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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 34, Number 8
BKCSDE 34(8)
August 20, 2013 

The α-Effect in Nucleophilic Substitution Reactions of Y-Substituted-Phenyl Diphenylphosphinates with HOO and OH
Hyo-Jeong Hong, Ae Ri Bae, Ik-Hwan Um*
The α-effect, Ground state, Transition state, Solvation effect, H-bonding interaction
Second-order rate constants (kHOO−) for the nucleophilic substitution reactions of Y-substituted-phenyl diphenylphosphinates (4a-4i) with HOO− in H2O have been measured spectrophotometrically. The α-nucleophile HOO− is 10-70 times more reactive than the reference nucleophile OH− although the former is ca. 4 pKa units less basic than the latter, indicating the α-effect is operative. The Brønsted-type plot for the reactions of 4a-4i with HOO– is linear with βlg = –0.51, a typical βlg value for reactions which were reported to proceed through a concerted mechanism. The Yukawa-Tsuno plot is also linear with ρ = 1.40 and r = 0.47, indicating that a negative charge develops partially on the O atom of the leaving group, which can be delocalized to the substituent Y through resonance interactions. Thus, the reactions have been proposed to proceed through a concerted mechanism. The magnitude of the α-effect (i.e., the kHOO−/kHO− ratio) decreases linearly as the leaving-group basicity increases. It has been concluded that solvation effect is not solely responsible for the α- effect found in this study but the transition-state stabilization through an intramolecular H-bonding interaction is also responsible for the α-effect.
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