Carbonic acid reactivity

Carbon dioxide, the final oxidation product of carbon, is not very reactive at ordinary temperatures. However, in water solution it forms carbonic acid [463-79-6] H2CO2, which forms salts and esters through the typical reactions of a weak acid. The first ionization constant is 3.5 x 10 at 291 K the second is 4.4 x 10 at 298 K. The pH of saturated carbon dioxide solutions varies from 3.7 at 101 kPa (1 atm) to 3.2 at 2,370 kPa (23.4 atm). A soHd hydrate [27592-78-5] 8H20, separates from aqueous solutions of carbon dioxide that are chilled at elevated pressures. 

In the diaziridine field many compounds are known bearing N-YL, A/-alkyl and A-acyl groups, but here no dramatic changes in reactivity are caused by A-substituents. N-Aryldiaziridines are underrepresented. The ring carbon is in the oxidation state of a carbonyl compound or, in the diaziridinones (5) and the diaziridinimines (6) that of carbonic acid. In single cases, diaziridine carbon bears chlorine or fluorine. 

CDI and the other A /V -carbonylbisazoles of sufficiently high reactivity react with alcohols ROH to produce diesters of carbonic acid RO-CO-OR, and with amines R R2NH to give diamides of carbonic acid (ureas) R N-CO-NR 2. By use of corresponding bifunctional partners, heterocyclic systems are accessible through insertion of the carbonyl group between two heteroatoms (see Chapter 7). 

A, A /-Carbonylbis[2(3//)-benzoxazolethione], obtained from mercaptobenzoxazole and dimeric phosgene as the more thermodynamically stable A-acyl product, also represents a reactive heterocyclic diamide of carbonic acid, and is therefore used in the same way for ester syntheses as A /V -carbonyldibenzimidazole.c 1321... 

Alternatively, some conclusions can be derived from the relative reactivities of car-banions. For example, DePuy and colleagues13 made use of a clever method involving reactions of silanes with hydroxide ion to deduce acidities of such weak acids as alkanes and ethylene. The silane reacts with hydroxide ion to form a pentacoordinate anion that ejects a carbanion held as a complex with the hydroxysilane rapid proton transfer gives the stable silanoxide ion and the carbon acid (equation 5). 

This chapter will begin with a brief overview of the development of carbanion chemistry followed by a section devoted to the structure and stability of carbanions. Methods of measuring carbon acidity and systematic trends in carbanion stability will be key elements in this chapter. Next, processes in which carbanions appear as transient, reactive intermediates will be presented and typical carbanion mechanisms will be outlined. Finally, some new developments in the field will be described. Although the synthetic utility of carbanions will be alluded to many times in this chapter, specific uses of carbanion-like reagents in synthesis will not be explored. This topic is exceptionally broad and well beyond the scope of this chapter. 

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