Acid-base reactions counterions

The conservation of charge is a fundamental law for all processes, such as the addition of nucleophiles to it systems or acid-base reactions. The first step of die basic hydrolysis of nitriles has die hydroxide ion adding to the it bond of die nitrile. For the purposes of mechanistic discussion, the hydroxide is shown widiout its counterion and die net charge on the reactant side of the equation is — 1. Consequently, the product of diis first step (and each subsequent step) must also have a net negative charge. 

The Reformatsky reaction is the reaction of an a-halo ester with an aldehyde or ketone in the presence of zinc metal as shown in Scheme 1. The usual product of the reaction is a -hydroxy ester, which may be dehydrated in subsequent steps to give an unsaturated ester. A zinc ester enolate (1), the so-called Reformatsky reagent, is an intermediate in the reaction and the sequence is thus classified as an aldol condensation. Compared to the usual base-promoted aldol procedures, the distinguishing features of the Reformatsky reaction are the use of a metal-halogen redox reaction rather than an acid-base reaction to form the enolate, and the fact that the counterion of the enolate is zinc. 

Direct diastereo- and enantioselective hydrohydroxyalkylations of butadiene, an abundant petrochemical feedstock, required the use of a mthenium catalyst modified by a chiral phosphate counterion derived from Hg-BINOL. The anion is attached to the metal center through the acid-base reaction of H2Ru(CO)(PPh3)3 with the indicated chiral phosphoric acid. With the chiral counterion as the sole chiral inducing element, primary benzylic alcohols hydrohydroxyalkylate butadiene with good levels of anri-diastereo- and enantioselectivity (Scheme 3) [37]. 

The acid-base reaction renders organic ions that have lower ionic mobility than the small inorganic ions currently used as PE partners. Thus, the electrostatic attraction between the ionized pending groups of the PE and the organic ions yields a high proportion of counterionic condensation with IQc in the range of 10 -10. Available results have shown that the Kcc are not affected by the dilution of the dispersions. 

Recently, Liu has developed a Bronsted acid activated trifunctional organocatalyst, based on the BINAP scaffold, that was used for the first time to catalyze aza MBH reactions between N tosylimines and MVK with fast reaction rates and good enantioselectivity at room temperature. This trifunctional catalyst containing a Lewis base, a Bronsted base and a Bronsted acid, required add activation to confer its enantioselectivity and rate improvement for both electron rich and electron deficient imine substrates. The role of the amino Lewis base of 27 was investigated and found to be the activity switch in response to an acid additive. The counterion of the acid additive was found to influence not only the excess ratio but also the sense of asymmetric induction (Scheme 13.23) [36]. 

An attempt has been made to study the working mechanism of ionites during the catalytic hydration of 4-cyanopyridine on AB-17-8 ionite in OH form [28-31a]. TWo mechanisms have been proposed the catalysis by counterions and the catalysis by fixed ionite ions. According to the former mechanism, the reaction proceeds by the homogeneous acid-base route ... 

There is very little published on chemical reactions occurring at the polymer/water interface in polymer colloids beyond the simple acid-base neutralizations of the counterions mentioned above. Some evidence has been developed to indicate that certain secondary reactions must have occurred either during the emulsion polymerization or subsequently on storage. 

The ketone is added to a large excess of a strong base at low temperature, usually LDA in THF at -78 °C. The more acidic and less sterically hindered proton is removed in a kineti-cally controlled reaction. The equilibrium with a thermodynamically more stable enolate (generally the one which is more stabilized by substituents) is only reached very slowly (H.O. House, 1977), and the kinetic enolates may be trapped and isolated as silyl enol ethers (J.K. Rasmussen, 1977 H.O. House, 1969). If, on the other hand, a weak acid is added to the solution, e.g. an excess of the non-ionized ketone or a non-nucleophilic alcohol such as cert-butanol, then the tautomeric enolate is preferentially formed (stabilized mostly by hyperconjugation effects). The rate of approach to equilibrium is particularly slow with lithium as the counterion and much faster with potassium or sodium. 

Carboxylic acids can be alkylated in the a position by conversion of their salts to dianions [which actually have the enolate structures RCH=C(0")2 ] by treatment with a strong base such as LDA. The use of Li as the counterion is important, because it increases the solubility of the dianionic salt. The reaction has been applied to primary alkyl, allylic, and benzylic halides, and to carboxylic acids of the form RCH2COOH and RR"CHCOOH. This method, which is an example of the alkylation of a dianion at its more nucleophilic position (see p. 458),... 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

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