Surprisingly low aqueous acidity at the alpha-positions of pyridiniums and pyrimidinium: the role of solvation.
The p K(a)'s of the 6-CH groups of N-methyl-2-methoxypyridinium, N-methyl-4-methoxypyridinium, and 1-methyl-2,4-dimethoxypyrimidinium ions in aqueous solution were determined to be about 33. The p K(a) values were nearly identical to each other and surprisingly similar to those reported for the neutral pyridones and uracil. Further determination of the enthalpies and entropies of the H-D exchange reactions revealed the role of solvation.
Department of Chemistry and Biochemistry, San Francisco State University, San Francisco, California 94132, USA.
Freeman M
FM
Wong
Christina C
CC
Capule
David X
DX
Chen
Scott
S
Gronert
Weiming
W
Wu
D000143:Acids / Q000737:chemistry*; D003653:Decarboxylation; D041961:Deuterium Exchange Measurement; D006863:Hydrogen-Ion Concentration; D007477:Ions / Q000737:chemistry; D007700:Kinetics; D015394:Molecular Structure; D009963:Orotic Acid / Q000031:analogs & derivatives / Q000737:chemistry; D011726:Pyridinium Compounds / Q000737:chemistry*; D011743:Pyrimidines / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry*; D013696:Temperature; D013816:Thermodynamics
C512852:1,3-dimethylorotic acid; D000143:Acids; D007477:Ions; D011726:Pyridinium Compounds; D011743:Pyrimidines; D012997:Solvents; D009963:Orotic Acid