The effect of solvent on the structure of the transition state for the S(N)2 reaction between cyanide ion and ethyl chloride in DMSO and THF probed with six different kinetic isotope effects.
The secondary alpha- and beta-deuterium, the alpha-carbon, the nucleophile carbon, the nucleophile nitrogen, and the chlorine leaving group kinetic isotope effects for the S(N)2 reaction between cyanide ion and ethyl chloride were determined in the very slightly polar solvent THF at 30 degrees C. A comparison of these KIEs with those reported earlier for the same reaction in the polar solvent DMSO shows that the transition state in THF is only slightly tighter with very slightly shorter NC-C(alpha) and C(alpha)-Cl bonds. This minor change in transition state structure does not account for the different transition structures that were earlier suggested by interpreting the experimental KIEs and the gas-phase calculations, respectively. It therefore seems unlikely that the different transition states suggested by the two methods are due to the lack of appropriate solvent modeling in the theoretical calculations. Previously it was predicted that the transition state of S(N)2 reactions where the nucleophile and the leaving group have the same charge would be unaffected by a change in solvent. The experimental KIEs support this view.
Fang
Yao-ren
YR
Department of Chemistry and Biochemistry, Laurentian University, Sudbury, Ontario, P3E 2C6, Canada.
MacMillar
Susanna
S
Eriksson
Jonas
J
Kołodziejska-Huben
Magdalena
M
Dybała-Defratyka
Agnieszka
A
Paneth
Piotr
P
Matsson
Olle
O
Westaway
Kenneth Charles
KC