Cation reaction with water

When bubbler systems are used for collection, the gaseous species generally undergoes hydration or reaction with water to form anions or cations. For example, when SOj and NH3 are absorbed in bubblers they form HSO3 and NH4, and the analytical techniques for measurement actually detect these ions. Table 13-1 gives examples of gases which may be sampled with bubbler systems. 

Coming back to the mechanism of dediazoniation, mechnism B in Scheme 9-2 is consistent with all experimental data known in 1973. Mechanism B was, indeed, mentioned in that paper (Zollinger, 1973 a) as an explanation, but not proposed as the explanation because it violated the common knowledge mentioned above. If that reverse reaction of the phenyl cation is faster than the forward reaction with water or metal halides, the rate is dependent on the concentrations of compounds involved only in the second step of the mechanism, even if that step is much faster than the first (forward) step. 

Ce4+ is a versatile one-electron oxidizing agent (E° = - 1.71 eV in HC10466 capable of oxidizing sulfoxides. Rao and coworkers66 have described the oxidation of dimethyl sulfoxide to dimethyl sulfone by Ce4+ cation in perchloric acid and proposed a SET mechanism. In the first step DMSO rapidly replaces a molecule of water in the coordination sphere of the metal (Ce v has a coordination number of 8). An intramolecular electron transfer leads to the production of a cation which is subsequently converted into sulfone by reaction with water. The formation of radicals was confirmed by polymerization of acrylonitrile added to the medium. We have written a plausible mechanism for the process (Scheme 8), but there is no compelling experimental data concerning the inner versus outer sphere character of the reaction between HzO and the radical cation of DMSO. 

The guanine radical cations (G +) are detected by their reactions with water, which leads after treatment with piperidine or ammonia to selective strand cleavage [14]. A similar charge detection method was used by J.K. Barton, G.B. Schuster and I. Saito as described in their articles in this volume. The cleavage products were separated and quantified by gel electrophoresis. A typical example is shown in Fig. 7 where the GGG unit acts as a thermodynamic sink for the positive charge, and the efficiency of the charge transfer can be measured by the product ratio Pggg/Pg-... 

Thus reaction of the 1-propyl cation (13) with water (reaction type a) will yield propan-l-ol (14), elimination of a proton from (13) will yield propene (15, reaction type b), while rearrangement of (13, reaction type d)—in this case migration of He—will yield the 2-propyl cation... 

Type (b) reaction on this rearranged cation (16) will yield more propene (15), while type (a) reaction with water will yield propan-2-ol... 

The cations that will reaction with water to form H+ (or H30+) ions are... 

Ferrocenyl-substituted allenyl cations 28 were generated when 1,3-diferrocenyl-substituted secondary and ferrocenyl-substituted tertiary alcohols 29 were treated with trifluoroacetic acid27. These were rapidly converted into trifluoroacetoxyallylic ions by solvent addition the ions gave ferrocenyl-substituted enones by reaction with water (equation 8). 

Cation derived from a strong base Reaction with water neither ion Solution neutral Examples NaCl, K2SO4, Ca(N03)2 Reaction with water only the anion Solution basic Examples NaCHsCOO, KF, Mg(HS04)2... 

Cation derived from a weak base Reaction with water only the cation Solution acidic Examples NH4CI, NH4NO3, NH4CIO4 Reaction with water both ions Solution neutral if ka = kb, acidic if ka> kb, basic if kb > ka Examples NH4CN (basic), (NH4)2S (basic), NH4NO2 (acidic)... 

The mechanism which could explain the formation of these products is described in Scheme 27. In an EC mechanism, the intermediate radical cation 48a could undergo a follow-up reaction with water as a nucleophile to form radical 48b which could than dimerize through S-N or S-S bond formation or react with 48a to yield 50 and 51 as the fianl one-electron oxidation products. In an ECE mechanism, intermediate 48b is further oxidized to 48c which reacts with acetonitrile as a solvent to give 49 as the final two-electron oxidation product. The cation intermediate 48c can react with the parent molecule 48 through [2 -f 3]-cycloaddition to give the final products 50 and 51. The [2 -f 3]-... 

The oxides of low-valency metals (i.e., with cations in oxidation number < -i-4) are typically ionic compounds [76]. They are most frequently easily obtained in crystalline forms. In ionic metal oxides the coordination of the cations (four to eight) is generally higher than their valency (one to four) and this also occurs for the coordination of 0 oxide ions (three to six). The bulk basic nature of the ionic metal oxides is associated with the strong polarization of the metal-oxygen bond, to its tendency to be dissociated by water and to the basic nature of the products of their reaction with water (i.e., the metal hydroxides) [67]. 

Schindewolf and WUnschel [112] have studied solvated electron reactions in liquid ammonia and water with several univalent anions and divalent cations. Ions such as NO3, N02, and BrO in water showed diffusion-limited behaviour and the ions Cd2+, Ni2+, Co2+, and Zn2+ in water displayed diffusion-limited behaviour or faster. Schindewolf and WUnschel considered that reactions of none of these ions were quite diffusion-limited in liquid ammonia. Applying the hydrodynamic correction suggests that the anionic reaction with solvated electrons may just be diffusion-limited, but the cations reaction with solvated electrons remains slower than diffusion-limited. 

It might seem surprising that a nucleophilic reaction with water competes with proton loss from the phenanthrenonium ion considering the stability of the aromatic product. As discussed by Richard24 (and considered further below) this reflects a higher intrinsic reactivity of the cations toward nucleophilic attack which compensates for the thermodynamic disadvantage of this reaction. For the phenanthrenonium ion the ratio of rate constants for deprotonation and nucleophilic attack on the cation (kp/kH2o) is 25 25 for the 1-protonated naphthalene it is 1600,106 for 9-protonated anthracene, 1.8.75... 

Comparisons of structurally related hydroxy- and methoxy-substituted cations show that hydroxy is more stabilizing by between 4 and 5 log units. This difference was recognized 20 years ago by Toullec who compared pifas for protonation of the enol of acetophenone and its methyl ether145 (-4.6 and 1.3, respectively) based on a cycle similar to that of Scheme 15, but with the enol replacing the hydrate, and a further cycle relating the enol ether to a corresponding dimethyl acetal and methoxycarbocation.146 Toullec concluded, understandably but incorrectly, that there was an error in the pA a of the ketone (over which there had been controversy at the time).147,148 In a related study, Amyes and Jencks noted a difference of 105-fold in reactivity in the nucleophilic reaction with water of protonated and O-methylated acetone and concluded that the protonated acetone lacked a full covalent bond to... 

By contrast, measurement of pATR = 4.7 for the Fe(CO)3-cooordinated cyclo-hexadienyl cation 44 (Scheme 26) indicates a 107-fold more favorable equilibrium constant for carbocation formation than for the uncoordinated cation.197 However, a more dramatic effect of coordination is to render nucleophilic reaction with water more favorable than loss of a proton. A pXa = 9 can be estimated by computing the energy differences between coordinated and uncoordinated benzene and coordinated cyclohexadiene. This compares with the value of —24.5 for the uncoordinated cyclohexadienyl cation. The large difference must reflect the unfavorable effect of Fe(CO)3 coordination on benzene, an effect analogous to that found by Mayr for Fe (CO)3 coordination on the tropylium ion.196 As expected, both the coordinated cyclohexadienyl and tropylium ions are highly stereoselective toward exo attack by water. 

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