Physical organic chemistry

Transcription

1 Physical organic chemistry Second edition Neil S. Isaacs

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3 Foreword to first edition Foreword to second edition Symbols and abbreviations Mechanistic designations vi i ix xxvi i xxvii r Models of chemical bonding Covalency and molecular structure The valence bond (VB) model The molecular orbital (MO) model Approximate molecular orbital theory The Hückel molecular orbital (HMO) method Properties of Hückel molecular orbitals The relationship between MO and VB models Advanced MO methods Properties of covalent bonds Bond lengths Interbond angles Force constants Bond and molecular dipole moments Molecular and bond polarizabilities Bond dissociation enthalpies (BDE) Group additivities to bond enthalpies Intermolecular forces Electrostatic forces Ion-pairs Short-range intermolecular forces The hydrogen bond Charge-transfer complexes Crowns, cryptates, calixarenes and cyclodextrins 7 6

4 s Kinetics and thermodynamics Enthalpy Endothermic reactions Entropy The Gibbs function, G Factors that contribute to entropy Chemical equilibrium Some useful thermodynamic relationships Temperature dependence The application of thermodynamics to rate processes Activation The potential energy surface The transition state model Properties of the transition state Activation parameters Heat capacity of activation Variation of rate with pressure The uses of activation parameters The empirical treatment of rates of simpl e irreversible reactions The rate-determining step Relative rates Entropies and volumes of reaction The isokinetic relationship The location of the transition state The Hammond Postulate Reactivity and selectivity Kinetic and thermodynamic control of products The principle of least motion The principle of microscopic reversibility Limitations of the transition-state theory Reagents and reaction mechanisms Polar and radical pathways Polar reactions Nucleophiles Electrophiles Radicals Reactivity A classification of fundamental reaction types Bond formation and bond breaking Transfer reactions Elimination (E) and addition (Ad) 136

5 3.2.4 Pericyclic reactions Oxidations and reductions Reaction mechanism The advantages of synchronous reactions Electron supply and demand Transition-state properties and structural change Correlation of structure pith reactivity Electronic demands The Hammett equation Substituent constants a Theories of substituent effects The resonance effect The inductive effect Interpretation of a-values Unshared-pair (n) substituents, -7C Alkyl groups Electron-withdrawing groups, Cationic centres Reaction constants, p Deviations from the Hammett equation Random deviations Mechanistic change ' Enhanced resonance Variable resonance interactions Dual-parameter correlations : the flowering of LFER Inductive substituent constants The Taft model Other chemical model systems : modern o and QR scales Molecular orbital considerations Cross-interaction terms The sign of p;; 187 f Solvent effects The structure of liquids Solutions Solvation Polarity Polarizability 199

6 5.3.3 Hydrogen bonding Donor-acceptor interactions Thermodynamic measures of solvation Free energies of solution and transfer functions Activities of solutes `Solvation' in the gas phase ' The effects of solvation on reaction rates and equilibria Solvent effects on rates Empirical indexes of solvation Scales based on physical properties Scales based on solvent-sensitive reaction rates Scales based on spectroscopic properties Scales for specific solvation Relationships between empirical solvation scales The use of solvation scales in mechanistic studies Multiparameter solvation analysis Acids and bases, electrophiles and nucleophiles Acid-base dissociation The strengths of oxygen and nitrogen acids The effect of pressure on acid-base dissociation The interpretation of KA Linear free-energy relationships Rates of proton transfers Structural effects on amine protonation Linear free-energy relationships Acidities of carbon acids The measurement of weak acidity Factors that influence carbon acidity Electronic effects of adjacent - R and - I groups Stabilization by d-orbitals s-character of carbon hybridization Aromaticity Rates of ionization of carbon acids Gas-phase acidity and basicity Theories of proton transfer Highly acidic and highly basic solutions Highly acidic solutions Highly basic media Nucleophilicity and electrophilicity Measurement of nucleophilicity : nucleophilicit y and basicity 266

7 Hard and soft acids and bases : frontier orbita l interactions Nucleophilicity scales The relationship between nucleophilicity an d nucleofugacity The `a-effect' Ambident nucleophiles The measurement of electrophilicity Brr nsted relationships in nucleophilic reactions The Leffler index 28 2 Kinetic isotope effects Isotopic substitution Theory of isotope effects : the primary effect Transition-state geometry Secondary kinetic isotope effects `Inductive' and `steric ' isotope effects Heavy atom isotope effects The tunnel effect Solvent isotope effects Fractionation factors Solvent isotope effects in mixed isotopic solvents : the proton inventory technique Examples of solvent isotope effects Steric and conformational properties The origins of steric strain Examples of steric effects upon reactions Ortho effects F-strain effects Bond-angle strain Steric inhibition of resonance Steric acceleration Steric enhancement of resonance Calculation of steric effects : the molecula r mechanics method Measurement of steric effects upon rates The Taft-Ingold hypothesis 332

8 8.3.2 Other steric parameters Examples of steric LFER Conformational barriers to bond rotation Spectroscopic detection of individual conformers Acyclic compounds Cyclic compounds Rotations about partial double bonds Inversion at Group V elements Chemical consequences of conformational isomerism : the Winstein-Holness-Curtin-Hammett principle Homogeneous catalysis Acid and base catalysis Specific and general catalysis Mechanisms of acid catalysis Methods of distinguishing between Al and A 2 reactions Linear free-energy relationships ; the Brenste d Catalysis Law Interpretation of the Brensted coefficients Nucleophilic catalysis Potential-energy surfaces for proton transfers Solvent isotope effects Electrophilic catalysis The mechanisms of some catalysed reactions Substitutions a- to a carbonyl group Keto-enol equilibria Hydrolyses of acetals, ketals, orthoesters and related compounds Dehydration of aldehyde hydrates and related compounds The formation of oximes, semicarbazones and hydrazones Decarboxylation Acid-catalysed alkene-alcohol interchange Some acid-catalysed rearrangements Rate-limiting proton transfers Catalysis by non-covalent binding Host-guest interactions 41 1

9 to Substitution reactions at carbon Substitutions at saturated carbon Nucleophilic substitution (S N 2) The bimolecular reaction, S N Solvolytic reactions-the S N 1 spectrum Measurement of solvent participation Kinetic isotope effects The structures of intermediates in S N1 reactions The phenomenon of `return' Rearrangement criteria for return The `special' salt effect : an ion exchange in an ion-pair Structural effects upon ionization Leaving-group effects Bridgehead systems Linear free-energy relationships Intramolecular assistance in ionization Activation parameters The S N 1 reactions Aliphatic S N 2 reactions in the gas phase Electrophilic substitutions at saturated carbon The S E 1 mechanism The S E2 mechanism Electrophilic substitution via enolization Nucleophilic displacements at a vinyl carbon Electrophilic displacements at an aromatic carbon Timing of bond-breaking and making The general mechanism for electrophili c aromatic substitution The nature of the electrophilic reagents Kinetic isotope effects Kinetics of S E 2-Ar reactions Structural effects on rates The ortho-para selectivity ratio, so, n = (2fo/fp) The nature of the intermediate Ipso attack The MO interpretation of aromatic reactivity Nucleophilic substitution at an aromatic centre The addition-elimination pathwa y (S NAr-Ad, E) The unimolecular mechanism The aryne mechanism (E-Ad) Nucleophilic substitution via ring opening : the S N(ANRORC) route Nucleophilic substitutions at carbonyl carbon Basic hydrolysis of carboxylic esters 511

10 Acidic hydrolysis of esters Stereoelectronic factors in the decomposition of the tetrahedral intermediate Other mechanisms for ester hydrolysis Hydrolysis of amides, acyl halides and anhydrides Properties of tetrahedral intermediates Nucleophilic catalysis in carbonyl substitutions 53 6 Problems it Elimination reactions Base-promoted eliminations in solution Kinetic criteria of mechanisms Structural effects on rates of elimination Kinetic isotope effects Variation of the base-solvent system Competition between elimination an d substitution Orientation in product formation Stereochemistry of E2 reactions Frontier orbital considerations Elcb reactions Ester hydrolysis by the Elcb mechanism Intramolecular pyrolytic eliminations (the E ; reactions) Ester pyrolysis The Chugaev reaction Amine oxide, sulphoxide and selenoxid e pyrolyses Pyrolysis of alkyl halides a-eliminations Oxidative eliminations Oxidations of alcohols by chromium (VI) The Moffatt oxidation Polar addition reactions Electrophilic additions to alkenes Kinetics Effect of structure Isotope effects 607

11 Orientation and stereochemistry The nature of the intermediates in Ad E reactions Miscellaneous additions Hydroboration Addition with ring closure ; halolactonization Addition of carbocations Additions to dienes, alkynes and allenes Nucleophilic additions to multiple bonds Michael addition Carbonyl additions Additions to heterocumulenes Frontier orbital considerations Vinyl substitution via addition/elimination Examples Stereochemistry 63 6 Problems 13 Intramolecular reactions Neighbouring-group participation The scope of neighbouring-group effects Methods for recognizing neighbouring-group participation The kinetic criterion Linear free-energy relationships Kinetic isotope effects Solvent effects Participation in carbonyl reactions The stereochemical criterion The rearrangement criterion Factors influencing neighbouring-group participation Observation and isolation of cycli c intermediates a- and z-participation : the question of nonclassical ions Enzymic reactions The structures of enzymes A model for enzyme action Mechanisms of some enzyme-catalysed reactions Enzymes that use cofactors Enzyme model systems 69 3

12 r Perigclic reactions Classification of pericyclic reactions The theory of pericyclic reactions Conservation of orbital symmetry : correlation diagrams The frontier orbital concept The aromaticity concept Suprafacial and antarafacial geometries Thermal cycloadditions : their scope and characteristics The Diels-Alder reaction Stereo- and regiospecificity in Diels-Alde r reactions Retro Diels-Alder reactions The nature of the Diels-Alder transition state Related six-electron cycloadditions Thermal (2 + 2) cycloadditions Cycloadditions of cumulenes Two-step cycloadditions (2 + 2) Cycloreversions ,3-Dipolar cycloadditions Electrocyclic reactions Cheletropic reactions Sigmatropic reactions Concertedness in sigmatropic rearrangements Acid catalysis of the Diels-Alder reaction 75 3 rj Reactions via free radicals The generation of radicals Primary processes Secondary routes The detection of radicals Direct observation ,2.2 Indirect methods By chemical characteristics Reactions of radicals Radical coupling Displacement (abstraction, transfer) reactions Additions to 7c-systems Fragmentation of radicals Radical rearrangements Radical cyclization reactions Linear free-energy relationships Electron transfer reactions 806

13 15.4 Factors influencing the reactivities of radicals Radical stability Polar influences Solvent effects on radical reactions Steric effects in radical reactions Frontier-orbital considerations The stereochemistry of radicals 82 4 r6 Organic photochemistry Excited electronic states Absorption of light by molecules Vertical and horizontal excitation Spin multiplicity: singlet and triplet states Sensitization and quenching Techniques of photochemistry Photochemistry of the carbon-carbon double bond Geometrical isomerization Photochemical pericyclic reactions The di-rr-methane rearrangement Photoadditions to alkenes Photoreactions of carbonyl compounds Carbon-carbon bond cleavage Cycloadditions Photochemistry of aromatic compounds Photosubstitutions at the aromatic ring The photo-fries rearrangement Valence isomerization Photocycloadditions Photo-oxidations with oxygen 864 Index 871