Chlorine isotope effects in the solvolysis of substituted 1-phenylethyl chlorides

McLennan, Duncan J.; Stein, Allan R.; Dobson, B.

doi:10.1139/v86-199

Cited by 4 publications

(3 citation statements)

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“…The chlorine leaving group kinetic isotope effects obtained by the IRMS method in two different experiments are given in Table . The agreement between the two isotope effects is similar to that reported by Stein and McLennan, who found that repeated measurements of a chlorine isotope effect differ by ∼0.0004.…”

Section: Results and Conclusionsupporting

confidence: 89%

“…Chlorine kinetic isotope effects are a sensitive tool for distinguishing between alternative mechanisms . Although the magnitude of these isotope effects is small (large chlorine kinetic isotope effects are close to 1%), they can be used to indicate whether a bond to a chlorine atom is broken or weakened in the transition state. , Chlorine has two stable isotopes in a proportion suitable for direct measurement of their isotopic ratio, so determining a chlorine isotope effect is both feasible and attractive. , However, determining chlorine isotope effects is impeded by the tedious procedure required for the conversion of chlorine atoms into the gaseous methyl chloride that is required for determining the 35 Cl/ 37 Cl isotopic ratio by isotope ratio mass spectrometry (IRMS). ,− This obstacle could be avoided if the isotopic composition of a reactant could be determined by direct analysis. In fact, this can be accomplished by determining the 35 Cl/ 37 Cl isotopic ratio of silver chloride samples by fast atom bombardment-isotope ratio mass spectrometry (FAB-IRMS).…”

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A New Method of Determining Chlorine Kinetic Isotope Effects

et al. 1998

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Two methods have been used to measure the chlorine leaving group kinetic isotope effect for the S(N)2 reduction of benzyl chloride to toluene by sodium borohydride in DMSO at 30.000 °C. The reaction was monitored by titrating the unreacted borohydride ion. One method involved determining the chlorine isotope effect using the classical IRMS method, which requires the conversion of the chloride ions into gaseous methyl chloride that is analyzed in an isotope ratio mass spectrometric analyses (Hill, J. W.; Fry, A. J. Am. Chem. Soc. 1962, 84, 2763. Taylor, J. W.; Grimsrud, E. P. Anal. Chem. 1969, 41, 805.). Two different measurements using this method yielded isotope effects of k(35)/k(37) = 1.007 19 ± 0.000 19 and 1.007 64 ± 0.000 19. The second method was a new technique where the ratio of the chlorine isotopes was obtained by fast atom bombardment mass spectrometry on the silver chloride recovered from the reaction, i.e., from the first step in the classical procedure. Therefore, the new method is much simpler and avoids the time-consuming preparation, purification, and recovery of the gaseous methyl chloride. Although the experimental error is larger (k(35)/k(37) = 1.008 03 ± 0.00 10 and 1.008 02 ± 0.000 65) when the new technique is used to analyze the silver chloride samples from the same set of experiments that were used to measure the isotope effect by the classical method, the chlorine isotope effect found by the two methods is identical within experimental error. This large chlorine kinetic isotope effect indicates there is considerable C(α)-Cl bond rupture in the S(N)2 transition state.

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Section: Results and Conclusionsupporting

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A New Method of Determining Chlorine Kinetic Isotope Effects

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“…A continuing research interest of ours has been the ion-pair mechanism for nucleophilic substitutions, especially bimolecular substitutions at saturated carbon (1,2). As part of the mechanistic evidence, P-deuterium kinetic isotope effects were desired.…”

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β-Deuterium kinetic isotope effects for identity processes: bromide ion substitution at 1-bromo-1-arylethanes and 2-bromooctane

Stein¹

1994

Can. J. Chem.

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. While deuterium kinetic isotope effects for solvolyses have been extensively studied, other nucleophilic substitutions have received less attention, and identity processes, that is, substitutions where the nucleophile and leaving group are the same, have rarely been examined. Identity reactions must pass through a truly symmetrical stage, a transition state or an intermediate, so that data will be of interest to both theoretical and experimental chemists. Values of kHlkD have been determined by polarimetry for bromide exchange-racemization at ArCHBrCH3/CD3 (Ar = C6H5, 4-Br-and 4-Me-C6H4, and 3,4,-dimethyl-C6H3) in acetone, acetonitrile, and nitromethane. Observed values are analogous to values seen in solvolyses. They range from 1.01 to 1.35 and, in some cases, increase markedly as the concentration of Bu4NBr decreases. Solvolyses are either first order or pseudo first order whereas plotting observed racemization rate versus [Bu4NBr] allows separation of first-and second-order components; those species giving more stable carbocations in the more dipolar solvents, the systems showing kHlkD variation with Br-concentration, alone show an appreciable first-order component. The second-order kHlkD ratio averages 1.062 2 4 . 0 1 8 at temperatures ranging from 25 to 50°C for all substrates in the three solvents, very analogous to the values seen for racemization of 1,1,1 -d3-2-bromooctane or solvolysis of ethyl substrates but considerably lower than the typical solvolysis values of 1.15-1.25 for secondary, and 1.35-1.5 for tertiary substrates. The first-order kHlkD values obtained are higher, 1.1-1.5. These and other results are discussed. de Bu4NBr. Les solvolyses sont soit du premier ordre ou du pseudo premier ordre alors que, dans notre cas, la courbe de la rackmisation en fonction de la concentration de Bu4NBr permet de sCparer les composantes du premier et du deuxieme ordre; seules les especes conduisant B carbocations les plus stables dans les solvants les plus dipolaires, les systemes presentant une variation du kH/kD en fonction de la concentration du Br-, prksentent une portion importante des rCaction du premier ordre. Pour tous les substrats, dans les trois solvants, ides temptratures allant de 25 a 50°C, le valeur du rapport kH/kD des rkactions du deuxikme ordre rend une moyenne de 1,062 5 0,018 et elles sont tres semblables ? I celles observkes lors de la ractmisation du I ,1,1-d3-2-bromoCthane ou de la solvolyse des substrats avec des groupes Cthyles; elles sont toutefois beaucoup plus basses que les valeurs typiques de 1,15 1,25 ou 1,35 B 1,5 obsewtes lors de solvolyses de substrats secondaires ou tertiaires. Les valeurs des kH/kD du premier ordre sont plus ClevCes (1,l-1,5). On discute de ces valeurs et d'autres rtsultats.[Traduit par la redaction]

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Solvolysis of 1-alkyl-1-chloro-1-(4-methyl)phenylmethanes. Nucleophilic solvent intervention and extendedYBnCl scale

Liu

Chang

et al. 1997

J. Phys. Org. Chem.

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Chlorine isotope effects in the solvolysis of substituted 1-phenylethyl chlorides

Cited by 4 publications

References 33 publications

A New Method of Determining Chlorine Kinetic Isotope Effects

A New Method of Determining Chlorine Kinetic Isotope Effects

β-Deuterium kinetic isotope effects for identity processes: bromide ion substitution at 1-bromo-1-arylethanes and 2-bromooctane

Solvolysis of 1-alkyl-1-chloro-1-(4-methyl)phenylmethanes. Nucleophilic solvent intervention and extendedYBnCl scale

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