Other Reactions of Nucleophiles and Carbonyl Compounds

We showed in Figs. 3-2 and 3-3 that the tetrahedral intermediate which is initially formed from the reaction of a nucleophile with a carbonyl compound may further react in a number of different ways. In this section, we will consider some reactions which proceed along the pathway indicated in Fig. 3-3. The hydration of ketones is a reaction analogous to the hydrolysis of an ester, with the first step of the reaction involving nucleophilic attack of water on the carbonyl group. The tetrahedral intermediate is trapped by reaction with a proton to yield the hydrated form of the ketone, the geminal diol (Fig. 3-15). Similar reactions occur with alcohols as nucleophiles to yield, initially, hemiacetals. 

A slightly different pattern of reactivity is seen when 3.8 interacts with antimony(m) fluoride in methanol. In this case, the product contains a ligand derived from the hemia-cetal, but the hydroxy group is deprotonated and co-ordinated to the metal centre, to give an N,iV,0-bonded anionic ligand (Fig. 3-17). It is not known whether the co-ordination of the oxygen to the metal centre occurs after the hydration reaction (in which case we are 

The effect of charge on the cation is important, and is clearly seen in the hydration of 4-pyridinecarbaldehyde, 3.9. In aqueous solution the free ligand is only partially hydrated, and at equilibrium the ratio of the carbonyl form to the hydrate is about 55 45 (Fig. 3-18). Upon co-ordination of the nitrogen atom of the pyridine to ruthenium(in) in the complex [Ru(NH3)5(3.7)]3+, the ligand is over 90 % hydrated at equilibrium. 

A direct metal involvement in the hydration of an aldehyde is seen in the ruthenium(n) complex of the hydrated form of 2-pyridinecarbaldehyde (3.10). 

3 Reactions of Co-ordinated Carbonyl Compounds with Nucleophiles 

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