The Role of Single-Electron Transfer in SN2-Type Substitution Reactions of Anions with Alkyl Halides

Article abstract of DOI:10.1021/jo00281a035

Rate constants (k_obsd) for reactions of electrophiles with: (a) various carbanions, including 9-substituted fluorenide ions (9-GFl^-), 9-substituted xanthenide ions, α-cyano carbanions, anb β-diketo enolate ions, (b) phenothiazinide (PTZ^-) and carbazole (Cb^-) nitranions, (c) 4-substituted phenoxide ions, and (d) thiphenoxide ions, have been campared with rate constants for single-electron transfer (k_SET) calculated using a Marcus-type equation.For both 9-GFl^- carbanion and 2-GPTZ^- nitranion families reacting with SET acceptors, such as 1,1-dinitrocyclohexane, the k_obsd/k_SET ratio is near unity.For 9-GFl^- carbanions reacting with Ph2CHCl the k_obsd/k_SET ratios range from 2 to 10⁵, those for 2-PTZ^- nitranions from 370 to 1.3*10⁴, and those for Cb^- nitranions from 10⁸ to 10¹⁰.For reactions of 4-GC6H4O^- oxanions with Ph2CHCl the ratios range from 10³ for G = Me2N to 10¹³ for G = CN; for PhS^- ion reacting with n-BuBr the ratio is 10¹⁷.The significance of these results with respect to the role of an outer sphere SET mechanism in substitution reactions of anions with alkyl halides is discussed.