Rate laws base hydrolysis

For example the hydrolysis of optically active 2 bromooctane in the absence of added base follows a first order rate law but the resulting 2 octanol is formed with 66% inversion of configuration... 

General acid/base catalysis is less significant in natural fresh waters, although probably of some importance in special situations. This phenomenon can be described fairly well via the Bronsted law (relating rate constants to pKa and/or pKb of general acids and bases). Maximum rates of general acid/base catalysis can be deduced from a compound s specific acid/base hydrolysis behavior, and actual rates can be determined from relatively simple laboratory experiments (34). 

Studies of the base-hydrolysis mechanism for hydrolysis of technetium complexes have further been expanded to an octahedral tris(acetylacetonato)techne-tium(III) [30], Although a large number of studies dealing with base hydrolysis of octahedral metal(III) complexes have been published [31], the mechanism of the tris(acetylacetonato)metal complex is still unclear. The second-order base hydrolysis of the cationic complex tris(acetylacetonato)silicon(IV) takes place by nucleophilic attack of hydroxide ion at carbonyl groups, followed by acetylacetone liberation, and finally silicon dioxide production [32], The kinetic runs were followed spectrophotometrically by the disappearance of the absorbance at 505 nm for Tc(acac)3. The rate law has the following equation ... 

AF values for cyanide attack at [Fe(phen)3] +, [Fe(bipy)3] + and [Fe(4,4 -Me2bipy)3] " in water suggest a similar mechanism to base hydrolysis, with solvation effects dominant in both cases. Cyanide attack at [Fe(ttpz)2] , where ttpz is the terdentate ligand 2,3,5,6-tetrakis(2-pyridyl)pyr-azine, follows a simple second-order rate law activation parameters are comparable with those for other iron(II)-diimine plus cyanide reactions. Interferences by cyanide or edta in spectro-photometric determination of iron(II) by tptz may be due to formation of stable ternary complexes such as [Fe(2,4,6-tptz)(CN)3] (2,4,6-tptz= (66)). ... 

There are a number of ways in which the rate law for base hydrolysis can depart from the simple second-order rate law discussed above in addition to the special cases in liquid ammonia already referred to. 

If instead of pre-equilibrium deprotonation the hydroxide ion was associated with the cationic substrate and thereby generated a more labile species, the rate law discussed above would also apply. Such a mechanism was originally suggested by Chan to account for the departures from the first-order [OH-] dependence that he observed in the base hydrolysis of [Co(NH3)5C1]2+,331 but this was later shown to be an artefact.332 The mechanism has been reinvoked recently to account for similar departures for the simple rate law for the loss of Me2SO from [Co(tren)(NH3)(Me2SO)]3+ (Me2SO trans to the tertiary nitrogen333). 

Finally, an aspartic acid residue is necessary for full catalysis and this residue is thought to use its CO2 group as a general base. A chemical model shows that the hydrolysis of p-nitrophenyl acetate in aqueous acetonitrile containing sodium benzoate and imidazole follows the rate law ... 

Both nucleophile and base are essentially similar reagents as they each have a lone pair of electrons to bond to either proton or electrophile. A lone pair donor acting as a nucleophile or as a general base (Scheme 16) in an overall hydrolysis reaction leads to identical rate laws. 

Intramolecular general acid catalysis is normally detected in the first instance by a horizontal portion on the plot of logiokobs versus pH, governed by the add dissociation constant of the substrate. Thus, the hydrolyses of various salicyl acetals (including the P-D-glucopyranoside below pH 10, " where a base-catalysed process occurs) obey the rate law of eqn (3.7), where ko is the first-order rate constant for hydrolysis of the neutral molecule and ka is the second-order rate constant for the acid-catalysed hydrolysis of the neutral molecule ... 

General acid-base catalysis is defined experimentally by the appearance in the rate law of acids and/or bases other than lyonium or lyate ions. For example, the hydrolysis of enol ethers 1.2 (Scheme 2.2) is general acid-catalyzed. In strong acid the rate expression will be the same as in Scheme 2.1, but near neutral pH the rate is found to depend also on the concentration of the buffer (HA + A ) used to maintain the pH. Measurements at different buffer ratios show that the catalytic species is the acid HA. (If more than one acid is present there will be an additional term kHAi[HA ][1.2] for each.)... 

Region II (pH 10 to 12.5 for (CO)5Cr=C(SMe)Ph, pH 10 to 11.5 for (CO)5W=C(SMe)Ph)). Nucleophilic attack by water is rate limiting. Just as is the case for the hydrolysis of alkoxy carbene complexes, there is also general base catalysis of water attack, hence feobsd is given by equation (79) ( f [B] term not shown in Schemes 11 and 12). This rate law implies that the relationships of equations (80) and (81) hold the fe2 H+ is again negligible and omitted from equation (81). 

In general, kB (for base hydrolysis) is some 104 times kA (for acid hydrolysis). The interpretation of this rate law has occasioned an enormous amount of experimental study and discussion, but as yet there is nothing approaching a complete and generally accepted interpretation. Here, we can but touch on a few main aspects of the problem. 

---