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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 13, Number 1
BKCSDE 13(1)
January 20, 1992 

Theoretical Studies on the Gas-Phase Pyrolysis
Chang Kon Kim, Bon-Su Lee, Ikchoon Lee*
The gas-phase pyrolysis reactions of 2-alkoxypyrimidines(Ⅱ), 2-alkoxypyrazines(Ⅲ), 4-ethoxypyrimidine(Ⅳ) and 3-ethoxypyridazine(Ⅴ) are investigated theoretically using the AM1 MO method. These compounds pyrolyze in a concerted retro-ene process with a six-membered cyclic transition state (TS). The relative order of reactivity if (Ⅳ)〉(Ⅱ)〉(Ⅲ)〉(Ⅴ), which can be rationalized by the two effects arising from electron-withdrawing power of the aza-substituent: (ⅰ) Electron withdrawal efrom the C-O bond accelerates the rate and (ⅱ) electron withdrawal from the N1-atom, that is participating in the six-membered TS, deactivates the reaction. We are unable to explain the experimental result of the greatest reactivity for pyridazine, (Ⅴ), with our AM1 results. The reactivity increase accompanied by successive methylation of the ethoxy group, ethoxy〈 iso-propoxy 〉tert-butoxy, is due to a release of steric crowding in the activation process.
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