A kinetic study of the mechanisms of esterification of alcohols by trifluoroacetic acid

Rates of trifluoroacetylation of a structurally diverse range of aliphatic alcohols in neat trifluoroacetic acid at 35°C have been measured using ¹H n.m.r. spectroscopy. The reaction mechanism changes from reverse A_AC2 for primary and secondary alcohols (except Ph₂CHOH) to reverse A_AL1 for t-butanol.     

Zum Kondensationsverhalten von Silanolen. V. Die basenkatalysierte Kondensation von Organodimethylsilanolen in Dioxan/Wasser

On the Condensation Behaviour of Silanols. V. Base Catalysed Condensation of Organodimethylsilanols in Dioxane/Water The base catalyzed condensation has been followed by GLC for silanols of the type XMe₂SiOH (X = alkyl, halogenoalkyl, aryl, benzyl) in aqueous dioxane. The obtained kinetic data and equilibrium constants are discussed with respect to their substituent dependence and the reaction mechanism.     

Transition metal free oxidative esterification of alcohols with toluene

Using Bu₄NI as the catalyst and tert-butyl hydroperoxide as the oxidant, direct esterification of alcohols with toluene derivatives was achieved. Mechanistic investigations indicate that the alcohols are sequentially oxidized to aldehydes, carboxylic acids, and then to benzyl esters. Bu₄N⁺ functions as a phasetransfer reagent and iodide catalyzes the reaction.     

Evidence of protonation during the oxidation of some aryl alcohols by permanganate in perchloric acid medium and mechanism of the oxidation processes

The oxidation of benzyl alcohol and methoxy-, chloro-, and nitro- substituted benzyl alcohols by permanganate has been studied in aqueous and acetic acid medium in presence of perchloric acid. The reaction is first-order in [MnO₄^?] and [XC₆H₄CH₂OH], but the order is complex with respect to [H⁺]. Different thermodynamic parameters have been evaluated. The reaction occurs through the protonation of alcohol in a fast preequilibrium followed by a slow rate-determining oxidation step. A two-electron transfer oxidation step has been suggested for benzyl alcohol and chloro- and nitro- substituted alcohols, while the oxidation of methoxy compounds involves a one-electron transfer via a free-radical mechanism. © 1995 John Wiley & Sons, Inc.     

Absolute Rate Constants for the Addition of Benzyl (PhĊH2) and Cumyl (PhĊMe2) Radicals to Alkenes in Solution

Absolute rate constants and their temperature dependence were determined by time-resolved electron spin resonance for the addition of the radicals Ph?H₂ and Ph?Me₂ to a variety of alkenes in toluene solution. To vinyl monomers CH₂=CXY, Ph?H₂ adds at the unsubstituted C-atom with rate constants ranging from 14 M ^?1S ^?1 (ethoxyethene) to 6.7 · 10³ M ^?1S ^?1 (4-vinylpyridine) at 296 K, and the frequency factors are in the narrow range of log (A/M ^?1S ^?1) = 8.6 ± 0.3, whereas the activation energy varies with the substituents from ca. 51 kJ/mol to ca. 26 kJ/mol. The rate constants and the activation energies increase both with increasing exothermicity of the reaction and with increasing electron affinity of the alkenes and are mainly controlled by the reaction enthalpy, but are markedly influenced also by nucleophilic polar effects for electron-deficient substrates. For 1,2-disubstituted and trisubstituted alkenes, the rate constants are affected by additional steric substituent effects. To acrylate and styrenes, Ph?Me₂ adds with rate constants similar to those of Ph?H₂, and the reactivity is controlled by the same factors. A comparison with relative-rate data shows that reaction enthalpy and polar effects also dominate the copolymerization behavior of the styrene propagation radical.     

Hydrogen migrations in mass spectrometry. VI—the chemical ionization mass spectra of substituted benzoic acids and benzyl alcohols

The H₂ and CH₄ chemical ionization mass spectra of a series of series of substituted benzoic acids and substituted benzyl alcohols have been determined. For the benzoic acids the major fragmentation reactions of the protonated molecule involve elimination of H₂O or elimination of CO₂, the latter reaction involving migration of the carboxylic hydrogen to the aromatic ring. For the benzyl alcohols the major fragmentation reactions of [MH]⁺ involve loss of H₂O or CH₂O, analogous to the CO₂ elimination reaction for the benzoic acids. It is shown that the CO₂ and CH₂O elimination reactions occur only when a conjugated aromatic ring system is present, and that for the carboxylic acid systems, methyl groups and, to a lesser extent, phenyl groups are capable of migrating. The only discernible effect of substituents on the fragmentation of [MH]⁺ is an enhancement of the H₂O loss reaction in the benzoic acid system when an amino, hydroxyl, or halogen substituent is ortho to the carboxyl function. This ‘ortho’ effect, which differs in scope from that observed in electron impact mass spectra, is attributed to an intramolecular catalysis by the ortho substituent of the 1,3 hydrogen migration in the carbonyl protonated acid followed by H₂O elimination. Apparently, this route is favoured over the direct elimination of H₂O from the carbonyl protonated acid, since the latter has a high activation energy barrier because of unfavourable orbital symmetry restrictions.     

Oxidation of benzyl alcohols by nitrous and nitric acids in strong sulfuric acid media

We have studied the oxidation of benzyl alcohols by nitrous and nitric acid in sulfuric acid media. The oxidation by nitrous acid is rapid and has an activation energy of 10.6 ± 0.8 kcal mol^?1. A Hammett plot of logk₂ vs. σ⁺ is linear with a ρ value of ?1.4. The oxidation by nitric acid in sulfuric acid media is autocatalytic. From the kinetic and product analyses, it is concluded that a common oxidant, the nitrosonium ion is involved when either nitrous or nitric acid is used. A mechanism is proposed which involves the abstraction of hydride from the alcohols as the rate determining step. It is demonstrated that the autoxidation of the alcohols is catalyzed by nitrous acid or nitric oxide.     

Dediazoniations of arenediazonium ions part 25 Influence of Substituents on the Exchange of the Diazonio Group for External Molecular Nitrogen and on the N(α), N(β)-Rearrangement in the Diazonio Group

The N(α), N(β)-rearrangement of the two N-atoms which can be observed in solutions of [β-¹⁵N]-labelled p-substituted benzenediazonium ions follows dual substituent parameter treatments. The reaction yields a negative field and a positive resonance reaction constant (p_F =?3.35, p_R = 2.47). The magnitude of these constants is, within experimental error, the same as the respective reaction constants for solvolytic dediazoniation. The exchange of the diazonio group of ¹⁵N-labelled p-substituted benzenediazonium ion yields, however, field and resonance reaction constants which are close to zero. This result is attributed to cancellation of the reaction constants for the forward and reverse steps in the complex mechanism of the exchange reaction.     

Kinetics and mechanism of reaction between zirconium‐lactate and pentane‐2,4‐dione in excess lactate

The kinetics of the reaction of pentane‐2,4‐dione (HA) with Zr^IV has been studied at 25.0°C in an excess lactate (L^?) media. The equilibrium reaction was found to be: Zr₂L₆+HA?HL+Zr₂AL₅. The equilibrium was approached from either direction and a plausible mechanism has been proposed with kinetic constants, but individual reactivities of the keto and the enol tautomers of pentane‐2,4‐dione could not be apportioned. However, it was found that both the uncatalyzed and acid‐catalyzed paths contribute to the reverse reaction. But 2‐thenoyltrifluoroacetone (HT) forms a stronger chelate with Zr^IV, so its reaction with less reactive Zr₂L₅(OH₂) could not be detected; reactivity of the more reactive Zr₂L₅(OH₂)(OH) could be found. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 725–729, 2011     

Activation of the aromatic system by the SO2CF3 group: Kinetics study and structure–reactivity relationships

The rate constants for the reaction of 2,6‐bis(trifluoromethanesulfonyl)‐4‐nitroanisole with some substituted anilines have been measured by spectrophotometric methods in methanol at various temperatures. The data are consistent with the S_NAr mechanism. The effect of substituents on the rate of reaction has been examined. Good linear relationships were obtained from the plots of log k₁ against Hammett σ_para constants values at all temperature with negative ρ values (?1.68 to ?1.11). Activation parameters Δ^≠H varied from 41.6 to 54.3 kJ mol^?1 and Δ^≠S from ?142.7 to ?114.6 J mol^?1 K^?1. The δΔ^≠H and δΔ^≠S reaction constants were determined from the dependence of Δ^≠H and Δ^≠S activation parameters on the σ substituent constants, by analogy with the Hammett equation. A plot of Δ^≠H versus Δ^≠S for the reaction gave good straight line with 177°C isokinetic temperature. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 203–210, 2010     

Highly Efficient Direct Aerobic Oxidative Esterification of Cinnamyl Alcohol with Alkyl Alcohols Catalysed by Gold Nanoparticles Incarcerated in a Nanoporous Polymer Matrix: A Tool for Investigating the Role of the Polymer Host

The selective aerobic oxidation of cinnamyl alcohol to cinnamaldehyde, as well as direct oxidative esterification of this alcohol with primary and secondary aliphatic alcohols, were achieved with high chemoselectivity by using gold nanoparticles supported in a nanoporous semicrystalline multi‐block copolymer matrix, which consisted of syndiotactic polystyrene‐co‐cis‐1,4‐polybutadiene. The cascade reaction that leads to the alkyl cinnamates occurs through two oxidation steps: the selective oxidation of cinnamyl alcohol to cinnamaldehyde, followed by oxidation of the hemiacetal that results from the base‐catalysed reaction of cinnamaldehyde with an aliphatic alcohol. The rate constants for the two steps were evaluated in the temperature range 10–45 °C. The cinnamyl alcohol oxidation is faster than the oxidative esterification of cinnamaldehyde with methanol, ethanol, 2‐propanol, 1‐butanol, 1‐hexanol or 1‐octanol. The rate constants of the latter reaction are pseudo‐zero order with respect to the aliphatic alcohol and decrease as the bulkiness of the alcohol is increased. The activation energy (E_a) for the two oxidation steps was calculated for esterification of cinnamyl alcohol with 1‐butanol (E_a=57.8±11.5 and 62.7±16.7 kJ mol^?1 for the first and second step, respectively). The oxidative esterification of cinnamyl alcohol with 2‐phenylethanol follows pseudo‐first‐order kinetics with respect to 2‐phenylethanol and is faster than observed for other alcohols because of fast diffusion of the aromatic alcohol in the crystalline phase of the support. The kinetic investigation allowed us to assess the role of the polymer support in the determination of both high activity and selectivity in the title reaction.     

Kinetics,mechanism, and products of the reaction of diphenylcarbonyl oxide with carboxylic acids

The kinetics of the reactions of acetic, benzoic, formic, oxalic, malic, tartaric, trifluoroacetic, and hydrochloric acids with diphenylcarbonyl oxide Ph₂COO was studied. The carbonyl oxide Ph₂COO was generated by flash photolysis of diphenyldiazomethane Ph₂CN₂ in solutions of acetonitrile and benzene at 295 K. The apparent rate constants of the reaction range from 4.6·10⁸ for (COOH)₂ in MeCN to 7.5·10⁹ L mol^–1 s^–1 for acetic acid in a benzene solution. The reaction mechanism was proposed, according to which at the first stage the carbonyl oxide is reversibly solvated by the solvent. Then the solvated carbonyl oxide reacts with the acid molecule by the mechanism of insertion at the O—H bond.     

pH-Metrische Studien an den binären Komplexen von Oxovanadium(IV) mit Hippur- und Anthranilsäure und Pyridin-2-aldoxim

pH-metric studies on the interaction of oxovanadium(IV) with hippuric and anthranilic acids and pyridine-2-aldoxime indicate the formation of monohydroxo derivatives of 1:1 chelates. The equilibrium constants for the reaction, VO²⁺+HA+H₂O?VO(OH)A+2H⁺ have been calculated as 4.47±0.07 and 6.32±0.05 in the 1:1 VO²⁺-hippuric or anthranilic acid systems resp. and for the reaction, VO²⁺+H₂ A ⁺+H₂O?VO(OH)A+3H⁺ as 8.40±0.09 in the 1:1 VO²⁺-pyridine-2-aldoxime hydrochloride system at 30±0.5°C (μ=0.1-KNO₃).     

Ritter reactions. II. Reductive deamidation of N-bridgehead amides.

Adamantyl- or homoadamantyl-derived N-bridgehead amides are converted in high yields into hydrocarbon derivatives on prolonged reflux in ethanol and 50% sulphuric acid (1:1 by volume). This process probably involves A_AL¹ hydrolysis to the tertiary carbonium ion, followed by hydride abstraction from the ethanol solvent.     

Production of Biodiesel Through Esterification Reaction Using Choline Exchanging Polytungstoboronic Acids as Temperature-Responsive Catalysts

High proton content polytungstoboronic acid H₇BW₁₁TiO₄₀ (abbreviated as H₇BW₁₁Ti) had been prepared through titanium mono-substituted of H₅BW₁₂O₄₀, which was evaluated in esterification of palmitic acid with methanol. H₇BW₁₁Ti could promote esterification reaction with almost 100% conversion of 417 mol/mol h TOF within very short time of 1 h at methanol/acid ratio of 25:1 under reaction temperature of 65?°C, which was attributed to the higher Brønsted acidity and cooperation of Lewis acidic site generating from Ti substituent. Limitation by its homogeneous performance, solidification of H₇BW₁₁Ti was designed via partial exchanging protons with choline cation (abbreviated as Ch⁺) to prepare a series of [(CH₃)₃NCH₂CH₂OH]_nH_7?nBW₁₁TiO₄₀ (abbreviated as Ch_nH_7?nBW₁₁Ti, n?=?0?~?7). Among all solid catalysts, the highest efficiency had been achieved by with 99.0% conversion and 429 mol/mol h TOF within 50 min at 65?°C at methanol/acid ratio of 15:1. The comparable activity was attributed to the cooperation of Ch⁺—self-assembling to form nanoreactor with amphiphilic surrounding to concentrate substrates and to resist to water-poison, temperature-responsive property to control form changing between heterogeneous to homogenous. This temperature-responsive HPA catalyst was available for production of biodiesel through esterification reaction. In addition, Separation of such heteropolyacid catalysts was easy by the lowering the reaction temperature to room temperature without appreciable loss of its high performance, which were reused for more than six times.     

Novel preparation of ion-supported triphenylphosphines and their synthetic utility

Novel ion-supported Ph₃P compounds, 4-(diphenylphosphino)- benzyltrimethylammonium bromide (A) and N-methyl-N-[4-(diphenylphosphino) -benzyl]pyrrolidinium bromide (B), were prepared. Because of their stability in air, ion-supported Ph₃P A and B could be used for the halogenation of alcohols, the esterification of carboxylic acid with the Mitsunobu reaction, the Mizoroki-Heck reaction, and the Sonogashira reaction. The advantages of using these ion-supported Ph₃P A and B are the simple isolation of the products by ether extraction due to their poor solubility in ether, and the easy recovery of the co-product, ion-supported Ph₃PO, by filtration in high yields (>90%), which could be regenerated and reused for the same reactions, in the halogenation of alcohols and the esterification of carboxylic acid with the Mitsunobu reaction. On the other hand, ionic liquid reaction media containing Pd(OAc)₂ or PdCl₂ and ion-supported Ph₃P A or B as catalysts could be reused for the same Mizoroki-Heck reaction and the Sonogashira reaction maintaining high yields, using iodotoluene with methyl acrylate and phenylacetylene, respectively.     

1H-NMR of the esterification of syndiotactic poly(methacrylic acid) with carbodiimides—II. Esterification with benzylalcohol and trifluoroethanol

The partial esterification of syndiotactic poly(methacrylic acid) with benzyl alcohol or trifluoroethanol and dicyclohexylcarbodiimide as a condensing agent has been studied, evaluating triad and pentad probabilities by ¹H-NMR. The mechanism of this esterification leads to a tendency toward alternation for esterified and unesterified monomer units along the chain and to a limiting conversion. Only a moderate approximation of triad and pentad probabilities by conditional probabilities of first and second order was possible. The esterification of poly(methacrylic acid) with trifluoroethanol has been carried out also in conc H₂SO₄ and leads to a random distribution of monomer units.     

1H-, 31P- and 13C-NMR spectra of some 2-arylsulphonylamido-2-thiono-5-methyl-1,3,2-dioxaphospholanes

Some 2-arylsulphonylamido-2-thiono-5-methyl-1,3,2-dioxaphospholanes (1), containing two chiral centres, give NMR spectra in which splittings of the ¹H and ¹³C signals of the 5-methyl substituent and of the ³¹P signal indicate that they exist in an approximately 70:30% ratio of two racemic mixtures, Z and E respectively, diastereomeric to each other. These splittings were invariably observed for compounds 1a, containing a monosubstituted sulphonylamido group (Y = H) and various substituents at position 4′ of the aryl group, X = H, CH₃, OCH₃, F, CI and Br. The methylenic protons in position 4 of the phospholane ring are diastereotopic and therefore magnetically non-equivalent. Benzene-induced shifts were measured for the Z isomers of compounds 1a, X = H, F and for compound 1b, containing an N-disubstituted sulphonylamido group (Y = CH₃) and X = H; the tentative interpretation of these shifts places the more shielded diastereotopic methylene proton on the same side of the ring as the 5-methyl substituent. The H? P and H? H coupling constants indicate that the preferred conformation of the dioxaphospholane ring should be a ‘twisted envelopey’ shape with the 5-methyl substituent in the equatorial position.     

Stereochimie de l'action des organomagnesiens et des organolithiens sur les dimethyl-1,2 alcoxy-1 silacyclobutanes

The reaction of organomagnesium organolithium compounds (alkyl, aryl, allyl and benzyl derivatives) with 1,2-dimethyl-1-alkoxy-l-silacyclobutanes proceeds with retention of configuration at the silicon atom. Stereochemical results are discussed in terms of the S_N2Si mechanism. The proposed configurations are supported by ¹H and ¹³C NMR data.