Hydroxide ion, reactions

Nucleophilic attack at the C-5 carbon atom of 1,2,4-thiadiazoles has been proposed as a reaction mechanism for many of the ring transformations described throughout this chapter. Thus, 2-methyl-3,5-diphenyl-1,2,4-thiadiazolium fluorosulfate (33) is cleaved by alkoxide giving the benzimidate (34) in the case of hydroxide ions, reaction continues to the benzoylamidine stage (35) (Scheme 10) <82AHC(32)285>. 

It is important to recognize the difference between the order of a reaction with respect to a specific reactant and the overall order of a reaction. The /order of a reaction with respect to a particular reactant is the power to which the concentration of that reactant must be raised to have direct proportionality between concentration and reaction rate. According to Equation 8-2 the rate of the chloromethane-hydroxide ion reaction is first order with respect to chloromethane and first order with respect to hydroxide ion. In Equation 8-1 the rate is first order with respect to chloromethane and zero order with respect to hydroxide ion because [OH0]0 = 1. The overall order of reaction is the sum of the orders of the respective reactants. Thus Equations 8-1 and 8-2 express the rates of overall first-order and second-order reactions, respectively. 

Petrongolo C, Ranghino G, Scordamaglia R. Ab initio study of P-lactam antibiotics. I. Potential energy surface for the amidic carbon-nitrogen bond breaking in the P-lactam+hydroxide ion reaction. Chem Phys 1980 45 279-290. 

The CTAB-catalyzed reaction between p-nitrophenyl diphenyl phosphate and hydroxide or fluoride ion is also inhibited by phenyl, diphenyl, and p-t-butylphenyl phosphates (Bunton et al., 1969). The inhibition by these bulky anions decreases, however, with decreasing pH, and at lower pH values, where the hydroxide ion reaction becomes negligible, the reaction of p-nitrophenyl diphenyl phosphate with p-t-butylphenyl, phenyl, and inorganic phosphate ions is enhanced significantly by CTAB (Fig. 12 and Table 8). The order of the micellar rate enhancement for these nucleophilic reactions (p-t-BuC6H40P03 >C6H60P0 > HOPOf ). The cationic micellar catalysis is thus not dependent on the nucleophilicity of the anions but is explicable in terms of hydrophobic interactions between the nucleophiles and the micelle (Bunton et al., 1969). 

Every chemical reaction requires some time for its completion, but some reactions are very fast and some are very slow. Reactions between ions in solution without change in oxidation state are usually extremely fast. An example is the neutralization of a strong acid by a strong base, which proceeds as fast as the solutions can be mixed. Presumably nearly every time a hydronium ion collides with a hydroxide ion reaction occurs, and the number of collisions is very great, so that there is little delay in the reaction. 

Some measurements of the rates of this process" show that it is not a simple hydroxide ion reaction, in the pH range 10-12. Determination of the acidity constants is necessary. 

Very similar results have been obtained by a number of workers for reactions involving hydrophilic anions in cationic micelles [72,73,76]. An important point is that the ion-exchange constants determined from rates or equilibria of hydroxide ion reactions in cationic micelles agree reasonably well with independent estimates from physical measurements on the relative affinities of various anions for cationic micelles [74,86]. 

Such a rate expression is consistent with (but does not absolutely prove) that the reaction occurs in one stage. (For more details of the bromopropane/hydroxide ion reaction, see page 249). 

We have now seen aU three of the situations illustrated by arrows with double-sided arrowheads, namely the redistribution of tt bonds and/or lone pairs, formation of a new a bond (generally from a lone pair or v bond), and breaking of a <7 bond (generally to form a new lone pair or sometimes a new tt bond). Often, as in the case of the acetic acid-hydroxide ion reaction, more than one arrow is used in a given mechanism step. Now that you have seen all of the important types of arrows, we can point out their most important common feature ... 

There are acids that do not contain hydrogen ions and bases that do not contain hydroxide ions. Reactions between them are also called neutralization reactions. The H -plus-OH neutralization is the most common and the only one we will consider here. 

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