Secondary Kinetic Isotope Effects in Bimolecular Nucleophilic Substitutions. V. The Role of the Solvent in the Reaction Between Methyl Iodide and Pyridine

Abstract

Secondary kinetic deuterium isotope effects are presented for the reaction of methyl-d3 iodide and pyridine in four different solvents. Calculations on mass and moment of inertia change with deuteration in the initial state and an assumed tetrahedral transition state, together with internal rotational effects, are used to rationalize the inverse isotope effects. It is concluded from the variation of the isotopic rate ratio, that the transition state structure varies with solvent.Les effets secondaires isotopiques cinetiques de deuterium sont presentes pour la reaction de I'iodure de methyle d3 et la pyridine dans quatre solvants differents. Les calculs de masses et de moments d'inertie qui varient avec la deuteration dans I'etat initial et un etat de transition tetrahedrique suppose, conjointement avec les effets de rotation interne, sont utilise pour rationaliser les effets isotopiques inverses. On conclut de la variation du rapport des vitesses isotopiques que la structure de I'etat de transition varie avec le solvant.

SN2 and E2 Branching of Main-Group-Metal Alkyl Intermediates in Alkane CH Oxidation: Mechanistic Investigation Using Isotopically Labeled Main-Group-Metal Alkyls

“…In contrast, from 8 , the E2-type reaction involves cleavage of a C–H bond and the ratio of 4 to 7 would be expected to be more similar to that from the putative intermediate, EtTl­(TFA) 2 . No attempts at highly accurate quantitative analyses of the difference among 4 , 10 , and 7 from 2 or 8 were made, as secondary kinetic isotope effects (SKIEs) could be possible on both the S N 2 and E2-type pathways. , The SKIEs observed with 2 and 8 are consistent with literature reports for S N 2 and E2 reactions from alkyl halides. − When these functionalizations are carried out at room temperature, the intermediate species 5 and 6 are observed immediately after mixing, in both cases, in an ∼1:1 ratio (Figure , spectrum B for 2 and spectrum D for 8 ). This is evidence for the formation of free ethylene, as CH 2 CD 2 generated from either 2 or 8 should add to Tl­(TFA) 3 in a 1:1 ratio as the SKIE for addition would be expected to be small.…”

Isotope effects in nucleophilic substitution reactions. II. Secondary α-deuterium kinetic isotope effects: a criterion of mechanism?

“…Second, the reverse of these reactions, the Menshutkin reaction, occurs by an SN2 process (12) and the principle of microscopic reversibility requires that the forward reaction also be an SN2 process. Finally, all the experimental evidence except for the secondary a-deuterium kinetic isotope effects, is equally consistent with a mechanism where the substitution occurs in a one-step SN2 reaction within the triple ion, 'A,' eq.…”

Secondary Kinetic Isotope Effects in Bimolecular Nucleophilic Substitutions. VI. Effect of α and β Deuteration of Alkyl Halides in their Menschutkin Reactions with Pyridine in Nitrobenzene

“…With such a large excess of pyridine the reaction tends to be a pyridinolysis, since the pyridine will tend to solvate the transition state preferentially, compared to nitrobenzene. This effective change to a solvent of more specific solvating power (16) will give a transition state for the reaction more reactant-like (17) and therefore more crowded with a larger isotope effect.…”

“…The runs were followed conductometrically in a manner very similar to that previously described (17). except for those involving methyl bromide.…”