassium permanganate. Nitrite can be determined directly by alkylation with an alkaline solution of pentafluorobenzyl bromide [690]. The yield of derivative was about 80t.with a detection limit of 0.46 ng in 0.1 ml of aqueous sample. Pentafluorobenzyl p-toluenesulfonate has been used to derivatize carboxylate and phenolate anions and to simultaneously derivatize bromide, iodide, cyanide, thiocyanate, nitrite, nitrate and sulfide in a two-phase system using tetrapentylammonium cWoride as a phase transfer catalyst [691]. Detection limits wer Hi the ppm range. 

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