Aromatic Electrophilic Substitution by Hydrogen. III. The Mechanism of the Acid-catalyzed Decarboxylation of Aromatic Aldehydes<sup>1,2</sup>

Schubert, Werner; Donohue, Jere; Gardner, Jessica D.

doi:10.1021/ja01630a001

Cited by 18 publications

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“…An important aspect of the model is the relative rates of decarbonylation (Reaction 17) and decarboxylation (Reaction 2) reactions to yield benzene. Decarbonylation of aromatic aldehydes is thought to proceed via protonation of the ipso-carbon . Decarboxylation of carboxylic acids has been studied with a variety of acids, where the mechanism is likely quite dependent on structure. − The model demonstrates that most of the benzene may be accounted for via decarboxylation of benzoic acid, as the decarboxylation rate constant is approximately 2 orders of magnitude larger than that of decarbonylation at both temperatures.…”

Section: Resultsmentioning

confidence: 99%

Production of Carboxylic Acids from Aldehydes under Hydrothermal Conditions: A Kinetics Study of Benzaldehyde

Fecteau

Gould

Glein

et al. 2018

ACS Earth Space Chem.

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Aldehydes represent an intermediate redox state of organic carbon and can be precursors to carboxylic acids via disproportionation. A model aldehyde, benzaldehyde, was subjected to hydrothermal experiments (250–350 °C, saturation pressure) to assess the kinetics and mechanisms of the reactions leading to carboxylic acids. The concentration dependence demonstrates the kinetics are second-order in benzaldehyde, consistent with a disproportionation reaction, which is reminiscent of the base-promoted Cannizzaro reaction known at lower temperatures. Arrhenius parameters for these rate constants trend well with data from most, but not all, previous studies for the reaction under supercritical conditions. The rate constants yielded an entropy of activation (ΔS ‡) of −161 J mol–1 K–1, consistent with a bimolecular transition state at the rate-limiting step. Experimental yields of benzoic acid and benzyl alcohol were not equal, unlike what is expected for the disproportionation reaction. A kinetic model that included many secondary reactions was developed and was able to reproduce the observed yields of benzyl alcohol and benzoic acid, indicating that disproportionation and subsequent secondary reactions can explain the observed product yields. Experiments at different pH values demonstrated that while the reaction can be expedited by hydroxide, as traditionally understood, at lower pH the rate is independent of pH, indicating the neutral hydrate is capable of hydride donation to another benzaldehyde molecule in the rate-limiting step. Aldehyde disproportionation offers an alternative pathway to ketone oxidation for the formation of carboxylic acids in geologic environments where they are abundant, such as sedimentary basins.

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Section: Resultsmentioning

confidence: 99%

Production of Carboxylic Acids from Aldehydes under Hydrothermal Conditions: A Kinetics Study of Benzaldehyde

Fecteau

Gould

Glein

et al. 2018

ACS Earth Space Chem.

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“…. Decarbonylation of substituted benzaldehydes in concentrated solutions of sulfuric acid at 100°C (218,219)…”

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Application Of The H₀ Acidity Function To Kinetics And Mechanisms Of Acid Catalysis

Long¹,

Paul²

1957

Chem. Rev.

201

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Halochromie und Basizität enolisierbarer β‐Diketone)

1954

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Enolisierbare P-Diketone zeigen ,,amphotere Halochromie", d. h. das Absorptions-Maximum der Enolfonn wird sowohl in alkalischer Losung wie auch in starker Schwefelsaure nach lingeren Wellen verschoben (mi-bzw. Oniurn-Halochromie). Bei trans-fixierten P-Diketonen gehen die Absorptionskurven im %-Bereich zwischen etwa +15 und -10 durch einen mehr oder wenigor ausgepriigten isosbestischen Punkt ; bei offenkettigen und ,,cis-fixierten" P-Diketonen iiberlagert daa Keto-Enol-Gleichgewicht die aniphoteren Neutralisierungs-Gleichgewichte. Durch Isolierung und Analpse kristalliner Perchlorate und Sulfate lieB sich erweisen, daB sich mit den genannten Sauren Enol-Oxonium-Salze bilden. Diese besitzen ein ehnlichea mesomeres Chromophor-System wie die Enolate, was die a nlichkeit der Absorptions-Kurven erkliirt.Die ,,Oniurn-Halochromie" von enolisierbaren P-Benzoyl-ketonen fiihrt zu einer groBeren bathochromen Verschiebung und Inten-sit&-Erhohung des Absorptions-Maximums als die ,,mi-Halochromie" der gleichen P-Diketone; ale Grund dafiir wird angesehen, daB sich nur im ersten Falle die Phenylringe am Zustandekommen des neuen Chromophors beteiligen.Aus den optischen Daten in Schwefelsiiuren verschiedener Konzentration wird die ,,Basizitiit" der P-Diketone berechnet.Beim Stehenlassen der stark sauren Lijsungen der P-Diketone treten hderungen der Spektren ein, die auf einer Saure-Spaltung in je eine Molekel Mono-Keton und Carbonsiiure beruhen.Die vorliegende Untersuchung befafit sich mit der Frage, in welcher Form sich enolisierbare P-Diketone in starken Siiuren, insbesondere in Schwefelsaure, losen. Es wird gezeigt, daD sich die P-Diketone gegen starke Sauren wie relativ starke B asen verhalten, waa sich in UV-optisch verfolgbaren Neutralisierungs-Gleichgewichten tiufiert, die weitgehend den Neutralisierungs-Gleichgewichten in alkalischen Losungen ahneln.Bevor wir auf unsere Versuche und Ergebnisse eingehen, sei k u n geschildert, was bisher uber die ,,baakchen" Eigenschaften von Mono-Carbonylverbindungen und die hderungen ihrer Lichtabsorption in aaurem Medium bekannt ist.Von g e s a t t i g t e n Mono-Carbonylverbindungen, wie Aceton, ist es bekannt, da13 sie Ansolvosiiurcn (,,Antibasen"l) ) an daa Carbonyl-Sauerstoff-Atom unter Bildung stiichiometrisch zusammengesetzter Molekiilverbindungen anlagern2). Die nucleophilen (,,basischen") Eigenschaften der Carbonylgruppe iiuBern sich ferner darin, daB sich zwischen den einmmen Elektronenpaaren des Carbonyl-Sauerstoff-Atom und den Wesserstoff-Atomen von OH-gruppenhaltigen Ltisungsmitteln Wwerstoff briicken bilden8). Diese *) Aus den Dissertat.

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Aromatic Electrophilic Substitution by Hydrogen. III. The Mechanism of the Acid-catalyzed Decarboxylation of Aromatic Aldehydes^1,2

Cited by 18 publications

References 0 publications

Production of Carboxylic Acids from Aldehydes under Hydrothermal Conditions: A Kinetics Study of Benzaldehyde

Production of Carboxylic Acids from Aldehydes under Hydrothermal Conditions: A Kinetics Study of Benzaldehyde

Application Of The H₀ Acidity Function To Kinetics And Mechanisms Of Acid Catalysis

Halochromie und Basizität enolisierbarer β‐Diketone)

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Aromatic Electrophilic Substitution by Hydrogen. III. The Mechanism of the Acid-catalyzed Decarboxylation of Aromatic Aldehydes1,2

Cited by 18 publications

References 0 publications

Production of Carboxylic Acids from Aldehydes under Hydrothermal Conditions: A Kinetics Study of Benzaldehyde

Production of Carboxylic Acids from Aldehydes under Hydrothermal Conditions: A Kinetics Study of Benzaldehyde

Application Of The H0 Acidity Function To Kinetics And Mechanisms Of Acid Catalysis

Halochromie und Basizität enolisierbarer β‐Diketone)

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Aromatic Electrophilic Substitution by Hydrogen. III. The Mechanism of the Acid-catalyzed Decarboxylation of Aromatic Aldehydes^1,2

Application Of The H₀ Acidity Function To Kinetics And Mechanisms Of Acid Catalysis