Keynotes in Organic Chemistry

Transcription

1 Keynotes in Organic Chemistry Second Edition ANDREW F. PARSONS Department of Chemistry, University of York, UK Wiley

2 Contents Preface xi 1 Structure and bonding Ionic versus covalent bonds The octet rule Formal charge Sigma (ct ) and pi (Jt ) bonds Hybridisation Inductive effects, hyperconjugation and mesomeric effects Inductive effects Hyperconjugation Mesomeric effects Acidity and basicity Acids Bases Lewis acids and bases Basicity and hybridisation Acidity and aromaticity Acid-base reactions 16 Worked example 17 Problems 18 2 Functional groups, nomenclature and drawing organic compounds Functional groups Alkyl and aryl groups Alkyl substitution Naming carbon chains Special cases Drawing organic structures 27 Worked example 28 Problems 29 3 Stereochemistry Isomerism Conformational isomers Conformations of ethane (CH3CH3) Conformations of butane (CH3CH2CH2CH3) Conformations of cycloalkanes 34

3 vi Contents Cyclohexane Configurational isomers Alkenes Isomers with chiral centres 38 Worked example 44 Problems 45 4 Reactivity and mechanism Reactive intermediates: ions versus radicals Nucleophiles and electrophiles Relative strength Carbocations, carbanions and carbon radicals Order of stability Steric effects Oxidation levels General types of reaction Polar reactions (involving ionic intermediates) Radical reactions Pericyclic reactions Ions versus radicals Reaction selectivity Reaction thermodynamics and kinetics Thermodynamics Kinetics Kinetic versus thermodynamic control Orbital overlap and energy Guidelines for drawing reaction mechanisms 67 Worked example 68 Problems 69 5 Halogenoalkanes Structure Preparation Halogenation of alkanes Halogenation of alcohols Halogenation of alkenes Reactions Nucleophilic substitution Elimination Substitution versus elimination 89 Worked example 91 Problems 92 6 Alkenes and alkynes Structure Alkenes 97

4 Contents vii Preparation Reactions Alkynes Preparation Reactions 110 Worked example 113 Problems Benzenes Structure Reactions Halogenation Nitration Sulfonation Alkylation: The Friedel-Crafts alkylation Acylation: The Friedel-Crafts acylation Reactivity of substituted benzenes Reactivity ofbenzene rings: Activating and deactivating substituents Orientation of reactions Nucleophilic aromatic substitution (the SNAr mechanism) The formation of benzyne Transformation of side chains Reduction of the benzene ring The synthesis of substituted benzenes Electrophilic substitution of naphthalene Electrophilic substitution of pyridine Electrophilic substitution of pyrrole, furan and thiophene 136 Worked example 136 Problems Carbonyl compounds: aldehydes and ketones Structure Reactivity Nucleophilic addition reactions Relative reactivity of aldehydes and ketones Types of nucleophiles Nucleophilic addition of hydride: Nucleophilic addition of carbon nucleophiles: reduction 143 formation of C-C bonds Nucleophilic addition of oxygen nucleophiles: formation of hydrates and acetals Nucleophilic addition of sulfur nucleophiles: formation of thioacetals Nucleophilic addition of amine nucleophiles: formation of imines and enamines 152

5 viii Contents 8.4 a-substitution reactions Keto-enol tautomerism Reactivity of enols Acidity of a-hydrogen atoms: enolate ion formation Reactivity of enolates Carbonyl-carbonyl condensation reactions Condensations of aldehydes and ketones: the aldol condensation reaction Crossed or mixed aldol condensations Intramolecular aldol reactions The Michael reaction 163 Worked example 164 Problems Carbonyl compounds: carboxylic acids and derivatives Structure Reactivity Nucleophilic acyl substitution reactions Relative reactivity of carboxylic acid derivatives Reactivity of carboxylic acid derivatives versus carboxylic acids Reactivity of carboxylic acid derivatives versus aldehydes/ketones Nucleophilic substitution reactions of carboxylic acids Preparation of acid chlorides Preparation of esters (esterification) Nucleophilic substitution reactions of acid chlorides Nucleophilic substitution reactions of acid anhydrides Nucleophilic substitution reactions of esters Nucleophilic substitution and reduction reactions of amides Nucleophilic addition reactions of nitriles a-substitution reactions of carboxylic acids Carbonyl-carbonyl condensation reactions The Claisen condensation reaction Crossed or mixed Claisen condensations Intramolecular Claisen condensations: the Dieckmann reaction A summary of carbonyl reactivity 181 Worked example 182 Problems Spectroscopy Mass spectrometry (MS) Introduction Isotope patterns Determination of molecular formula 188

6 Contents ix Fragmentation patterns Chemical ionisation (CI) The electromagnetic spectrum Ultraviolet (UV) spectroscopy Infrared (IR) spectroscopy Nuclear magnetic resonance (NMR) spectroscopy !H NMR spectroscopy C NMR spectroscopy 202 Worked example 203 Problems Natural products and synthetic polymers Carbohydrates Lipids Waxes, fats and oils Steroids Amino acids, peptides and proteins Nucleic acids Synthetic polymers Addition polymers Condensation polymers 217 Worked example 218 Problems 219 Appendix 1: Bond dissociation enthalpies 221 Appendix 2: Bond lengths 223 Appendix 3: Approximate pka values (relative to water) 225 Appendix 4: Useful abbreviations 227 Appendix 5: Infrared absorptions 229 Appendix 6: Approximate NMR chemical shifts 231 Appendix 7: Reaction summaries 235 Appendix 8: Glossary 241 Further reading 249 Outline answers 251 Index 277