ACID BASE EQUILIBRIA INVOLVING CARBONYL COMPOUNDS
q The qualitative criterion for whether an acid/base reaction goes to completion or not is: The stronger acid is converted to the weaker acid. Since the pKa of the ketone is ca. 19 and that of water is ca. 16, water is the stronger acid. Therefore the equilibrium lies to the left.
q The quantitative criterion for obtaining the equilibrium constant K for this acid/base reaction is:
K = K(Reactatnt Acid)/K(Product Acid). In the present application the K for the reactant acid, the ketone, is 10-19, while the K for the product acid, water, is 10-16. The ratio is therefore 10-19/10-16 or 10-3.
q A much stronger base than hydroxide ion is needed to do this.
q Quantitative conversion is important because if all of the base used (like hydroxide ion) is not consumed in forming the enolate, it will be available to compete with the enolate: both are good nucleophiles.
q Although the simple amide (NH2_ anion) is strong enough, it is also a pretty good nucleophile and adds readily to the carbonyl carbon. This process is irreversible because the result anion is an alkoxide anion, whereas the reactant anion was an amide type anion (much less stable). Consequently, the hindered amide base LDA is used because it is equally strong as a base and steric hindrance slows down the addition to the carbonyl carbon in comparison with abstracting the small proton.