Reactions of cations with water

Any cation in liquid water will be surrounded by water molecules oriented with the oxygen atom facing the cation (Fig. 5.2). The number of water molecules in the coordination sphere will normally be close to the ideal coordination number expected for the cation (Appendix 4) but if the hydrated complex is to be stable, the bonding strength of the cation must match that of the water molecule. This is best illustrated by examples. 

Cations with still larger charges or lower coordination numbers lose progressively more and more H ions until with typically only one or two are left, (P02(0H)2) or (P030H) , and with none are left (SO, Fig. 4.1(b)). At this stage these are no longer thought of as deprotonated hydrated cations but as oxyanions or protonated oxyanions. 

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Decomposition or alteration of a chemical snbstance by water. (2) In aqueous solutions of electrolytes, the reactions of cations with water to produce a weak base or of anions to produce a weak acid. 

Reaction of A with water under kinetic control conditions leads to the 5)5,19-cyclo-6 -ol (69a), whereas under conditions of thermodynamic control A rearranges to the isomeric cation B which reacts with water to give the B-homo-7)5-ol (70a). 

It is not intended to extend this discussion of reactions of carbocations with water to consideration of the alcoholic solvents trifluoroethanol (TFE) and hexa-fluoroisopropanol (HFIP), which have been extensively studied and reviewed by McClelland and Steenken.3 However, an important point of interest of these solvents is that their reactivities toward carbocations are greatly reduced compared with water (by up to a factor of 104 in TFE and 108 in HFIP) and that differences in rate constants can be observed between cations which would react indiscriminately at the solvent relaxation limit in water. The following comparisons of rate constants for carbocations with similar pAR values reacting with hexafluoroiso-propanol241,242 reinforces the conclusion that structural variations in the cation lead to changes in intrinsic barrier and, for example, that phenyl substitution is probably associated with such an increase in going from benzyl to benzhydryl (although the benzyl cation itself is not shown). 

A notable difference between reactions of carbocations with water as a nucleophile and a base is the significantly higher intrinsic barrier for the latter. This difference has been demonstrated most explicitly by Richard, Williams, and Amyes22,246 for reaction of the a-methoxyphenethyl cation 68 with methanol (rather than water) acting as the base and nucleophile. The two reactions and the intrinsic barriers calculated from their rate and equilibrium constants are shown in Scheme 32. Values of A = 6.8 and 13.8 kcal mol 1 are found for the substitution and elimination, respectively. 

Radical 80 has been prepared as its perchlorate salt by anodic oxidation in ethyl acetate in the presence of hthium perchlorate. The reactivity toward nucleophiles of material so prepared was investigated nitrite and nitrate ions give 2-nitrodibenzo[l,4]dioxin although the mechanisms of the reactions are not clear. Pyridine gives 7V-(2-dibenzo[l,4]dioxinyl)pyridinium ion (84). Other nucleophiles acted as electron donors and largely reduced 80 back to the parent heterocycle they included amines, cyanide ion and water. In an earlier study, the reaction of 80 with water had been examined and the ultimate formation of catechol via dibenzo[l,4]dioxin-2,3-dione was inferred. The cation-radical (80) has been found to accelerate the anisylation of thianthrene cation-radical (Section lII,C,4,b) it has been found to participate in an electrochemiluminescence system with benzo-phenone involving phosphorescence of the latter in a fluid system, and it has been used in a study of relative diffusion coefficients of aromatic cations which shows that it is justified to equate voltammetric potentials for these species with formal thermodynamic redox potentials. The dibenzo[l,4]dioxin semiquinone 85 has been found to result from the alkaline autoxidation of catechol the same species may well be in-... 

SECTION 16.9 The acid-base properties of salts can be ascribed to the behavior of their respective cations and anions. The reaction of ions with water, with a resultant change in pH, is called hydrolysis. The cations of... 

Several interesting variations on the reaction have recently been introduced. While the reaction with water is often carried out using a sulfonated, and hence water-soluble, tri-arylphosphine, the use of a palladium(O) complex of l,3,5-triaza-7-phosphaadamantane (77, Scheme 25) was recently reported. Monflier and co-workers reported that in addition to running the reaction under a CO2 atmosphere, the reaction of butadiene with water is facilitated by the use of cationic or neutral (nonionic) surfactants. The role of cationic surfactants such as dodecylammonium hydroxide is conceived to be threefold ... 

ACID-BASE PROPERTIES OF SALTS (SECTION 16.9) The acid-base properties of salts can be ascribed to the behavior of their respective cations and anions. The reaction of ions with water, with a resultant change in pH, is called hydrolysis. The cations of the alkali metals and the alkaline earth metals as well as the anions of strong acids, such as Cl , Br , F, and NO3 , do not undergo hydrolysis. They are always spectator ions in acid-base chemistry. A cation that is the conjugate acid of a weak base produces H upon hydrolysis. 

Show a mechanism (a sequence of molecular-level events) of the reaction of BaO with water to produce aqueous barium cations and hydroxide anions. 

The reactions of alcohols with hydrogen halides to give alkyl halides (Chapter 4) are nucleophilic substitution reactions of alkyloxonium ions m which water is the leaving group Primary alcohols react by an 8 2 like displacement of water from the alkyloxonium ion by halide Sec ondary and tertiary alcohols give alkyloxonium ions which form carbo cations m an S l like process Rearrangements are possible with secondary alcohols and substitution takes place with predominant but not complete inversion of configuration... 

In a classic study in 1940, Crossley and coworkers demonstrated that the rates of nucleophilic substitution of the diazonio group of the arenediazonium ion in acidic aqueous solution were independent of the nucleophile concentration, and that these rates were identical with the rate of hydrolysis. Since that time it has therefore been accepted without question that these reactions proceed by a DN + AN mechanism, i.e., that they consist of a rate-determining irreversible dissociation of the diazonium ion into an aryl cation and nitrogen followed by rapid reactions of the cation with water or other nucleophiles present in solution (Scheme 8-6). 

J.12 C6HsNH3C1 is a chloride salt with an acidic cation, (a) If 50.0 g of C6H5NH3C1 is dissolved in water to make 150.0 mL of solution, what is the initial molarity of the cation (b) Write the chemical equation for the proton transfer reaction of the cation with water. Identify the acid and the base in this reaction. 

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