E2 Mechanism Research Articles

Dealkylation of alkylcobalt complexes by the mercury(II) electrophile

The dealkylation of a variety of RCo(BDMBg) + complexes by Hg 2+ is discussed. (The ligand BDMBg - is formed by the condensation of two moles of 2,3-butanedionemonoxime with one mole of 1,3-propanediamine or 1,2-ethanediamine.) The products of the dealkylation reaction are Co III(BDMBg)(H 2O) 2+ 2 and RHg +. All reactions are first order in the [Hg 2+]. The second order rate constants ( k 2) vary from 5.9 M -1 sec -1 (R = CH 3) to 5.7 x 10 -3 M -1 sec -1 (R = n-C 3H 7). The relative rates for R are: CH 3 ⪢ C 2H 5 τ C 6H 5CH 2 τ n-C 4H 9 ⋍ n-C 3H 7. This order and other evidence are indicative of an S E2 mechanism with an attack by Hg 2+ on the carbon bonded to the cobalt.

NUCLEOPHILIC SUBSTITUTION AND ELIMINATION REACTIONS ON NITROGEN ATOM—REACTION OF N,N-DIBENZYL-O-p-NITROBENZOYLHYDROXYLAMINE WITH VARIOUS NUCLEOPHILES IN DIPOLAR APROTIC SOLVENT

Abstract The reactions of N,N-dibenzyl-O-p-nitrobenzoylhydroxylamine (DNH) with nucleophiles were found to give benzaldehyde, benzylamine and p-nitrobenzoate ion in good yields, respectively. It has been suggested that these reactions would proceed via E2 mechanism on the nitrogen atom on the basis of kinetic isotope experiment. Furthermore, SN2 mechanism on the nitrogen atom has been inferred for the reaction of DNH with cyanide ion.

Elimination and addition reactions. Part XXIII. Mechanisms of elimination in nitro-compounds bearing phenoxy and phenylthio leaving groups

Rates of elimination in 1-nitro-2-phenoxyethane and in 1-nitro-2-phenylthioethanes (PhS·CHY·CH2·NO2; Y = H or Ar) have been measured in ethanolic sodium ethoxide, aqueous sodium hydroxide, and aqueous triethylamine. The phenoxy-compound reacts by rate-determining loss of phenoxide from the carbanion [(E1) anion mechanism], while for the α-arylphenylthio-compounds, ionisation is rate-determining and rate constants agree with predictions made from ionisation rates of model substrates. The simple phenylthio-compound (Y = H) undergoes elimination with a primary deuterium isotope effect of 7·4 but rate constants are larger than predicted for rate-determining ionisation and the E2 mechanism is tentatively assigned.

Acylarylnitrosamines. Part VII. [2H] Labelling experiments and the mechanism of formation of benzyne from N-nitrosoacetanilide and benzenediazonium acetate

Reaction of N-nitroso[2′,4′,6′-2H3]acetanilide, formed in situ from [2′,4′,6′-2H3]acetanilide (99%[2H3]) and p-chlorobenzoyl nitrite, with tetraphenylcyclopentadienone and acetic acid in benzene gave biphenyl, from which no deuterium had been lost, and 1,2,3,4-tetraphenylnaphthalene which had lost one g atom of deuterium. Similar results were obtained from reaction of [2,4,6-2H3]benzenediazonium fluoroborate (99%[2H3]) with potassium acetate in benzene. The corresponding reaction using anthracene as a benzyne trap gave similar result. These results exclude the operation of a ‘pre-equilibrium’ type of E1cB mechanism in the formation of benzyne from the benzenediazonium acetate ion pair, but do not allow a distinction to be made between E1cB (irreversible) or concerted E2 mechanisms, particularly in view of the low (1·5–1·8) isotope effects observed with [2-2H]benzenediazonium salts.

Reactivity of vinyl sulphonic esters. Part XVI. Solvolytic reactivity of β-halogenovinyl derivatives

The solvolytic reactivity of trans-(I) and cis-(II)β-halogenovinyl 2,4,6-trinitrobenzenesulphonates, R2(Hal)C:C(O3SAr)R1[(I; Hal = Cl, Br, or I, R1,R2= alkyl or aryl), (II; Hal = Cl or Br, R1,R2= aryl)], has been investigated in acetic acid and in nitromethane–methanol. The kinetics and products indicate that the solvolysis in both solvents proceeds by SN1 and SN1–E1 mechanisms in the case of some β-iodovinyl derivatives. The reactivity of esters (I) strongly depends on the nature of the halogen and follows the order IBr > Cl. The trans-esters (I; R1,R2= aryl) are more reactive than the corresponding cis-isomers when Hal = Br [k(I)/k(II)= 10–15] and almost equally reactive when Hal = Cl. The substitution products from (I; Hal = I) retain the trans-geometry, whereas the substitutions of esters (I) and (II)(R1,R2= aryl), when Hal = Br or Cl, are not stereospecific. Anchimeric assistance due to iodine and, to a much lesser extent, to bromine are suggested to be involved. The results are also discussed in terms of bridged versus open geometry for the intermediate vinyl cations and compared with those for electrophilic additions of halogens to the CC triple bond.

Role of membrane phase in the transmission mechanism of colicin E1

Role of membrane phase in the transmission mechanism of colicin E1

Properties of the fluorescence probe response associated with the transmission mechanism of colicin E1.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTProperties of the fluorescence probe response associated with the transmission mechanism of colicin E1S. K. Phillips and W. A. CramerCite this: Biochemistry 1973, 12, 6, 1170–1176Publication Date (Print):March 1, 1973Publication History Published online1 May 2002Published inissue 1 March 1973https://doi.org/10.1021/bi00730a024RIGHTS & PERMISSIONSArticle Views62Altmetric-Citations57LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (855 KB) Get e-Alerts Get e-Alerts

The Formation of Alkenes by the Thermal Elimination Reaction of N-Methyl-4-alkoxypyridinium Iodides. II. An Investigation of the Mechanism

A comparison of the products from elimination reactions of a number of compounds containing various leaving groups with those containing the N-methyl oxypyridinium leaving group suggests that the elimination is not occurring by means of a simple E1 mechanism. Changing the anion of the salt from iodide to methyl-sulphate and tetrafluoroborate affects the product composition indicating that the anion is taking part in the reaction. The mechanism of this reaction appears to be on the E1-E2 borderline.

A re-examination of the ‘unified’SN2–E2 mechanism

A re-examination of the ‘unified’S_N2–E2 mechanism

Isomerization of 3?-acetyl-?-3,4-epoxycarane

3β-Acetyl-α-3,4-epoxycarane under the influence of bases is easily rearranged to theα,β-unsaturated ketol, namely 3β-acetyl-4-caren-3-ol, by the E2 mechanism of theβ-elimination reaction.

Study of iododemercuration of symmetrical organomercury compounds

Iododemercuration of symmetrical organomercury compounds, R 2Hg, was investigated in a number of solvents (CCl 4, dioxane, DMF, HMPA). The relative reactivity of the compounds was obtained in these solvents. It was shown that the S Ei reaction rate could not serve as a measure of the nucleophilic nature of the organic group. High reaction rates with I 2 in the aprotic dipolar media were obtained for the compounds containing strong electron-attractive substituents. In such cases the reactions are assumed to follow the S E1(N) mechanism.

Iododemercuration of (Pentafluorophenyl)- and (pentachlorophenyl)mercury bromides

Reaction of (perfluorophenyl)- and (perchlorophenyl) mercury bromides with iodine in DMF follows the first order which is discussed in terms of the S E1(N) mechanism. In benzene an analogous reaction proceeds only in the presence of iodide ions. In both solvents the reaction with I − 3 follows the second order kinetics. A mechanism is suggested in which iodinating agent plays the role of a nucleophilic catalyst, S E1(I − 3).

Cleavage of allylic tin compounds by iodine

A kinetic study has been made of the cleavage of allylic derivatives of tin of the type Ph 3SnCH 2CHCHR (R = H, Me, Ph) by iodine in a variety of solvents and in the presence of NaI. The rate-determining step is thought to involve unimolecular decomposition of the π-complex Ph 3SnCH 2CHCHR · I 2. In weakly polar media, the 3-methylallyl(=2-butenyl)tin compound yields 1-methylallyl iodide in quantitative yield whereas in highly polar media such as DMSO, appreciable amounts of 3-methylallyl iodide are formed. For the 3-phenylallyl series only 3-phenylallyl iodide is obtained. The reaction is suggested to have an S′ E type mechanism in weakly polar solvents with the incursion of an S E2 mechanism in more polar media. The conversion of 1-methylallyl into 3-methylallyl iodide has been briefly studied.

Radical exchange reactions between organomercury compounds

Reactions of diphenylmercury with R′ 2Hg (where R′ = CN, CCl 3, CCPh) in dimethylformamide were studied by a polarographic method and have been found to yield PhHgR′, obeying the second-order kinetics. Addition of iodide displays negligible effect on the reaction rates. When R′ = CFClCOOEt, CF 2COOEt, CH 2COOCH 3, CH 2C 6H 5, the reaction of Ph 2Hg with R′ 2Hg does not occur. p-OCH 3 groups in Ph 2Hg increase the reaction rates. The results obtained are discussed in terms of the S E2 mechanism.

β-Elimination of 9-fluorenylmethanol in aqueous solution: an E1cB mechanism

9-Fluorenylmethanol undergoes base-catalysed β-elimination in aqueous sodium hydroxide to yield dibenzofulvene. Comparison of rate constants for elimination, kE, and for exchange of tritium at the 9-position (β to the hydroxy-group), kxT, together with the observation of kinetic induction periods upon elimination of [9-1H1]9-fluorenylmethanol in D2O and [9-2H1]9-fluorenylmethanol in H2O, indicate that a carbanion is formed in the course of reaction. A kinetic analysis of the induction period for reaction of the protio-compound in D2O allows determination of a rate constant for its exchange, kxH, and an isotope effect upon elimination, kEH/kED= 7·2. Comparison of kEH/kED in D2O with the solvent isotope effect upon elimination of the protio-compound in H2O and the deuterio-compound in D2O allows a distinction to be drawn between E2 and E1cB mechanisms of elimination. It is concluded that the observed value of 0·92 for the solvent isotope effect is too small to incorporate the primary effect of 7·2 required by the E2 mechanism, but is of the right magnitude for an E1cB mechanism.For an E1cB mechanism, kEH/kED corresponds to the isotope effect upon ionization of 9-fluorenylmethanol to form the carbanion, and use of the Swain–Schaad relation between deuterium and tritium isotope effects, together with the observed value of kxH, permits calculation of a rate constant for tritium exchange, kxT, which agrees well with the independently measured value. Less satisfactory agreement for the corresponding comparison in H2O is ascribed to experimental error, and either to the presence of ‘internal return’ from a specifically solvated carbanion, or to the contribution of a minor component of E2 elimination.

Relationships between E2 and E1cB mechanisms of β-elimination

It is shown that for base-catalysed β-elimination of 9-fluorenylmethyl derivatives, the stability of the 9-fluorenylcarbanion is too great to be consistent with a continuity of transition states between E2 and E1cB mechanisms. A model is formulated in which the reactants, products, and transition states for concerted elimination are represented on a common potential-energy surface with the carbonium ion and carbanion intermediates of stepwise reaction paths. The effect of energy changes in each stable species upon the energy and structure of the E2 transition state is considered within the framework of the Hammond postulate. The model provides an interpretation of the effects of substituents and reaction conditions upon β-eliminations. It is concluded that a more stable carbanion causes the E2 transition state to approach the carbanion in structure, leading to an apparent relation between E2 and E1cB mechanisms. At the borderline between the mechanisms a continuity of transition states, or operational indistinguishability between them, can be expected only when the reaction of the carbanion approaches diffusion control.

Steroids and Walden inversion. Part LXV. Comparative experiments on the deamination of cis- and trans-4-t-butylcyclohexylamine

Deamination of trans-4-t-butylcyclohexylamine (NH2eq) in 50% acetic acid at 20° gives alcohols (63%) with 93% retention, and acetates (35%) with 73% retention; in acetic acid at 20° alcohols (42%) are formed with 95% retention, and acetates (52%) are formed with 78% retention. Very little elimination (2%, 6%) occurs. Deamination of cis-4-t-butylcyclohexylamine (NH2ax) in 50% acetic acid at 20° gives alcohols (35%) with only 54% retention, and acetates (18%) with only 41% retenion; in acetic acid at 20° alcohols (19%) are produced with 78% retention, whilst acetates (31%) are produced with only 44% retention. Considerable elimination (47% 50%) occurs.These results provide further evidence that the deamination is a complex reaction, greatly influenced by steric environment, and proceeding by a combination of SNi, SN2, and E2 mechanisms and mechanisms involving an intermediate solvated carbonium ion.

Rates of solvolysis and of base-catalysed elimination of substituted αα-dimethylphenethyl chlorides

Rates of solvolysis in methanol, and of concurrent solvolysis and sodium methoxide-catalysed elimination in methanol, of substituted αα-dimethylphenethyl chlorides have been measured at 66·5°, and product distributions have been determined in these reactions. For solvolysis, log k1 values correlate with σ° values and ρ=–1·15. The product distribution appears to be constant (∼50% ether, ∼30% conjugated olefin, and ∼20% non-conjugated olefin) when the substituent is varied. In elimination the conjugated olefin is the major product, and its proportion increases when electron-withdrawing substituents are introduced. For elimination to conjugated olefin ρ=+1·00 and for elimination to non-conjugated olefin ρ=–0·10. These results are not consistent with an E2 mechanism which has considerable E1 character, i.e., which has C–X bond fission much more advanced than C–H bond rupture in the transition state.

The influence of solvent composition on the rates of E2 elimination and similar reactions in alcohol–water mixtures

The overall rates of E2 elimination of hydrogen bromide from β-phenylethyl bromide and ethylene dibromide by alkoxide and hydroxide ions in methanol– and ethanol–water mixtures have been measured. Similar studies have also been made of the hydrolysis of chloroform and ethylene chlorohydrin which have mechanisms very similar to the E1cB mechanism of elimination. The results for the E2 reactions show a pattern similar to that previously observed for second-order substitution reactions and are given a similar interpretation. The other reactions show no constant pattern of behaviour as the solvent is changed from alcohol to water. The possibility of using the reaction pattern in alkaline alcohol–water mixtures to distinguish between E2 and E1cB mechanisms of elimination is discussed.

Nichtstationäre Kinetik bei SN1, SN2, E1, E2 und E1cb‐Mechanismen

AbstractFor the SN1, SN2, E1, E2, AND E1cb mechanisms exact solutions of the kinetic equations are compared with solutions obtained under the hypothesis of stationarity. Exact integrals are calculated numerically for a set of the essential reaction rate constants. Comparison with the stationary state solution demonstrates that both types of solutions are approximately equal in many cases if the transient particle concentration remains low, whereas the conventional requirement | \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop x\limits^. $\end{document} | = 0 is unimportant. For approximation of exact kinetics by stationary kinetics, however, neither condition is sufficient nor necessary. Practical criterions for recognition of deviations from stationary kinetics are given.