Use of kinetic isotope effects in mechanism studies. 3. Measurement of hydrogen isotope effects on the primary chlorine isotope effect during elimination reactions

Koch, H.; Koch, Judith G.; Tumas, William; McLennan, Duncan J.; Dobson, B.; Lodder, Gerrit

doi:10.1021/ja00547a033

Cited by 13 publications

(7 citation statements)

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“…The ethoxide-promoted dehydrochlorination of QHjCHClCFjCl (III) occurs without prior exchange with ethanol and has kH/kP = 3.06 at 25 °C and k3S/k37 = 1.01229 at 0 °C. 19 The hydrogen isotope effects are almost temperature independent with £aH =< E® and AH/AD = 4.8,20 and the chlorine isotope effect drops to 1.01003 for Ill-d.…”

Section: Resultsmentioning

confidence: 94%

Proton-transfer reactions. 4. Near-unity kinetic isotope effects for hydron exchange and dehydrofluorination reactions

Koch¹,

Dahlberg²,

Lodder³

et al. 1983

J. Am. Chem. Soc.

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Section: Resultsmentioning

confidence: 94%

Proton-transfer reactions. 4. Near-unity kinetic isotope effects for hydron exchange and dehydrofluorination reactions

Koch¹,

Dahlberg²,

Lodder³

et al. 1983

J. Am. Chem. Soc.

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“…One method to probe this effect is the study of kinetic solvent isotope effects (KSIE), kR0D/kR0H, for the various reactions. Effects of 2.2 to 2.4 were obtained for reaction of alkoxide with C6H5CC1=CF2, and values of 2.3 to 2.8 were found for elimination and exchange reactions of our highly halogenated systems.23,24 A KSIE of 1.9 was obtained23 for methoxide-promoted dehydroiodination of C6H5-CL(CH3)CH2I, a compound which is thought to react by an E2 mechanism.25, 26 Gold and Grist27 have analyzed possible origins for the KSIE in methanol and conclude that effects are due to a multiply Me X-…”

Section: Discussion and Resultsmentioning

confidence: 95%

Proton-transfer reactions. II. Effects of internal return on reactivity difference between alkoxide-promoted eliminations in tert-butyl alcohol and ethyl alcohol

1981

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“…To satisfy the Swain-Schaad relationship the experimental (k H /k T ) obs of 20.6 should be 17% to 26% greater with (k H /k T ) obs of 23 or 26 [11]. Dahlberg was able to model their results with an internal-return mechanism [34]. The methoxide-promoted elimination reactions of four YC 6 H 4 C i HXCH 2 X can be modeled by using all three hydrogen isotopes, Table 18.2.…”

Section: Proton Transfer Associated With Methoxide Promoted Dehydrohamentioning

confidence: 99%

Formation of Hydrogen‐bonded Carbanions as Intermediates in Hydron Transfer between Carbon and Oxygen

Koch

2006

Hydrogen‐Transfer Reactions

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This chapter is divided into three sections: Proton transfer from carbon acids to methoxide ion; proton ttransfer from methanol to carbanion intermediates; proton transfer associated with methoxide promoted dehydrohalogenation reactions. Proton Transfer from Carbon Acids to Methoxide IonKnowledge of equilibrium pK a values of carbon acids is important for an understanding of organic chemistry; however, they are not always reliable indicators of relative rates for proton transfer reactions. Ritchie [1] predicted in 1969 that carbon acids whose conjugate bases have localized charge will show "kinetic acidities" greater than their thermodynamic acidities. This is illustrated by the weaker acid pentafluorobenzene-t [PFB-t], pK a = 25.8 [2], with a methanolic sodium methoxide catalyzed protodetritiation rate, k = 2.57 · 10 -2 M -1 s -1 [3], that is 15 times faster at 25 C than that for 9-phenylfluorene-9-t [9-PhFl-9-t], pK a = 18.5 and k = 1.54 · 10 -3 M -1 s -1 [4]. Primary kinetic isotope effects [PKIE] can be used to determine C-H bond breaking in the rate limiting steps of any reaction mechanism. Normal hydrogen isotope effects are largely due to zero-point energy differences of the stretching frequencies of C-H vs. C-D bonds. If only stretching frequencies are considered the value for k H /k D would be 6.2 for the C-i H bond. With allowance for bending vibrations this could result in a k H /k D = 10, k H /k T = 27 and k D /k T = 2.7 at 25 C [5]. Melander [6] and Westheimer [7] suggested that lower values of k H /k D could be due to residual zero-point energy in an asymmetric transition structure. For this reason the magnitude of k H /k D was often used to assign early or late transition structures for hydron-transfer reactions. Smaller values due to asymmetric transition structures should still obey the Swain-Schaad relationship [8], k H /k T = (k H /k D ) 1.442 . The k D /k T = 1.0 associated with the exchange reactions of PFB i H suggest that the C-H bond is not broken in the rate limiting step and this differs significantly from the k D /k T of 2.54 measured from the reactions of 9-PhFl-9-i H.

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Use of kinetic isotope effects in mechanism studies. 3. Measurement of hydrogen isotope effects on the primary chlorine isotope effect during elimination reactions

Cited by 13 publications

References 0 publications

Proton-transfer reactions. 4. Near-unity kinetic isotope effects for hydron exchange and dehydrofluorination reactions

Proton-transfer reactions. 4. Near-unity kinetic isotope effects for hydron exchange and dehydrofluorination reactions

Proton-transfer reactions. II. Effects of internal return on reactivity difference between alkoxide-promoted eliminations in tert-butyl alcohol and ethyl alcohol

Formation of Hydrogen‐bonded Carbanions as Intermediates in Hydron Transfer between Carbon and Oxygen

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