The decomposition of methyl hemiacetals of benzaldehyde in aqueous solution: a study of the effect of aromatic substitution

Engell, Karen M.; McClelland, Robert A.; Sørensen, P. E.

doi:10.1139/cjc-77-5-6-978

Cited by 3 publications

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“…The second issue in the photolysis of 10 is the absence of any evidence for a hemiacetal intermediate. Rates for breakdown of hemiacetals derived from benzaldehydes and acetophenones are dominated by hydroxide ion catalysis at alkaline pH values, and the second-order rate constants for hydroxide ion catalysis are an order of magnitude lower for aldehyde-derived hemiacetals than those for those derived from ketones. ,, Hence if a hemiacetal intermediate analogous to 11 were formed in the photolysis of 10 at pH 8.5, it should have had a lifetime of at least several seconds, substantially longer than the observed lifetime of the aci -nitro intermediate from 10 (∼0.4 s at 1 °C), and therefore should have been observable in our FTIR spectra, inter alia by the presence of a strong nitroso monomer band. The absence of such a band thus excludes a hemiacetal intermediate and 10 appears exclusively to undergo photocleavage by a “normal” pathway analogous to that shown in Scheme , i.e., direct collapse of a bicyclic intermediate equivalent to 3 to yield the end products.…”

Section: Resultsmentioning

confidence: 99%

Photolytic Cleavage of 1-(2-Nitrophenyl)ethyl Ethers Involves Two Parallel Pathways and Product Release Is Rate-Limited by Decomposition of a Common Hemiacetal Intermediate

et al. 2003

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Time-resolved FTIR spectroscopic studies of the flash photolysis of several 1-(2-nitrophenyl)ethyl ethers derived from aliphatic alcohols showed that a long-lived hemiacetal intermediate was formed during the reaction. Breakdown of this intermediate was rate-limiting for product release. One of these compounds (methyl 2-[1-(2-nitrophenyl)ethoxy]ethyl phosphate, 9) was studied in detail by a combination of time-resolved FTIR and UV-vis spectroscopy. In addition, product studies confirmed clean photolytic decomposition to the expected alcohol, 2-hydroxyethyl methyl phosphate, and the 2-nitrosoacetophenone byproduct. At pH 7.0, 1 degrees C, the rate constant for product release was 0.11 s(-1), very much slower than the 5020 s(-1) rate constant for decay of the photochemically generated aci-nitro intermediate (pH 7.0, 2 degrees C). Time-resolved UV-vis measurements showed that the hemiacetal intermediate is formed by two competing pathways, with fast (approximately 80% of the reaction flux) and slow (approximately 20% of the flux) components. Only the minor, slower path is responsible for the observed aci-nitro decay process. These competing reactions are interpreted with the aid of semiempirical PM3 calculations of reaction barriers. Furthermore, AMSOL calculations indicate that the pK(a) of the nitronic acid isomer formed by photolysis is likely to determine partition into the alternate paths. These unusual results appear to be general for 1-(2-nitrophenyl)ethyl ethers and contrast with a related 2-nitrobenzyl ether that photolyzed without involvement of a long-lived hemiacetal.

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Section: Resultsmentioning

confidence: 99%

Photolytic Cleavage of 1-(2-Nitrophenyl)ethyl Ethers Involves Two Parallel Pathways and Product Release Is Rate-Limited by Decomposition of a Common Hemiacetal Intermediate

et al. 2003

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“…b Buffer p K a values: formate, 3.85; acetate, 4.75; phosphate, 6.75; succinate (H 2 A), 4.21; succinate (HA - ), 5.64 (ref ). c Literature values (25 °C, M -1 s -1 ): BMA, 60.5 (ref ); BBA, 73 (ref ); BTBA, 2000 (ref ); BTBA, k H = 2950, k HA = 0.325; acetate, 0.029; phosphate (ref ).…”

Section: Resultsmentioning

confidence: 99%

“…Finally, hydrolyses of primary benzaldehyde acetals show induction periods by stopped-flow spectroscopy for primary alkyl acetals that are too fast to be observed by conventional methods. , These short induction periods are attributed to formation of transient hemiacetal intermediates within the first few seconds of the reaction and do not perturb the relatively slow overall hydrolyses. However, Jensen , and co-workers and Capon report significant induction periods (1−2 min) for some primary acetals and significant deviations from first-order kinetics in the pH range 5−7 for the hydrolysis of BTBA, a break in the log k obs −pH profile for BTBA, and a marked increase, by a factor of approximately 10, in the observed second-order rate constant for BTBA hydrolysis at pH > 7.…”

Section: Introductionmentioning

confidence: 99%

Effect of Alkyl Group Size on the Mechanism of Acid Hydrolyses of Benzaldehyde Acetals

et al. 2003

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Hydrolyses of benzaldehyde acetals, PhCH(OR)(2), are specific hydrogen-ion catalyzed when R = methyl, n-butyl, but with secondary and tertiary alkyl derivatives, R = i-propyl, s-butyl, t-butyl, t-amyl, hydrolyses are general-acid catalyzed. The Brønsted alpha values for both secondary and tertiary alkyl groups are in the range: alpha = 0.57-0.61. A simple iterative procedure was developed to estimate the individual rate constants for general-acid catalysis by the diacid and monoacid forms of succinic acid buffer. Plots of log k(obs) (at [buffer] = 0 M) against pH are linear for the secondary and tertiary acetals, and plots of log k(H) for the H(3)O(+)-catalyzed reaction, (13)C and (1)H chemical shifts, and (1)J(CH) coupling constants against the Charton steric parameter, nu, for alkoxy groups are linear. The second-order rate constant, k(H), increases about 100-fold on going from R = Me to R = t-amyl, indicating the significant role of steric effects on reactivity. Steric effects upon (13)C NMR chemical shifts and coupling constants indicate that increasing the bulk of the alkoxy moiety increases the electron density at the carbon reaction center, which accelerates hydrolysis. Analysis of the Jencks-More-O'Ferrall free energy diagram for the reaction provides support for concerted proton transfer and C-O bond breaking in the transition state for hydrolyses of benzaldehyde acetals with secondary and tertiary alkyl groups in contrast to specific hydrogen catalysis with R = Me and n-Bu. All our results are consistent with rate-determining acid hydrolysis of benzaldehyde dialkyl acetals to hemiacetal intermediates that breakdown rapidly to benzaldehyde.

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Hydrodynamic behaviour of an ensemble of encapsulated liquid drops in creeping motion: a fluid-mechanic based model for liquid membranes

Ferreira¹,

Soares²,

Chhabra³

2003

Fluid Dyn. Res.

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The free surface cell model is used with the Stokes approximation, to characterize the Newtonian ow of an ensemble of concentric two-phase liquid drops (i.e. globules) in steady, creeping motion through an immiscible continuous liquid phase. The drag on each globule is determined analytically, and its dependence on globule concentration in the ensemble is investigated. Closed form expressions are derived for the stream functions both in the continuous phase and within each globule phase. The e ect of globule concentration on streamline patterns and ow velocity is also investigated. The drag and internal circulation patterns are analysed in the limiting situations of a single globule, a rigid sphere ensemble and a uid sphere ensemble, and their dependence on globule concentration and the other ow parameters is determined.

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The decomposition of methyl hemiacetals of benzaldehyde in aqueous solution: a study of the effect of aromatic substitution

Cited by 3 publications

References 0 publications

Photolytic Cleavage of 1-(2-Nitrophenyl)ethyl Ethers Involves Two Parallel Pathways and Product Release Is Rate-Limited by Decomposition of a Common Hemiacetal Intermediate

Photolytic Cleavage of 1-(2-Nitrophenyl)ethyl Ethers Involves Two Parallel Pathways and Product Release Is Rate-Limited by Decomposition of a Common Hemiacetal Intermediate

Effect of Alkyl Group Size on the Mechanism of Acid Hydrolyses of Benzaldehyde Acetals

Hydrodynamic behaviour of an ensemble of encapsulated liquid drops in creeping motion: a fluid-mechanic based model for liquid membranes

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