- Concurrent Methoxide Ion Attack at the 5- and 7-Carbons of 4-Nitrobenzofurazan and 4-Nitrobenzofuroxan. A Kinetic Study in Methanol
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In methanolic solution,methoxide ions attack both the 5- and 7-carbons of 4-nitrobenzofurazan (5) and 4-nitrobenzofuroxan (8) to give the Meisenheimer-type complexes 6, 9 and 7, 10.The kinetics of the interactions have been studied by the stopped-flow method.The formation of the 5-methoxyl adducts 6 and 9 is found to always precede that of the thermodynamically more stable 7-methoxyl isomers 7 and 10.The results indicate that the para-like 4-nitro group is more efficient than the furazan and the furoxan rings in delocalizing the negative charge in 7 and 10.The kinetic and thermodynamic parameters Δ H0, Δ S0, Δ H(excit.), and Δ S(excit.) or the various reactions have been determined.It appears that only a large positive entropy change, Δ S0 (about +80 J/mol*K), is responsible for both the formation and the greater stability of the 7-methoxyl complexes 7 and 10 as compared to that of their 5-methoxyl analogues.Stabilization of 7 and 10 by association of the 4-nitro group with the potassium counterion is suggested.
- Terrier, Francois,Chatrousse, Alain-Pierre,Millot, Francois
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- Acceleration of the methanolysis of phosphate diesters promoted by La(OTf)3 - The analysis of non-integer s spH/rate profiles resulting from changes in metal ion speciation
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In a previous publication (A.A. Neverov and R.S. Brown. Inorg. Chem. 40, 3588 (2001)) we reported very effective catalysis of the methanolysis of some phosphate diesters (methyl p-nitrophenyl phosphate (1), bis(p-nitrophenyl) phosphate (2), and diphenyl p
- Gibson, Graham T.T.,Neverov, Alexei A.,Teng, Allen Chun-Tien,Brown
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p. 1268 - 1276
(2007/10/03)
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- Acidity, basicity, and the stability of hydrogen bonds: Complexes of RO- + HCF3
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Ion-molecule complexes of RO- (R = Me, Et, i-Pr) and HCF3 have been studied with Fourier transform ion cyclotron resonance spectrometry. The RO- complexation energies with HCF3 were measured relative to RO-·H2O. These complexes, [ROHCF3]-, have complexation energies on the order of -20 kcal/tool and have low deuterium fractionation factors and are, therefore, hydrogen bonded. The structure of the complexes was studied by isotopic equilibrium experiments and ab initio calculations. All of the complexes studied have the structure RO-·HCF3 even when HCF3 is a stronger acid than ROH. The structure of the complexes can be understood through electrostatic arguments rather than the difference in acidity between the ion and neutral.
- Chabinyc, Michael L.,Brauman, John I.
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p. 10863 - 10870
(2007/10/03)
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- Gas-Phase Nucleophilic Displacement Reactions
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Displacement reactions of each of a variety of anionic nucleophiles reacting with each of a variety of neutrals have been studied by pulsed ion cyclotron resonance (ICR) spectroscopy.Rate constants for these reactions are interpreted in terms of a three-step reaction sequence.RRKM calculations are used to obtain information about the energy of transition states.The origin of the barrier to reaction in solution is discussed.
- Olmstead, William N.,Brauman, John I.
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p. 1653 - 1662
(2007/10/03)
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- Kinetic analysis of elementary steps in nucleophilic vinylic substitution reactions of α-nitro-β-X-stilbenes (X = OCH2CF3, OCH3, NO2) with various nucleophiles.
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Rate constants of elementary steps in the addition-elimination mechanism of nucleophilic vinylic substitutions (SNV) were determined by studying the following reactions in 50% Me2SO-50% water at 20 °C: (1) α-nitro-β-(2,2,2-trifluoroethoxy)stilb
- Bernasconi, Claude F.,Schuck, David F.,Ketner, Rodney J.,Weiss, Minda,Rappoport, Zvi
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p. 11764 - 11774
(2007/10/02)
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- The gas-phase elimination reaction of 3-methoxycyclohexene: Regiochemistry
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Gas-phase elimination reactions of deuterium labeled 3-methoxycyclohexenes have been investigated. 1,4-Elimination is heavily favored over 1,2-elimination when strong bases such as hydroxide and amide are used. The 1,2-pathway becomes more competitive when weaker bases such as methoxide are employed, and the mechanism shifts from E1 cB to E2.
- Rabasco, John J.,Kass, Steven R.
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p. 4077 - 4080
(2007/10/02)
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- Kinetics of reactions of hydroxide ion and water with β-X-substituted α-nitrostilbenes (X = Cl, I, SEt, OMe, SCH2CH2OH) in 50% Me2SO-50% water. Search for the intermediate in nucleophilic vinylic substitution
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The hydrolysis of Ph(LG)C=Cph(NO2) (4-LG, Lg = Cl, I, Set, Ome) in basic solution yields the anion of 1,2-diphenyl-2-nitroethanone, PhC(=O)C(=NO2-)Ph (8-), by the addition-elimanation mechanism of nucleophilic v
- Bernasconi, Claude F.,Fassberg, Julianne,Killion Jr., Robert B.,Schuck, David F.,Rappoport, Zvi
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p. 4937 - 4946
(2007/10/02)
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- Reaction Rates of Trimethylethoxysilane and Trimethylmethoxysilane in Alkaline Alcohol Solutions
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The kinetics of the reaction (CH3)3SiOC2H5 + CH3O- (CH3)3SiOCH3 + C2H5O- have been investigated in both directions by means of FTIR spectroscopy.To obtain k1, trimethylethoxysilane was reacted with methoxide in a metha
- Jaeglid, Ulf,Lindqvist, Oliver
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p. 887 - 891
(2007/10/02)
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- KINETICS AND MECHANISM OF SOLVOLYSIS OF N-ARYL SULFURIC DIAMIDES
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The methanolysis and hydrolysis kinetics have been studied with the following sulfuric diamide derivatives: N-methyl-N-phenyl- (IIIa), N-methyl-N-(4-methoxycarbonylphenyl)- (IIIb), N-(4-methoxycarbonylphenyl)- (IIIc), N-methyl-N-(2-methoxycarbonylphenyl)- (IIId), N-(2-methoxycarbonylphenyl)- (IIIe), and N-methyl-N-(2,4-dibromophenyl)- (IIIf).The solvolyses of the neutral substrates IIIa and IIIb proceed by the addition-elimination mechanism.In the presence of the solvent lyate ions the solvolyses go by the E1cb mechanism.The solvolyses of the conjugated bases ofcompounds IIIa and IIIb are subject to general acid catalysis, the effects of the ring substituents being opposite to those in the addition-elimination mechanism.The solvolyses of compounds IIId and IIIf exhibit a distinct catalytic effect of neighbouring group; the reaction goes via a reactive intermediate, the transformation of the intermediate into the solvolysis product being subject to general acid and base catalysis.
- Kavalek, Jaromir,Kralikova, Ulrika,Machacek, Vladimir,Sedlak, Milos,Sterba, Vojeslav
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p. 202 - 222
(2007/10/02)
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- Study of Reactions Leading to Sulfine Formation. 3. Competition of Reaction Pathways in the Reaction of Methoxide Ion with Methyl 1-Naphthylmethanesulfinates
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In CD3O1-/CD3OD methyl 1-naphthylmethanesulfinates, NpCH2S(O)OCH3 (2), undergo both exchange of CH3O by CD3O by substitution at the sulfinyl group and elimination to form the sulfine, NpCH=S=O.With use of methyl (2-methoxy-1-naphthyl)methanesulfinate (2a) it has been shown that formation of the sulfine takes place by an (E1cB)irrev mechanism.The rates of substitution (ks) and elimination (ke) of a series of 2 have been determined in CD3O1-/CD3OD by 1H NMR spectroscopy, and the effect of several reaction variables on the competition between substitution and elimination has been examined.Salient results are as follows: (1) the rate of elimination is markedly increased by the presence of electron-withdrawing substituents on the aromatic ring, but the rate of substitution is increased only modestly by the same substituents; (2) substituents at the 2-position of the naphthyl group cause a large decrease in ks (steric hindrance to substitution at S=O) but have little effect on ke (elimination rate not sensitive to steric requirements of ortho substituents); (3) the activation energy for elimination is almost 9 kcal/mol greater than the activation energy for substitution.This large difference in activation energy contrasts with the 1-2 kcal/mol difference for elimination vs substitution found14 with alkyl halides.
- Kice, John L.,Lotey, Harvinder
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p. 3596 - 3602
(2007/10/02)
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- On the Mechanism of Base-Induced Gas-Phase Elimination Reactions of Ethers
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For the base-induced gas-phase elimination reactions of diethyl ether and cis- and trans-1-tert-butyl-4-methoxy-cyclohexane the kinetic isotope and leaving group effects have been determined as functions of the base strength using the method of Fourier transform ion cyclotron resonance mass spectrometry.The results are interpreted in terms of a variable E2 transition-state structure.Increasing the base strength causes the transition state to shift toward the carbanion or E1cb region of the E2 spectrum, which is also a general phenomenon in the condensed phase.Moreover, it appears that the elimination reactions most readily proceed via a transition state in which the β hydrogen and leaving group are periplanar.If the substrate does not easily allow such a relationship, the transition state is found to shift toward the carbenium ion or E1 region of the E2 spectrum where the geometric restrictions of the substrate are less perceptible.The concept of syn/anti dichotomy is used to explain the formation of tree and solvated alkoxide anions in the reactions induced by OH-.Anti elimination is believed to result in the formation of free alkoxide.Syn elimination, which takes advantage of the electrostatic interaction between the base and leaving group, is held responsible for the formation of solvated alkoxide.The importance of base/leaving group association in the transition state of the syn elimination is demonstrated by the low yield of solvated alkoxide in the reaction of OH-, solvated by a dimethylamine molecule, with diethyl ether.Finally, it seems that the selectivity of gas-phase elimination reactions is determined by not only the relative heights of the intrinsic reaction barriers, but also the relative stabilities of the ion/molecule complexes preceding the reaction barriers.
- Koning, Leo J. de,Nibbering, Nico M. M.
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p. 1715 - 1722
(2007/10/02)
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- Generation, Thermodynamics, and Chemistry of the Diphenylcarbene Anion Radical (Ph2C.-)
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Dissociative electron attachment with Ph2C=N produced Ph2C.- (m/z 166).The reactions of Ph2C.- with potential proton donors of known gas-phase acidity were used to bracket PA(Ph2C.-) = 380 +/- 2 kcal mol-1 from which ΔHf0(Ph2C.-) = 81.8 +/- 2 kcal mol-1 was calculated.The reactions of Ph2C.- with CH3OH and C2H5OH proceeded with major and minor amounts, respectively, of a H2.+-transfer channel, forming Ph2CH2, RCHO, and an electron.The kinetic nucleophilicity of Ph2C.- in SN2 displacement reactions with CH3X and C2H5X molecules was shown to be medium, which requires a significant intrinsic barrier in these reaction.The reactions of Ph2C.- with various aldehydes, ketones, and esters were fast and established two principal product-forming channels: (1) H+ transfer if the neutral reactant contains activated C-H bonds and (2) carbonyl addition followed by radical β-fragmentation of one of the groups originally attached to the carbonyl carbon.The order for the ease of radical β-fragmentation in the tetrahedral intermediates was RO > alkyl >> H, and CO2CH3 > CH3.Since the reactions of Ph2C.- with the simple esters HCO2CH3 and CH3CO2CH3 were fast, it should now be possible to examine the reactions of carbonyl-containing organic molecules, which are expected to react slower than these esters and obtain their relative reactivities.
- McDonald, Richard N.,Gung, Wei Yi
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p. 7328 - 7334
(2007/10/02)
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- Gas-Phase Hydrogen-Deuterium Exchange Reactions of Anions: Kinetics and Detailed Mechanism
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Rate coefficients and branching ratios for anionic hydrogen-deuterium exchange reactions between isotopically labeled hydroxide, amide, hydrogen sulfide, and methoxide ions and their corresponding neutral compounds have been measured in the gas phase by using the selected ion flow tube (SIFT) technique.The kinetic data for the methoxide-methanol systems lead to a determination of the relative base strength of CD3O(1-) and CH3O(1-).The reaction efficiencies and branching ratios of hydroxide-water, amide-ammonia, and amide-water systems are discussed in terms of the mechanism by which exchange occours; the multiply labeled systems reveal that more proton transfer occur than one might expect upon initial inspection of the data.The product distributions are discussed in terms of relative ion-molecule complex lifetimes and dissociation of the complex is competition with scrambling.Efficient H/D exchange was also observed to occur between compounds containing second-row elements and the implications of this are discussed.
- Grabowski, Joseph J.,DePuy, Charles H.,Doren, Jane M.Van,Bierbaum, Veronica, M.
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p. 7384 - 7389
(2007/10/02)
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- The Four-centre Reactions of Alkanol-Alkoxide Negative Ions with Alkyl Ethers and Orthoacetates. An Ion Cyclotron Resonance Study
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Alkoxide-alkanol negative ions 1O-...HOR2> react with carbon ethers (e.g., Me3COR3) by the four-centre reactions 1O-...HOR2> + Me3COR3 -> 2O-...HOR3> + Me3COR1, and 1O-...HOR2> + Me3COR3 -> 1O-...HOR3> + Me3COR2.The former reaction predominates when R1 2.In the case of di- and tri-ethers, the four-centre reaction can, in principle, compete with the alternative six-centre reaction.For example, for Me2C(OR3)2, 1O-...HOR2> + Me2C(OR3)2 -> Me2C=O + R1OR3 + 2O-...HOR3>.Evidence is presented which shows that the six-centre reaction does not occur.
- Hayes, Roger N.,Paltridge, R. Lee,Bowie, John H.
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p. 567 - 572
(2007/10/02)
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- Uncatalyzed and General Acid Catalyzed Decomposition of Alkyl Xanthates and Monothiocarbonates in Aqueous Solutions
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The decomposition of potassium alkyl xanthates and alkyl monothiocarbonates follows the rate law kobbsd = kH3O++> + kHA + kH2O in aqueos buffer solutions.The Broested coefficient βlg for the kH2O terms is -1.1 for the monothiocarbonates and -1.3 for the xanthates.These results complement the value of -1.1 observed for the alkyl monocarbonates and have been interpreted as late transition states in the decomposition direction.The reactions are also subject to general acid catalysis with α values of 0.9 +/- 0.2 and 0.8 +/- 0.1 for ethyl and methoxyethyl xanthates; α values for the monothiocarbonates are 0.58 +/- 0.05 and 0.57 +/- 0.01 for the methyl and methoxyethyl compounds, respectively.Solvent kinetic isotope effects (kD3O+/kH3O+) for ethyl and methoxyethyl xanthates are 2.53 and 2.12, respectevely; for ethyl, methyl, and methoxyethyl monothiocarbonate they are 1.95, 1.94, and 1.91, respectevely.The high Broensted value for ethyl xanthate and the resultant uncertainly in its numerical value coupled with the high inverse solvent isotope effect place this reaction on the borderline between a specific acid catalyzed mechanism and a concerted mechanism.The latter is favored because the value for kH3O+ is larger than that predicted for the protonation of the alcohol oxygen of the xanthates and monothiocarbonates; further, the formation of such protonated species is estimated to be sufficiently unfavorable as to require a breakdown step faster than a molecular vibration to account for the observed rates.Acetic acid does not catalyze the kH3O+ term for ethyl xanthate at very low pH; the small amount of catalysis at higher pH must therefore be true general acid catalysis and not a solvent effect.Broensted βlg values for the acid-catalyzed decompositions of the xanthates and monothiocarbonates are small.These results have been interpreted in terms of concerted general acid catalysis where little change in charge occurs on the leaving group oxygen in going from the starting material to the transition state.
- Millican, Robert J.,Angelopoulos, Maria,Bose, Anita,Riegel, Bram,Robinson, Darlyne,et al.
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p. 3622 - 3630
(2007/10/02)
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- A NONLINEAR HAMMETT RELATIONSHIP AS EVIDENCE FOR A CHANGE-OVER IN MECHANISM IN THE ALKALINE HYDROLYSIS OF METHYL CARBANILATES
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A nonlinear Hammett relationship could be used as evidence for a change-over in mechanism in the alkaline hydrolysis of methyl carbanilates.The electron-withdrawing substituted compounds hydrolyse via an A-E pathway (ρ 1.06) whereas the hydrolysis of the electron-donating substituted compounds involves an A-E scheme.
- Bergon, Michel,Calmon, Jean-Pierre
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p. 937 - 940
(2007/10/02)
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- Nucleophilic Reactions of Anions with Trimethyl Phosphate in the Gas Phase by Ion Cyclotron Resonance Spectroscopy
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The gas-phase ion-molecule reactions of several negative ions (SF6-, SF5-, SO2F-, F2-, F-, CF3Cl-, Cl-, CD3O-, DNO-, OH-, and NH2-) with trimethyl phosphate are investigated using ion cyclotron resonance techniques.Nucleophilic attack on OP(OCH3)3 occurs chiefly at carbon, resulting in displacement of O2P(OCH3)2-.This behavior contrast with that observed in solution, where attack at phosphorus is favored for hard nucleophiles.This difference is ascribed to solvation energetics for the intermediates involved in the two reactions.The failure of SF6- to transfer F- to OP(OCH3)3 places an upper limit of 11 +/- 8 kcal/mol on the fluoride affinity of OP(OCH3)3.The significance of the results for the negative chemical ionization mass spectrometry of phosphorus esters is briefly discussed.
- Hodges, Ronald V.,Sullivan, S. A.,Beauchamp, J. L.
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p. 935 - 938
(2007/10/02)
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