Carbonyl compounds acid-base behavior

Note that only the hydrogens on the a positions of carbonyl compounds arc acidic. Hydrogens at 7. d, and sc on, arc not acidic and can t be removed by base. Well account for this unique behavior of o hydrogens shortly. 

In their behavior toward acids and bases, trityl ethers resemble the acetals, glycosides and orthoesters, being fairly stable in alkali, but easily split by acidic reagents. Thus, in cases where carbonyl compounds are used to protect pairs of hydroxyl groups by the formation of acetals, tritylation may often be used to protect suitably situated individual hydroxyl groups. 

In addition to their behavior as bases, primary and secondary amines can also act as very weak acids because an N-H proton can be removed by a sufficiently strong base. We ve seen, for example, how diisopropylamine (pAia = 40) reacts with bntyllithium to yield lithium diisopropylamide (LDA Section 17.4). Dialkylamine anions like LDA are extremely powerful bases that are often nsed in laboratory organic chemistry for the generation of enolate ions from carbonyl compounds (Section 17.5). They are not, however, encountered in biological chemistry. 

Alkoxides like Al(OR)3 will behave as fairly strong acid catalysts, owing to the tendency of the aluminum atom in these compounds to accept a share in a pair of electrons. In view of this fact it is to be expected that aluminum alkoxides will cause the amphoteric aldehydes to behave as bases, so that a simple ester results. As this behavior involves only the basic characteristics of the carbonyl group itself it makes little difference whether we have one, two, or no a-hydrogen atoms in the aldehyde an ester is the result. The first step is a typical acid-base neutralization reaction with the formation of a coordinate covalent bond ... 

Recent work by Ford et al. demonstrates that a variety of metal carbonyl clusters are active catalysts for the water-gas shift under the same reaction conditions used with the ruthenium cluster (104a). In particular, the mixed metal compound H2FeRu3(CO)13 forms a catalyst system much more active than would be expected from the activities of the iron or ruthenium systems alone. The source of the synergetic behavior of the iron/ruthenium mixtures is under investigation. The ruthenium and ruthenium/iron systems are also active when piperidine is used as the base, and in solutions made acidic with H2S04 as well. Whether there are strong mechanistic similarities between the acidic and basic systems remains to be determined. 

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