Meldrum 's acid is the most easy to ionization of 1, 3 - dicarbonyl compounds. The pKa low of 4.97, as a common carboxylic acid. Mie acid an unusually high content of acid in the past 100 years has not get a perfect explanation completely, and therefore become a prominent problem in the theory of organic chemistry.
The first explanation is based on analysis of dipole. Arndt was proposed based on the interpretation of the stereo electronic effect. Arndt, points out that ordinary ester owing to two lone pair electrons in silane oxygen and conjugated carbonyl there are two possibilities: one is with carbonyl PI lowest empty orbital conjugate; One is with carbonyl sigma key antibonding orbital conjugate. Both can reduce the conjugated molecular energy, is a molecule in a more stable state, so the ordinary ester ground-state conformation is the advantage of Z type. As a result of the existence of ring, mie acid lactone structure forced half ester in the E type, thus preventing the lone pair electrons with carbonyl sigma key antibonding orbital conjugate effect, as a result of the carbonyl electrophilic sexual enhancement to a stable alpha carbon anion more effectively. Because it is two and a half ester work together in a methylene, the effect of equivalent to two times, so the mie acid is much higher than ordinary lactone acid.
This can explain better explain Meldrum 's acid high acidity, but the energy balance of later found this explanation explained only 50% of the reason, the remaining 50% can't explain. Someone in 2001 and 2004, respectively, in this acid using quantum chemistry do the different calculation, put forward from the perspective of more explanation. But so far there are still a lot of this problem space.
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