DOCSLIB.ORG

Kinetics and Mechanisms of Base-Catalysed Reactions

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Kinetics and Mechanisms of Base-Catalysed Reactions Scholars' Mine Masters Theses Student Theses and Dissertations 1966 Kinetics and mechanisms of base-catalysed reactions Rohit Panalal Sheth Follow this and additional works at: https://scholarsmine.mst.edu/masters_theses Part of the Chemical Engineering Commons Department: Recommended Citation Sheth, Rohit Panalal, "Kinetics and mechanisms of base-catalysed reactions" (1966). Masters Theses. 5735. https://scholarsmine.mst.edu/masters_theses/5735 This thesis is brought to you by Scholars' Mine, a service of the Missouri S&T Library and Learning Resources. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. KINETICS AND MECHANISMS OF BASE·CATALYSED REACTIONS By ROHIT PANALAL SHETH A THESIS submitted to the faculty of THE UNIVERSITY OF MISSOURI AT ·roLLA in partial fulfillment of the requirements for the degree· of MASTER OF SCIENCE IN CHEMICAL ENGINEERING Rolla, Missouri 1966 ~... ii TABLE OF CONTENTS Page TITLE PAGE • • • • • • • • • • • • • • • • • • • • • • • • • i TABLE OF CONTENTS • • • • • • • • • • • • • • • • • • • • .ii LIST OF FIGURES • • • • • • • • I I I I I I I I I • • • • • iv LIST OF TABLES • • • • • • • • • • • • • • • • • • • • • • • v GLOSSARY OF TERMS • • • • • • • • •• • • • • • • • • • • • • vi QIAPTER I. Introduction • • • • • • • • • • • • • • • • • • 1 CHAPTER II. Literature Review • • • • • • • • • • • • • • • 4 CHAPTER III. Theory •••• • • • • • • • • • • • • • • • • 27 CHAPTER IV. Experimental • • • • • • • • • • • • • • • • • 31 A. Purpose of Investigation •• • • • • • • • • • • • • 31 B. Plan of Experimentation • • • • • • • • • • • • • • 32 c. Experimental Set-Up • • • • • • • • • • • • • • • • 32 D. Analytical Techniques • • • •• • • • • • • . .· . • • 33 1. Gas Chromatography • • •• • • • • • • • • • • 33 (; 2. Operating Conditions • • • • • • • • • • • • • 34 3. Sampling • • • • • • • • • • • • • • • • • • • 34 E. Preparation of calibration Curve • • • • • • • • • 34 F. Experimentation • • • • • • • • • • • • • • • • • • 35 G. Data and Results • • • • • • • • • • • • • • • • • 35 CHAPTER V. Discussion • • • • • • • • • • • • • • • • • • • • 48 A. Discussion of Data and Results • • • • • • • • • • 48 B. Discussion of Michael Mechanism ••••• • • • • • 51 c. Signifi.cance of the Rate Constants for the Reverse Process and of the Principle of Microscopic Reversibility • • • • • • • • • • • • • • • • • • • 58 D. Objections (Reservations) • • • • • • • • • • • • • 61 iii Page CHAPTER VI. Conclusions • • • • • • • • • • • • • • • • • • 66 QIAPTER VI I. Summary • • • • • • • • • • • • • • • • • • • 68 APPENDIX A. List of Computer Programs • • • • • • • • • • • 70 1. A Program for the Calculations of Adduct Concentrations • • • • • • • • • • • • • • • • • • 70 2. A Program for the Calculations of Rate and Equilibrium Constants • • • • • • • • • • • • • • • 71 3. A Program for the Calculations of Activation Energies • • • • • • • • • • • • • • • • • • • • • 73 APPENDIX B. List of Computer Programs (Error Calculations). 75 1. A Program for Computing the Effect of Error in Temperature on Activation Energies •••••••• 75 2. A Program for Correcting Equilibrium Rate Constants •••••••••••••. •••••••• 78 APPENDIX c. List of Equipment and Materials ' • • • • • • • • 81 BIBLIOGRAPHY • • • • • • • • • • • • • • • • • • • • • • • • .83 ACKNOWLEDGEMENTS • •• • • • • • • • • • • • . .. ........ 86 • 87 VITA • • • • • •• • • • • • • • • • • • • • • • • • • • • • iv LIST OF FIGURES Figure Page 1. Standard Curve of Area-Ratio as a Function of ~1ole-Ratio • • • • • • • • • • • 36 2. Concentration of Adduct as a Function of Time (Run 1) • • • • • • • • • • 43 3. Concentration of Adduct as a Ftmction of Time (Run 2) •• • • • • • • • • 44 4. Concentration of Adduct as a Function of Time (Run 3) •• • • • • • • • • 45 S. Concentration of Adduct as a Function of Time (Rtm 4) • • • • • • • • • • 46 6. Concentration of Adduct as a Function of Time (Run 5) •• • • • • • • • • 47 v LIST OF TABLES Table Page I Reported Equilibrium Yield of Adduct at Various Temperatures • • • • •• • • • • • 9 II Experirental Data for Run 1 • • • • • • • • • 38 III Experiroontal Data for Run 2 • • • • • • • • • 39 IV Experimental Data for Run 3 • • • • • • • • • 40 v Experimental Data for Run 4 • • • • • • • • • 41 VI Experimental Data for Run 5 • • • • • • • • • 42 VII Dependence of Rate Constants on Temperature • 49 VIII Equilibrium Yields at Various Reaction .Temperatures • • , • • • • • • • • • • • • • 50 IX Activation Parameters • • • • • • • • • • • • 52 X Entropies of Activation of Some Well Studied Reactions • • • • • • • • • • • • • • 53 XI Acidities of Reaction Components • • • • • • 60 XII Rate Constants and Entropies of Activation as Functions of Ionization Constants ••••• 60 XIII Theoretical Values for ~S~Reaction • • • • • 62 vi GLOSSARY OF TERMS Ea Arrhenius activation energy 6F* Free energy of activation 6H* Heat of activation h Plank's constant Boltzman constant Rate constant Ke Equilibrium constant Ka Dissociation constant for adduct Kb Dissociation constant for base (~·Buok) Km Dissociation constant for malonic ester R Gas constant 6S* Entropy of activation T Absolute reaction temperatu~ SUBSCRIPTS f Forward process r Reverse process 1 Olapter I INTRODUCfiON Base-catalysed reactions normally involve complex series of transformations. Their kinetics is usually governed by a variety of equilibria, participating in the overall and con­ current processes as well as by competitive side-reactions that interfere with the normal course of such reactions. :-.Iechanisms which have been proposed• by and large, are primarily based on product, by-product and intermediate analyses together \vith some scattered kinetic and isotopic evidence. In the cases where kinetic studies have been attempted, highly complex mechanisms have usually been proposed due to the frequent necessity of using heterogeneous media or employing involved mathematical treatments to describe the kinetics of the base-catalysed reactions. One of the most common and the least thoroughly studied class of reactions in this general area is the "Michael Reaction". It is the name commonly assigned to the base~catalysed addition of an activated methylene compound, the addendum, to a suitably activated :olefin, the acceptor, to yield normal or abnormal and retrogression Michael adducts as illustrated below: 2 y R R y I I I I CIIR + CH CH CR I II I I (1) z Ol 0!2 z I I X X Normal Adduct R R R X I I I I CH CH •., CH + CH2 I I II I CH z CR z I X/" y y Abnormal Adduct Retrogression Ad ducts where, Y and Z may be COOR, COR, CONH2, N02, S02R, CN or OIO•Y and X may be the same or different than z. Even though the l--lichae 1 reaction has been known since 1887, there are no thorough kinetic studies reported of typical systems in any abstracted publication. In the late 1940's studies were begtm by Shafer, Loeb and Johnson (43) on the abnormal Michael reaction. Later studies by Korst (25) and by Wulfman (52) indicated the need for thorough kinetic studies of both abnormal and normal Michael Reactions before any definitive mechanism could be proposed for these reactions. Wulfrnan (52), in his original studies, observed initial pseudo "first-order" followed by pseudo zero-order kinetic paths for the normal Michael reaction. These observations were taken to imply a change from homogeneous to heterogeneous media as the reaction proceeds coupled with fortuitous relationships between the various terms required to describe reaction kinetics. 3 ~1ehta (31) avoided these problems by using dilute solutions and by deriving an improved mathematical model for the rate law, which took into account the acidity of the product and sol vent as we 11 as that of the starting material. The study by Mehta (31) of a typical Michael reaction - ethyl crotonate, dimethyl malonate, t-butyl alcohol (solvent) and potassium tertiary butoxide (catalyst) with various initial co~ cent rations of reactants indicated that the kinetics of the system was consistent with the generally accepted Michael mechanism. The work presented here was undertaken to test the validity of the existing Michael mechanism through the use of thermodynamic interpretation of the experimentally obtained kinetic data. The investigation involved the study of the effect of temperature variations on the rate of the Michael reaction. Activation energies and entropies of activation were determined by conventional methods and their magnitudes were applied to furnish valuable clues to the true mechanism of the Michael reaction. 4 Chapter II LITERATURE REVIEW The ~tichael reaction or addition, in its original scope, is the addition of an addendum or donor containing an active n~thylene group to a conjugated carbon-carbon double bond (4). It can be adequately described by the reversible and .base­ catalysed addition of diethyl malonate (I), an addendum containing an activated methylene group, to methyl crotonate (II), an acceptor \"i th an activated double bond, to yield 1,1 - dicarbethoxy - 3 - carbomethoxy - 2 - methyl -propane (III) (Chart 1). Product (III) is referred to as the normal Hichael adduct, and the reaction sequence leading to (III) is known as the normal ~1ichael reaction or addition. The scope of the Hichael Reaction has been surveyed by Conner and ~1cClellan (7) and there have been more recent extensions of their review. Some of the numerous variations of the Michael Reaction are
Load more

Recommended publications
  • Physical Organic Chemistry
    CHM 8304 Physical Organic Chemistry Catalysis Outline: General principles of catalysis • see section 9.1 of A&D – principles of catalysis – differential bonding 2 Catalysis 1 CHM 8304 General principles • a catalyst accelerates a reaction without being consumed • the rate of catalysis is given by the turnover number • a reaction may alternatively be “promoted” (accelerated, rather than catalysed) by an additive that is consumed • a heterogeneous catalyst is not dissolved in solution; catalysis typically takes place on its surface • a homogeneous catalyst is dissolved in solution, where catalysis takes place • all catalysis is due to a decrease in the activation barrier, ΔG‡ 3 Catalysts • efficient at low concentrations -5 -4 -5 – e.g. [Enz]cell << 10 M; [Substrates]cell < 10 - 10 M • not consumed during the reaction – e.g. each enzyme molecule can catalyse the transformation of 20 - 36 x 106 molecules of substrate per minute • do not affect the equilibrium of reversible chemical reactions – only accelerate the rate of approach to equilibrium end point • most chemical catalysts operate in extreme reaction conditions while enzymes generally operate under mild conditions (10° - 50 °C, neutral pH) • enzymes are specific to a reaction and to substrates; chemical catalysts are far less selective 4 Catalysis 2 CHM 8304 Catalysis and free energy • catalysis accelerates a reaction by stabilising a TS relative to the ground state ‡ – free energy of activation, ΔG , decreases – rate constant, k, increases • catalysis does not affect the end point
    [Show full text]
  • Aldehydes Can React with Alcohols to Form Hemiacetals
    340 14 . Nucleophilic substitution at C=O with loss of carbonyl oxygen You have, in fact, already met some reactions in which the carbonyl oxygen atom can be lost, but you probably didn’t notice at the time. The equilibrium between an aldehyde or ketone and its hydrate (p. 000) is one such reaction. O HO OH H2O + R1 R2 R1 R2 When the hydrate reverts to starting materials, either of its two oxygen atoms must leave: one OPh came from the water and one from the carbonyl group, so 50% of the time the oxygen atom that belonged to the carbonyl group will be lost. Usually, this is of no consequence, but it can be useful. O For example, in 1968 some chemists studying the reactions that take place inside mass spectrometers needed to label the carbonyl oxygen atom of this ketone with the isotope 18 O. 16 18 By stirring the ‘normal’ O compound with a large excess of isotopically labelled water, H 2 O, for a few hours in the presence of a drop of acid they were able to make the required labelled com- í In Chapter 13 we saw this way of pound. Without the acid catalyst, the exchange is very slow. Acid catalysis speeds the reaction up by making a reaction go faster by raising making the carbonyl group more electrophilic so that equilibrium is reached more quickly. The the energy of the starting material. We 18 also saw that the position of an equilibrium is controlled by mass action— O is in large excess.
    [Show full text]
  • Kinetic and Theoretical Insights Into the Mechanism of Alkanol Dehydration on Solid Brønsted Acid Catalysts
    Article pubs.acs.org/JPCC Kinetic and Theoretical Insights into the Mechanism of Alkanol Dehydration on Solid Brønsted Acid Catalysts William Knaeble and Enrique Iglesia* Department of Chemical and Biomolecular Engineering University of California, Berkeley, California 94720, United States *S Supporting Information ABSTRACT: Elementary steps that mediate ethanol dehydration to alkenes and ethers are determined here from rate and selectivity data on solid acids of diverse acid strength and known structure and free energies derived from density functional theory (DFT). Measured ethene and ether formation rates that differed from those expected from accepted monomolecular and bimolecular routes led to our systematic enumeration of plausible dehydration routes and to a rigorous assessment of their contributions to the products formed. H-bonded monomers, protonated alkanol dimers, and alkoxides are the prevalent bound intermediates at conditions relevant to the practice of dehydration catalysis. We conclude that direct and sequential (alkoxide-mediated) routes contribute to ether formation via SN2-type reactions; alkenes form preferentially from sequential routes via monomolecular and bimolecular syn-E2-type eliminations; and alkoxides form via bimolecular SN2-type substitutions. The prevalence of these elementary steps and their kinetic relevance are consistent with measured kinetic and thermodynamic parameters, which agree with values from DFT-derived free energies and with the effects of acid strength on rates, selectivities, and rate constants; such effects reflect the relative charges in transition states and their relevant precursors. Dehydration turnover rates, but not selectivities, depend on acid strength because transition states are more highly charged than their relevant precursors, but similar in charge for transition states that mediate the competing pathways responsible for selectivity.
    [Show full text]
  • Acid-Base Catalysis Application of Solid Acid-Base Catalysts
    Modern Methods in Heterogeneous Catalysis Research Acid-Base Catalysis Application of Solid Acid-Base Catalysts Annette Trunschke 18 February 2005 Outline 1. Introduction - basic principles 2. Substrates and products 3. Kinds of acid / base catalysts - examples 4. Characterization of surface acidity / basicity - examples 5. Acid catalyzed reactions 6. Base catalyzed reactions 7. Acid-base bifunctional catalysis 8. Summary and outlook Modern Methods in Heterogeneous Catalysis Research : 18/02/2005 2 1. Introduction - Basic definitions concept acid base - + Brønsted H2O OH + H - - Lewis FeCl3 + Cl [FeCl4] Hydrogen transfer reactions intermediates acid-catalyzed + H+ carbenium ions base-catalyzed + H+ carbanions Modern Methods in Heterogeneous Catalysis Research : 18/02/2005 3 1. Introduction - Basic definitions specific acid / base general acid / base catalysis catalysis + - active H3O or OH undissociated acid or base species groups; a variety of species may be simultaneously active reaction •in solution •gas phase reactions medium / •on the surface of •high reaction temperatures conditions a hydrated solid Advantages of solid acid-base catalysts: • Easier separation from the product • Possible reuse and regeneration • Fewer disposal problems • Non-corrosive and environmentally friendly (but not always!) Modern Methods in Heterogeneous Catalysis Research : 18/02/2005 4 2. Substrates and products solid acid catalysis solid base catalysis substrates •Alkanes, aromatics •Alkenes (components of crude •Alkynes petrolium) •Alkyl aromatics •Alkenes (products of •Carbonyl compounds petrolium cracking (FCC, steam cracking)) products •Gasoline components •Chemical intermediates •Chemical intermediates •Fine chemicals •Fine chemicals Industrial processes in 1999: 103 solid acids worldwide production by catalytic cracking: 500x106 tonnes/y 10 solid bases 14 solid acid-base bifunctional catalysts K.Tanabe, W.F.Hölderich, Applied Catalysis A 181 (1999) 399.
    [Show full text]
  • 629 Acid/Base Catalysis 154–157
    629 Index a – – Horvath–Kawazoe (HK) method acid/base catalysis 154–157 525–526 – bimolecular, acid catalyzed reaction 155 – Nonlocal Density Functional Theory – bimolecular, base catalyzed reaction 156 (NL–DFT) 530–532 – Brønsted–Lowry theory 155 – physical adsorption 514–516 – RNA hydrolysis catalyzed by enzyme – theoretical description of adsorption RNase-A 157 519–523 acids 132–143, See also solid acids and bases ––BETtheoryofadsorption 521 Acinetobacter calcoaceticus 181 – – Langmuir isotherm 519–521 activated carbon supports 439 – – standard isotherms 522–523 activation, catalyst 58–64 – – t-method 522–523 – induction periods as catalyst activation – xenon porosimetry 533–534 ® 59–60 Aerosil process 435 active sites 269–273 alcohols activity of catalyst, in homogeneous catalysis – carbonylations of 245–247 54–58 – synthesis 176–177 acylases 185–186, 256 aldolases 190–193 adaptive chemistry methods 370 alkene metathesis 402–406 adiabatic reactor 569 altered surface reactivity 38–39 adsorption entropy 27–30 alumina (Al2O3) and other oxides adsorption methods for porous materials 436–438 characterization 514–534 amidases 185–186 – adsorption isotherms 517–518 amidases catalyzed reactions 256 – application of adsorption methods amino acid dehydrogenases (AADHs) 518–519 177, 253 – Barret–Joyner–Halada (BJH) method amino acids synthesis 177–178 529–530 6-aminopenicillanic acid (6-APA) 256 – classification of porous materials 517 7-aminocephalosporanic acid (7-ACA) 256 – mercury porosimetry 533 ammonia synthesis 114–119, 289–299 – mesoporous
    [Show full text]
  • Factors Affecting the Relative Efficiency of General Acid Catalysis
    Factors Affecting the Relative Efficiency of General Acid Catalysis Eugene E. Kwan Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada* * Now at Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA Electronic mail: [email protected] Abstract Specific acid catalysis (SAC) and general acid catalysis (GAC) play an important role in many organic reactions. Elucidation of SAC or GAC mechanisms requires an understanding of the factors that affect their relative efficiency. A simple model reaction in which SAC and GAC occur concurrently is analyzed to provide guidelines for such analyses. Simple rules which predict the effect of pH on the relative efficiency of GAC are discussed. An explicit expression for determining the optimal pH at which the effects of GAC are maximized is provided as well as an analogous expression for the general base case. The effects of the pKa of the general acid and the total catalyst concentration on GAC efficiency are described. The guidelines are graphically illustrated using a mixture of theoretical data and literature data on the hydrolysis of di-tert-butyloxymethylbenzene. The relationship between the model and the Brønsted equation is developed to explain the common observation that GAC is most easily observable when the Brønsted coefficient α is near 0.5. The ideas presented enable systematic evaluations of experimental kinetic data to provide quantitative support for SAC or GAC mechanisms. This material is suitable for the curriculum of a senior-level undergraduate course in physical-organic chemistry. Keywords: Organic chemistry; physical chemistry; curriculum; acid-base chemistry; catalysis; kinetics.
    [Show full text]
  • Microwave-Assisted Homogeneous Acid Catalysis and Chemoenzymatic Synthesis of Dialkyl Succinate in a Flow Reactor
    catalysts Article Microwave-Assisted Homogeneous Acid Catalysis and Chemoenzymatic Synthesis of Dialkyl Succinate in a Flow Reactor Laura Daviot 1, Thomas Len 1, Carol Sze Ki Lin 2 and Christophe Len 1,3,* 1 Centre de Recherche de Royallieu, Université de Technologie Compiègne, Sorbonne Universités, Cedex BP20529, F-60205 Compiègne, France; [email protected] (L.D.); [email protected] (T.L.) 2 School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, China; [email protected] 3 Institut de Recherche de Chimie Paris, PSL Research University, Chimie ParisTech, CNRS, UMR 8247, Cedex 05, F-75231 Paris, France * Correspondence: [email protected]; Tel.: +33-144-276-752 Received: 12 February 2019; Accepted: 8 March 2019; Published: 16 March 2019 Abstract: Two new continuous flow systems for the production of dialkyl succinates were developed via the esterification of succinic acid, and via the trans-esterification of dimethyl succinate. The first microwave-assisted continuous esterification of succinic acid with H2SO4 as a chemical homogeneous catalyst was successfully achieved via a single pass (ca 320 s) at 65–115 ◦C using a MiniFlow 200ss Sairem Technology. The first continuous trans-esterification of dimethyl succinate with lipase Cal B as an enzymatic catalyst was developed using a Syrris Asia Technology, with an optimal reaction condition of 14 min at 40 ◦C. Dialkyl succinates were produced with the two technologies, but higher productivity was observed for the microwave-assisted continuous esterification using chemical catalysts. The continuous flow trans-esterification demonstrated a number of advantages, but it resulted in lower yield of the target esters.
    [Show full text]
  • Brønsted and Lewis Solid Acid Catalysts in the Valorization of Citronellal
    catalysts Article Brønsted and Lewis Solid Acid Catalysts in the Valorization of Citronellal Federica Zaccheria 1, Federica Santoro 1, Elvina Dhiaul Iftitah 2 and Nicoletta Ravasio 1,* 1 CNR Institute of Molecular Science and Technology, Via Golgi 19, 20133 Milano, Italy; [email protected] (F.Z.); [email protected] (F.S.) 2 Chemistry Department Faculty of Science, Brawijaya University Jl. Veteran, Malang 65145, Indonesia; [email protected] * Correspondence: [email protected]; Tel.: +39-02-5031-4382 Received: 30 July 2018; Accepted: 12 September 2018; Published: 22 September 2018 Abstract: Terpenes are valuable starting materials for the synthesis of molecules that are of interest to the flavor, fragrance, and pharmaceutical industries. However, most processes involve the use of mineral acids or homogeneous Lewis acid catalysts. Here, we report results obtained in the liquid-phase reaction of citronellal with anilines under heterogeneous catalysis conditions to give tricyclic compounds with interesting pharmacological activity. The terpenic aldehyde could be converted into octahydroacridines with a 92% yield through an intramolecular imino Diels–Alder reaction of the imine initially formed in the presence of an acidic clay such as Montmorillonite KSF. Selectivity to the desired product strongly depended on the acid sites distribution, with Brønsted acids favoring selectivity to octahydroacridine and formation of the cis isomer. Pure Lewis acids such as silica–alumina with a very low amount of alumina gave excellent results with electron-rich anilines like toluidine and p-anisidine. This protocol can be applied starting directly from essential oils such as kaffir lime oil, which has a high citronellal content.
    [Show full text]
  • Asymmetric Catalysis in Aqueous Media*
    Pure Appl. Chem., Vol. 79, No. 2, pp. 235–245, 2007. doi:10.1351/pac200779020235 © 2007 IUPAC Asymmetric catalysis in aqueous media* Shu– Kobayashi Graduate School of Pharmaceutical Sciences, The University of Tokyo, The HFRE Division, ERATO, Japan Science and Technology Agency (JST), Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Abstract: Lewis acid catalysis has attracted much attention in organic synthesis because of unique reactivity and selectivity attained under mild conditions. Although various kinds of Lewis acids have been developed and applied in industry, these Lewis acids must be gener- ally used under strictly anhydrous conditions. The presence of even a small amount of water handles the reactions owing to preferential reactions of the Lewis acids with water rather than the substrates. In contrast, rare earth and other metal complexes have been found to be water- compatible. Several catalytic asymmetric reactions in aqueous media, including hydroxy- methylation of silicon enolates with an aqueous solution of formaldehyde in the presence of Sc(OTf)3–chiral bipyridine ligand or Bi(OTf)3–chiral bipyridine ligand, Sc- or Bi-catalyzed asymmetric meso-epoxide ring-opening reactions with amines, and asymmetric Mannich- type reactions of silicon enolates with N-acylhydrazones in the presence of a chiral Zn cata- lyst have been developed. Water plays key roles in these asymmetric reactions. Keywords: reactions in aqueous media; Lewis acids; catalytic asymmetric reactions; hydroxymethylation of silicon enolates; Sc(OTf)3–chiral bipyridine ligand. INTRODUCTION Organic reactions in aqueous media are of current interest owing to the key role played by water as a solvent for green chemistry [1].
    [Show full text]
  • Ester Hydrolysis: Conditions for Acid Autocatalysis and a Kinetic Switch
    This is a repository copy of Ester hydrolysis: Conditions for acid autocatalysis and a kinetic switch. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/116706/ Version: Accepted Version Article: Bánsági, T. and Taylor, A.F. orcid.org/0000-0003-0071-8306 (2017) Ester hydrolysis: Conditions for acid autocatalysis and a kinetic switch. Tetrahedron, 73 (33). pp. 5018-5022. ISSN 0040-4020 https://doi.org/10.1016/j.tet.2017.05.049 Article available under the terms of the CC-BY-NC-ND licence (https://creativecommons.org/licenses/by-nc-nd/4.0/ Reuse This article is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs (CC BY-NC-ND) licence. This licence only allows you to download this work and share it with others as long as you credit the authors, but you can’t change the article in any way or use it commercially. More information and the full terms of the licence here: https://creativecommons.org/licenses/ Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ Accepted Manuscript Ester hydrolysis: Conditions for acid autocatalysis and a kinetic switch Tamás Bánsági, Annette F. Taylor PII: S0040-4020(17)30537-9 DOI: 10.1016/j.tet.2017.05.049 Reference: TET 28715 To appear in: Tetrahedron Received Date: 2 April 2017 Revised Date: 10 May 2017 Accepted Date: 12 May 2017 Please cite this article as: Bánsági Tamá, Taylor AF, Ester hydrolysis: Conditions for acid autocatalysis and a kinetic switch, Tetrahedron (2017), doi: 10.1016/j.tet.2017.05.049.
    [Show full text]
  • Paul M a & Long F A. HO and Related Indicator Acidity Functions. Chem. Rev. 57:1-45, 1957
    Number 4 January 23, 1978 Citation Classics Paul M A & Long F A. HO and related indicator acidity functions. Chem. Rev. 57:1-45, 1957. The authors review the literature pertaining to function (a logarithmic measure of acidity indicator acidity functions and their reducing to pH in dilute aqueous solutions) is applications to elucidating the mechanisms of derived from indicators of charge type: B° + H+ acid catalysis. [The SCI® indicates $$$ BH + that this paper was cited 699 times in “In concentrated aqueous solutions of strong the period 1961-1976.] acids, HO is observed to decrease (indicating increasing acidity) with increasing acid concentration C significantly more rapidly than does -log C, and furthermore the HO values for the different strong acids are spread apart. Since the rates of certain acid- Professor M. A. Paul (retired) catalyzed reactions were found to correlate and Professor FA. Long rather well with the behavior of HO whereas Department of Chemistry the rates of others showed poor correlation, Cornell University interest grew in the possibility of using the Ithaca, New York 14853 presence or absence of such correlation as evidence for the particular mechanism of acid catalysis involved. Our review was prepared January 31, 1977 at a time when interest in acid catalysis had been renewed by the ideas of C.K. Ingold and his associates on the mechanisms of homogeneous organic reactions in general. By “This article, together with a companion assembling and critically evaluating the article on the application of the HO acidity published data on acidity functions and their function to kinetics and mechanisms of acid applications to the study of acid catalysis (252 catalysis, 1 reviewed twenty-five years of references), we evidently helped many research stemming from the establishment by investigators in further research on the subject.
    [Show full text]
  • Catalysis 9.1 General Principles of Catalysis Turnover Number: the Average Number of Reactants That a Catalyst Acts on Before the Catalyst Loses Its Activity
    Chapter 9 Catalysis 9.1 General principles of catalysis Turnover number: the average number of reactants that a catalyst acts on before the catalyst loses its activity. The catalyst increases rate of reaction (decreases activation energy). The thermodynamics of a catalyzed reaction are unaffected by the catalyst. A heterogeneous catalyst: one that does not dissolve in the solution. Ex) Pd/C A homogeneous catalyst: one that dissolves in the solution. All calaysts operate by the same general principle – that is, the activation energy of the rds must be lowered in order for a rate enhancement to occur. 9.1.1 Binding the transition state better than the ground state Substrate: any material (reactant) used in a catalytic reaction Activated complex: simply referred to as the transition state Rate enhancement: the ratio of the rate constant for the catalyzed reaction to that for the uncatalyzed one. To achieve catalysis, the catalyst must stabilize the transition state more than it stabilizes the ground state. That is, the transition state must be bound better than the ground state. 1 2 9.1.3 A spatial temporal approach To achieve catalysis, the catalyst must stabilize the transition state more than it stabilizes the ground state. That is, the transition state must be bound better than the ground state. Another explanation for catalysis: spatial temporal postulate, many intramolecular reactions are often much faster than corresponding intermolecular reactions. -> the rate of reaction between functionalities A and B is proportional to the time that A and B reside within a critical distance. The longer that A and B spend together in the correct geometry for reaction, the greater that probability.
    [Show full text]