Combined dual substituent constant and activation parameter analysis assigns a concerted mechanism to alkaline ethanolysis at phosphorus of Y-substituted phenyl diphenylphosphinates.
Second-order rate constants have been measured for reactions of Y-substituted phenyl diphenylphosphinates (1a-h) with EtO(-)K(+) in anhydrous ethanol. A linear Brønsted-type plot is obtained with beta(Lg) = -0.54, a typical beta(Lg) value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with sigma(o) and sigma(-) constants are linear but exhibit many scattered points, while the corresponding Yukawa-Tsuno plot results in excellent linear correlation with r = 0.41. The r value of 0.41 indicates that the leaving group departs at the rate-determining step (RDS) whether the reactions proceed through either a concerted or a stepwise mechanism. However, a stepwise mechanism in which departure of the leaving group occurs at the RDS is excluded since the incoming EtO(-) ion is much more basic and a poorer leaving group than the leaving aryloxide. The DeltaH(++) values determined in the current reactions are strongly dependent on the nature of the substituent Y, while the DeltaS(double dagger) values remain constant on changing the substituent Y in the leaving group, i.e., from Y = H to Y = 4-NO(2) and Y = 3,4-(NO(2))(2). These DeltaH(++) and DeltaS(++) trends also support a concerted mechanism.
Um
Ik-Hwan
IH
Division of Nano Sciences and Department of Chemistry, Ewha Womans University, Seoul, 120-750, Korea. ihum@ewha.ac.kr
Park
Jee Eun
JE
Shin
Young-Hee
YH
D000468:Alkalies / Q000737:chemistry; D000431:Ethanol / Q000737:chemistry*; D007700:Kinetics; D015394:Molecular Structure; D010636:Phenols / Q000737:chemistry*; D010721:Phosphinic Acids / Q000737:chemistry*; D010758:Phosphorus / Q000737:chemistry*
D000468:Alkalies; D010636:Phenols; D010721:Phosphinic Acids; D010758:Phosphorus; D000431:Ethanol