Base hydrolysis reaction

Before leaving Co(III) chemistry we must consider the base hydrolysis reaction... 

Acid hydrolysis of an octahedral metal ion complex is typically a dissociative or SNl-type reaction. In the case of base hydrolysis, reactions tend to display SN2-type reaction mechanisms, although others take place by what is termed an SnI-conjugate base mechanism. The latter involves attack by an electrophile to abstract a proton... 

In this paper we will discuss the substitution reactions of complexes of the type, [Co en2 A X]+n in nonaqueous solvents and will show how the general conditions that apply here can be extended to aquation and other solvolytic reactions, and to the base hydrolysis reaction. 

The concept of preassembly as a requirement for substitution may throw light upon the vexed question of the mechanism of the base hydrolysis reaction. It has long been known that complexes of the type, [Co en2 A X]+n can react rapidly with hydroxide in aqueous solution. The kinetic form is cleanly second-order even at high hydroxide concentrations, provided that the ionic strength is held constant. Hydroxide is unique in this respect for these complexes. Two mechanisms have been suggested. The first is a bimolecular process the second is a base-catalyzed dissociative solvolysis in which the base removes a proton from the nitrogen in preequilibrium to form a dissociatively labile amido species (5, 19, 30). 

A molecule that can both donate and accept a proton is said to be amphiprotic. The acid dissociation reaction (10-8) has a larger equilibrium constant than the base hydrolysis reaction (10-9), so we expect that a solution of leucine will be acidic. 

For the kinetically inert low-spin Co(III) complexes the mechanism of exchange is certainly dissociative although kinetic studies can give results that are superficially misleading. For example, the base hydrolysis reaction... 

We have Investigated the kinetics of base hydrolysis reactions of the cobalt acldate complexes In aqueous solution and In several mlcroemulslon solutions In which detergent concentrations are at least twice the respective cmc. The results are complicated by the onset of a slow secondary reaction which Is presumably formation of Insoluble, polymeric hydroxo, or hydrated hydroxo compounds. 

Table II. The Lack of Influence of X on Stereochemistry of the Base Hydrolysis Reaction"...

On the basis of such a mechanism, a second rule must be obeyed by the base hydrolysis reactions of [Co(en)2LX] + complexes, which is that the per cent retention of configuration by a cis substrate must be greater than or equal to the per cent of intermediate A times the fraction of cis product from A. This rule is followed by the data given in Table IV. In anticipation of the discussion in the next section, it should be noted that since B may not be symmetrical, it can provide a path for the reaction of an optically active complex to give an optically active product of inverted configuration. 

In conclusion, the overoxidized PAn is the de-protonated QI form, which can either undergo a further hydrolysis reaction to produce the final product, BQ, in aqueous media or can be re-activated by re-protonation or re-reduction in non-aqueous media. The hydrolysis mechanism of overoxidized PAn or pemigraniline films is similar to that of the Schiff base hydrolysis reaction. 

Since the early 1950 s when systematic studies of the substitution reactions of non-labile octahedral complexes were begun by Taube, Brown, Ingold and Nyholm and Basolo, there has been a preoccupation with features of aquation and base hydrolysis reactions. This work has been excellently summarised in all its features in a progressive series of and at least to this con-... 

Micellar and microemulsion effects on reactivity in aquation and base hydrolysis reactions of iron(II)-diimine complexes have been much studied/ The latest contribution deals with the effects of added potassium chloride or bromide to micelles of the respective cetyltrimethylammonium halides. Effects on base hydrolysis of [Fe(phen)3] and its 4,7-diphenyl and 3,4,7,8-tetramethyl derivatives can be interpreted in terms of competitive binding to the micelles in a pseudophase-ion exchange model. In connection with these secondary effects of added halides it should be mentioned that further studies of kinetics of aquation of [Fe(bipy)3] and of [Fe(phen)3] in strong aqueous solutions of chlorides have been interpreted in terms of water and of chloride attack, with the postulation of transient diimine-chloride-iron(II) intermediates. ... 

Hydroxide ion is different from other reagents with respect to its reactivity toward cobalt(iii) ammine complexes. It reacts very rapidly (as much as 10 times faster than H2O) with cobalt(iii) ammine complexes in a base hydrolysis reaction (33). In this reaction, a first-order dependence on the substituting... 

Another pieee of evidenee to support the 7d CB mechanism is that if there is no N-H hydrogen present in a cobalt(m) complex, the complex reacts slowly with OIT. This suggests that acid-base properties of the complex are more important to the rate of reaction than are the nucleophilic properties of OIT. This base hydrolysis reaction of cobalt(ni) ammine complexes illustrates the fact that kinetic data often can be interpreted in more than one way and that rather subtle experiments must be performed to eliminate one or more possible meehanisms. 

Rudienium(Ill) complexes were observed to be sensitive to base hydrolysis as early as 1964. However the difficulties associated with the preparation of suitable complexes and the other attractions of Ru(III) amine chemistry (photochemistry, redox reactions, dinitrogen complexes etc.) tended to divert attention from any systematic study of the base hydrolysis reactions. The most important features that have emerged from the work carried out. Table 3.10, is that substitution invariably takes place with complete retention of configuration and the presence of an amine group tram to the leaving group generates... 

Rates of the acid, Hg ion-induced, and base hydrolyses of c/y-[Co(tmd)t-(RNH2)C1] + ions (R = H, Me, Et, Pr , Bu , Bu or Ph) are ca. 10 faster than for the analogous [Co(en)2(RNH2)Cl] + ions. This arises primarily from a reduction in the activation energy by ca. 2.5 kcal mol . A comparison of the rate data for acid, Hg -assisted, and base hydrolysis reactions is given in Table 6. cis-trans isomerization was not observed in the aquation reactions. 

Common intermediates are not involved in the Hg +-, Ag+-, and NO -induced aquation and base hydrolysis reactions of [Co(Metren)(NH3)X] + ions (X=CI, Br, N3, or NH3) which have structures (6)—(8). The stereochemistries of the HaO/OH-product ions were established using A-Mc C-enriched materials related to known crystal structures. An h mechanism is favoured for these reactions with H O involved prior to complete Co—X bond fission. 

Hypochlorous acid has the structure H—O—Cl. Write the base hydrolysis reaction of hypochlorite, OCl". Given that A a for HOCl is 3.0 X 10", find for hypochlorite. 

Write out all reactants and products formed in the two base hydrolysis reactions just mentioned. 

Quenching the silver-catalyzed base hydrolysis reaction with acid and isolating the reaction products as dinitro complexes also shows inversion and considerable isomerism to the trans species... 

The study was also consistent with the results of Chan and Tobe, who had shown that neither step of the two-step base hydrolysis reaction... 

The base hydrolysis of D -[Co(en)2Cl(OH)] produces mainly cis product with essentially complete (> 90 %) retention of configuration. The results of the D -[Co(en)2Cl2] base hydrolysis reaction indicate that the D -[Co(en)2Cl(OH)] ion predominates over the corresponding l optical isomer at all concentrations, while the reverse is true for the dihydroxo enantiomers—under conditions which give dihydroxo species to a greater extent than predicted by rate data in the literature. Some L -[Co(en2-(0H)2] and trflns-[Co(en)2(OH)2] appear to be formed above that predicted by the individual reaction sequences deduced from kinetics measurements in dilute solution. This effect had been noted earlier by Pearson, Meeker, and Basolo, but was later discounted by Chan and Tobe, who did not observe this phenomenon when hydroxide ion is slowly added at 0°. The results (Table 1) agree with both observations. 

The involvement of conjugate base species in such base hydrolysis reactions must be considered in arriving at a mechanistic path for the Bailar inversion. And even though a d and l inversion can be accommodated by an unsymmetrical trigonal bipyramidal conjugate base intermediate, " the concentration and silver ion dependence of the Bailar inversion require other explanations. Four such possibilities can be imagined for the concentration dependent reaction. 

Other Metal Ions.— Base hydrolysis of /rawj [Rh(en)aX2] ions leads to almost complete trans- cis isomerization when X = Cl or Br, and to ca. 50 % isomerization when X = I. Rates and activation parameters for the aquation and base hydrolysis reactions were considered previously (Table 22). An S Nlcb mechanism seems likely when X = Cl or Br, but an 5n11p mechanism cannot be ruled out when X = I. 

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