.alpha.-Deuterium isotope effect for displacement of the nitrate group

KENNETH CHARLES WESTAWAY and SYED FASAHAT ALI. Can. J. Chem. 57,1354Chem. 57, (1979. The nucleophilic substitution reactions of a series of 4-substituted phenylbenzyldimethylammonium ions with thiophenoxide ions at 0°C in N,N-dimethylformamide have been used to demonstrate how a change in the leaving group alters the structure of the SN2 transition state. Heavy atom (nitrogen) kinetic isotope effects, secondary a-deuterium kinetic isotope effects and Hammett p values provide qualitative descriptions of both the nucleophilea-carbon and a-carbon -leaving group bonds in the transition states of these reactions. The results indicate that changing to a better leaving group causes the bond between the a-carbon and the nucleophile to be much more fully formed while the bond to the leaving group is essentially unchanged. The results are discussed in the light of current theories of substituent effects on SN2 reactions and a possible explanation for the surprising results (i) that the greatest effect is in the bond more remote from the point of structural change and (ii) that more nucleophilic assistance is required to displace a better leaving group is given. KENNETH CHARLES WESTAWAY et SYED FASAHAT ALI. Can. J. Chem. 57,1354Chem. 57, (1979. On a utilise les reactions de substitutions nucleophiles d'une skrie d'ions X-4 phenylbenzyldimkthylammonium avec les ions thiophknolates a 0°C dans le N,N-dimkthylformamide pour dkmontrer I'influence d'un changement dans le nucleofuge sur la structure de I'ktat de transition SN2. Les effets isotopiques cinetiques des atomes lourds (azote), les effets isotopiques cinetiques secondaires des deutkrium en a et les valeurs p de Hammett fournissent des descriptions qualitatives des liaisons nuclkophile -carbone en alpha'et carbone en alpha groupe nuclkofuge dans les ktats de transition de ces reactions. Les r6sultats indiquent que si I'on utilise un meilleur nuclCofuge la liaison entre le carbone alpha et le nucleophile devient plus developpke alors que la liaison vers le groupe nuclkofuge est pratiquement inchangee. On discute des rksultats a la lumiere des thkoriques actuelles des effets de substituants sur les rkactions SN2 et I'on propose une explication plausible pour les resultats surprenants: (i) que I'effet est le plus grand dans la liaison la plus Bloignee du point de changement structural et (ii) qu'il faut plus d'aide de la part du nucleophile pour dtplacer un meilleur nuclkofuge.[Traduit par le journal] Introduction have been used in studies on a wide range of reac~l t h~~~h the mechanisms of the simple one-step tions in an effort to determine how the structure of (SJ) nucleophilic substitution reaction at saturated the reactants affects the structure of the S,2 transicarbon, eq. [I], have been investigated for over half tion states. In addition, several theoretical studies (18-23) have predicted the answer to this question.

Section: Ci-ch2ch20hmentioning

confidence: 99%

Section: Ci-ch2ch20hmentioning

confidence: 99%

Isotope effects in nucleophilic substitution reactions. III. The effect of changing the leaving group on transition state structure in S_N2 reactions

Westaway¹,

Ali²

1979

“…Young and Jencks (37) recently showed that a modified Yukawa-Tsuno expression, eq. [5]: [5] log (klk,) = pon + pr(o+ -on)…”

Section: -7mentioning

confidence: 99%

“…Equation [5] correlates the substituent effects on the reaction rates with the polar (pon) and resonance (pr(o+ -on)) effects of the substituents. If resonance between the a carbon and the benzene ring is important in this reaction series, pr should be large.…”

Section: -7mentioning

confidence: 99%

Isotope effects in nucleophilic substitution reactions. IV. The effect of changing a substituent at the α carbon on the structure of S_N2 transition states

Westaway¹,

Waszczylo²

1982

Kinetic studies, secondary α-deuterium kinetic isotope effects, primary chlorine kinetic isotope effects (1), Hammett ρ values determined by changing the substituent in the nucleophile, and activation parameters have been used to determine the detailed (relative) structures of the transition states for the SN2 reactions between para-substituted benzyl chlorides and thiophenoxide ion. A rationale for the U-shaped Hammett ρ plots observed when para-substituted benzyl compounds react with negatively charged nucleophiles is also presented.

“…The work reported in this paper was prompted by In principle, a detailed understanding of the the observations of Koshy and Robertson (6,7). mechanism of solvolytic reactions involving or-For a series of benzyl nitrates in water, the a-kH/k, ganic substrates of the general form RX should be ratios vary between 0.99 and 1.18 per deuterium forthcoming from a study of the a-deuterium atom.…”

Section: Introductionmentioning

confidence: 99%

Kinetic deuterium isotope effects and kinetic solvent isotope effects for the solvolyses in water and in water + acetonitrile mixtures related to a series of substituted benzyl nitrates

Ahsan¹,

Robertson²,

Blandamer³

et al. 1980

In aqueous solution, the a-deuterium isotope effects in the solvolyses of benzyl nitrates derivatives depend on the nature of the substituent in the benzene ring. In addition, the isotope effect for some derivatives depends on mole fraction of added acetonitrile while for others the isotope effect is insensitive to solvent composition. However, the kinetic solvent isotope effects for para-methyl and meta-trifluoromethyl derivatives remain unchanged when acetonitrile is added. These observations are accounted for in terms of a model which describes the solvolytic reaction as a two-stage process and contrasts the relative importance of bond-making and bond-breaking.MOHAMMED AHSAN, ROSS ELMORE ROBERTSON, MICHAEL JESSE BLANDAMER et JOHN MARSHALL WILLIAM SCOTT. Can. J. Chem. 58,2142Chem. 58, (1980. En solution aqueuse, les effets isotopiques des deuterium a observes lors des solvolyses de nitrates de benzyles dependent de la nature du substituant fixe sur le cycle. De plus les effets isotopiques de quelques derives dependent de la fraction molaire d'acetonitrile ajoutk alors que d'autres effets isotopiques sont indipendants de la composition du solvant. Cependant les effets isotopiques cinetiques du solvant pour les derives para-methyle et mp'ta-trifluorom6thyle demeurent les m6mes quand on ajoute de I'acetonitrile. On interprgte ces observations en fonction d'un modelequi dkcrit la reaction de solvolyse, une reaction endeux etapes ou s'opposent les importances relatives de la rupture et de la formation d'une liaison.[Traduit par le journal] IntroductionThe work reported in this paper was prompted by In principle, a detailed understanding of the the observations of Koshy and Robertson (6, 7). mechanism of solvolytic reactions involving or-For a series of benzyl nitrates in water, the a-kH/k, ganic substrates of the general form RX should be ratios vary between 0.99 and 1.18 per deuterium forthcoming from a study of the a-deuterium atom. Moreover it was noted (7) that the latter isotope effects as measured by the ratio kH/kD. A ratios correlated with the heat capacities of activasimilar comment applies to the information pro-tion, ACp+, calculated from the dependence of rate vided by the kinetic solvent isotope effect as mea-constant for solvolysis in water on temperature. sured by the ratio kHZO/kDzO. However, as more However, a reappraisal (8) of the latter dependence information became available the interpretation of has indicated that the calculated ACp+-quantity is these isotope effects became less clear-cut. Thus an artefact of the mechanism which is more corShiner (1) attempted to use kH/kDas an indicator of rectly described in terms of a two-stage process. the extent of nucleophilic involvement in the rate This raises the question of whether a similar model determining step for displacement of chloride ion. could account for the isotope effects. The data However, Bentley and Schleyer (2) argue that the reported in this paper for the solvolysis of a series reference value of kHlkD is not known for systems of s...