《有机化学》课程教学资源（参考书籍）Instant Notes in Organic Chemistry, Second Edition, G. L. Patrick

Instant Notes Organic Chemistry SECOND EDITION G.Patrick Also available as a printed book see title verso for ISBN details

This edition published in the Taylor&Francis e-Library.2005. "To purchase your own copy of this or any of Taylor&Francis or Routledge's collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk." Garland science/Bios Scientific publishers.2004 First published 2000 Second edition published 2004 All rights rese ved.No part of this book may be reproduced or transmitted,in any form or by any means without permission. A CIP catalogue record for this book is available from the British Library. ISBN 0-203-42761-0 Master e-book ISBN 030344168.0 Garland Science/BIOS Scientific Publisher 4 Park Sq are,Milton Park,Abingdon,Oxon OX14 4RN,UK and 2 West 3th Street.New York,NY 10019.USA World Wide Web home page:www.bios.co.uk Garland Science/BIOS Scientific Publishers is a member of the Taylor&Francis Group Distributed in the USA by Taylor 10650 Toebben Drive Distributed in Canada by Taylor Francis 74 Rolark Drive Distributed in the rest of the world by Thomson Publishing Services North Way Andover,Hampshire SP10 5BE,UK Tel:+44(0)1264 332424;E-mail:salesorder.tandf@thomsonpublishingservices.co.uk Library of Congress Cataloging-in-Publication Data Patrick,Graham L Organic chemistry /G.L Patrick-2nd ed. ISBN-25lk Paper) sand inde 022x72 Production Editor:Andrew Watts

© Garland Science/BIOS Scientific Publishers, 2004 First published 2000 Second edition published 2004 All rights reserved. No part of this book may be reproduced or transmitted, in any form or by any means, without permission. A CIP catalogue record for this book is available from the British Library. ISBN 1 85996 264 5 Garland Science/BIOS Scientific Publishers 4 Park Square, Milton Park, Abingdon, Oxon OX14 4RN, UK and 29 West 35th Street, New York, NY 10001–2299, USA World Wide Web home page: www.bios.co.uk Garland Science/BIOS Scientific Publishers is a member of the Taylor & Francis Group Distributed in the USA by Fulfilment Center Taylor & Francis 10650 Toebben Drive Independence, KY 41051, USA Toll Free Tel.: +1 800 634 7064; E-mail: taylorandfrancis@thomsonlearning.com Distributed in Canada by Taylor & Francis 74 Rolark Drive Scarborough, Ontario M1R 4G2, Canada Toll Free Tel.: +1 877 226 2237; E-mail: tal_fran@istar.ca Distributed in the rest of the world by Thomson Publishing Services Cheriton House North Way Andover, Hampshire SP10 5BE, UK Tel.: +44 (0)1264 332424; E-mail: salesorder.tandf@thomsonpublishingservices.co.uk Library of Congress Cataloging-in-Publication Data Patrick, Graham L. Organic chemistry / G.L. Patrick.—2nd ed. p. cm. — (Instant notes series) Includes bibliographical references and index. ISBN 1-85996-264-5 (alk. paper) 1. Chemistry, Organic—Outlines, syllabi, etc. I. Title: Instant notes organic chemistry. II. Title. III. Series. QD256.5.P37 2003 547—dc22 Production Editor: Andrew Watts This edition published in the Taylor & Francis e-Library, 2005. "To purchase your own copy of this or any of Taylor & Francis or Routledge's collection of thousands of eBooks please go to www.eBookstore. ISBN 0-203-42761-0 Master e-book ISBN ISBN 0-203-44168-0 (Adobe eReader Format) (Print Edition) tandf.co.uk

CONTENTS Preface Section A-Structure and bonding Atomic structure of carbon 1 AI gand hybridization sp 3 sp 公 Section B-Alkanes and cycloalkanes 19 B1 Definition B2 Drawing structures B3 Nomenclature Section C-Functional groups C1 Recognition of functional groups Aliphatic and aromatic functional groups Intermolecular bonding 779035 compounds with functional groups Section D-Stereochemistry D1 Constitutional isomers D2 Configurational isomers-alkenes and cycloalkanes D3 Configurational isomers-optical isomers D4 Conformational isomers Section E-Nucleophiles and electrophiles 63 Definition Charged species Neutral inorganic species Organic structures Section F-Reactions and mechanisms Mechanisms Section G-Acid-base reactions G1 Bronsted-Lowry acids and bases 79 G2 Acid strength G3 Base strength Lewis acids and bases Enolates

Preface ix Section A – Structure and bonding 1 A1 Atomic structure of carbon 1 A2 Covalent bonding and hybridization 3 A3 sp3 Hybridization 5 A4 sp2 Hybridization 8 A5 sp Hybridization 14 A6 Bonds and hybridized centers 17 Section B – Alkanes and cycloalkanes 19 B1 Definition 19 B2 Drawing structures 20 B3 Nomenclature 22 Section C – Functional groups 27 C1 Recognition of functional groups 27 C2 Aliphatic and aromatic functional groups 29 C3 Intermolecular bonding 30 C4 Properties and reactions 33 C5 Nomenclature of compounds with functional groups 35 C6 Primary, secondary, tertiary and quaternary nomenclature 43 Section D – Stereochemistry 45 D1 Constitutional isomers 45 D2 Configurational isomers – alkenes and cycloalkanes 46 D3 Configurational isomers – optical isomers 49 D4 Conformational isomers 56 Section E – Nucleophiles and electrophiles 63 E1 Definition 63 E2 Charged species 64 E3 Neutral inorganic species 66 E4 Organic structures 69 Section F – Reactions and mechanisms 73 F1 Reactions 73 F2 Mechanisms 75 Section G – Acid–base reactions 79 G1 Brønsted–Lowry acids and bases 79 G2 Acid strength 82 G3 Base strength 88 G4 Lewis acids and bases 94 G5 Enolates 95 CONTENTS

Contents Section H-Alkenes and alky repa 990 sym metric al alke op insymmet nes 112 cation stabilization 115 H6 Reduction and oxidation of alkenes 117 H7 Hydroboration of alkenes 121 H8 Electrophilic additions to alkynes 124 H9 Reduction of alkynes 127 H10 Alkylation of terminal alkynes 129 H11 Conjugated dienes Section I- Aromatic chemistry 185 of honz ne 139 is of m 147 Electrophilic substitutions of mono-substituted aromatic rings 150 Synthesis of di-and tri-substituted benzenes 160 7 Oxidation and reduction 164 Section] Aldehydes and ketones 167 Preparat on ilic additi ilic ad arged nucleophiles Nucleon ogen nucleophiles and sulf 187 Reactions of enolate ions g-Halogenation 198 Reduction and oxidation I11 aB-Unsaturated aldehvdes and ketones 202 Section K-Carboxylic acids and carboxylic acid derivatives 205 KI and 205 K2 209 K3 Reactivity 213 K4 Preparations of carboxylic acids 217 K5 Preparations of carboxylic acid derivatives 219 K6 Reactions 224 K7 Enolate reactions 234 Section L-Alkyl halides 1 and physical properties of alkyl halides ohilic suhs ution 242 3 Factors affecting S2 versus S1 reactions 247 Elimination 252 Elimination versus substitution 256 L6 Reactions of alkyl halides 258 Organometallic reactions 261

Section H – Alkenes and alkynes 99 H1 Preparation of alkenes and alkynes 99 H2 Properties of alkenes and alkynes 101 H3 Electrophilic addition to symmetrical alkenes 105 H4 Electrophilic addition to unsymmetrical alkenes 112 H5 Carbocation stabilization 115 H6 Reduction and oxidation of alkenes 117 H7 Hydroboration of alkenes 121 H8 Electrophilic additions to alkynes 124 H9 Reduction of alkynes 127 H10 Alkylation of terminal alkynes 129 H11 Conjugated dienes 131 Section I – Aromatic chemistry 135 I1 Aromaticity 135 I2 Preparation and properties 137 I3 Electrophilic substitutions of benzene 139 I4 Synthesis of mono-substituted benzenes 147 I5 Electrophilic substitutions of mono-substituted aromatic rings 150 I6 Synthesis of di- and tri-substituted benzenes 160 I7 Oxidation and reduction 164 Section J – Aldehydes and ketones 167 J1 Preparation 167 J2 Properties 169 J3 Nucleophilic addition 173 J4 Nucleophilic addition – charged nucleophiles 175 J5 Electronic and steric effects 181 J6 Nucleophilic addition – nitrogen nucleophiles 184 J7 Nucleophilic addition – oxygen and sulfur nucleophiles 187 J8 Reactions of enolate ions 191 J9 α-Halogenation 198 J10 Reduction and oxidation 200 J11 α,β-Unsaturated aldehydes and ketones 202 Section K – Carboxylic acids and carboxylic acid derivatives 205 K1 Structure and properties 205 K2 Nucleophilic substitution 209 K3 Reactivity 213 K4 Preparations of carboxylic acids 217 K5 Preparations of carboxylic acid derivatives 219 K6 Reactions 224 K7 Enolate reactions 234 Section L – Alkyl halides 239 L1 Preparation and physical properties of alkyl halides 239 L2 Nucleophilic substitution 242 L3 Factors affecting SN2 versus SN1 reactions 247 L4 Elimination 252 L5 Elimination versus substitution 256 L6 Reactions of alkyl halides 258 L7 Organometallic reactions 261 vi Contents

Contents Section M-Alcohols.phenols.and thiols M2 paration of phenols M3 Pro perties of alcohols and phenols 266 M4 Reactions of alcohols 270 M5 Reactions of phenols M6 Chemistry of thiols Section N-Ethers,epoxide and thioeth repar s,epoxide and thioethers 283 Section O-Amines and nitriles 29 OT Preparation of amines 02 Properties of amines Reactions of amines 03 04 Chemistry of nitriles 311 Section P-Organic spectroscopy and analysis ult a violet spectroscopy 317 P3 Infra-red s 322 Proton nuclear etic resonance spectroscopy 324 P5 C nuclear magnetic resonance spectroscopy P6 342 Further reading Index 34

Section M – Alcohols, phenols, and thiols 263 M1 Preparation of alcohols 263 M2 Preparation of phenols 264 M3 Properties of alcohols and phenols 266 M4 Reactions of alcohols 270 M5 Reactions of phenols 277 M6 Chemistry of thiols 281 Section N – Ethers, epoxides, and thioethers 283 N1 Preparation of ethers, epoxides, and thioethers 283 N2 Properties of ethers, epoxides, and thioethers 286 N3 Reactions of ethers, epoxides, and thioethers 289 Section O – Amines and nitriles 295 O1 Preparation of amines 295 O2 Properties of amines 299 O3 Reactions of amines 305 O4 Chemistry of nitriles 311 Section P – Organic spectroscopy and analysis 315 P1 Spectroscopy 315 P2 Visible and ultra violet spectroscopy 317 P3 Infra-red spectroscopy 322 P4 Proton nuclear magnetic resonance spectroscopy 324 P5 13C nuclear magnetic resonance spectroscopy 339 P6 Mass spectroscopy 342 Further reading 347 Index 349 Contents vii

PREFACE a comprehensiveset of basic chemistry,wh h will be suitable for und studen chemistry-re course: or cou chemistry as an ancillary subject.The book concentrates on core topics which are most likely to be common to those organic chemistry courses which follow on from a foun- dation or introductory general chemistry course Organic chemistry is a subject which can lead some students to the heights of ecstasy,yet drive others up the wall.Some students 'switch on'to it imme diately,while others can make neither head nor tail of it,no matter how hard they try.Certainly,one of the major problems in studying the subject is the vast amount of material which often has to be covered.Many students blanche at the prospect of having to learn a seemingly endless number of reactions and when it comes to drawing mechanisms and curly arrows,they see only a confusing maze of squiggly lines going everywhere yet nowhere.The concepts of organic reaction mechanisms are often the most difficult to master. These difficulties are often compounded by the fact that current textbooks in organic chemistry are typically over 1200 pages long and can be quite expen- sive to buy. This book attempts to condense the essentials of organic chemistry into a manageable text of 310 pages which is student friendly and which does not cost an arm and a leg.It does this by concentrating purely on the basics of the subject without going into exhaustive detail or repetitive examples. Furthermore,key notes at the start of each topic summarize the essential facts covered and help focus the mind on the essentials. Organic chemistry is a peculiar subject in that it becomes easier as you go along!This might seem an outrageous statement to make,esp pecially to a first- year studenthmotermsth the ru of nmenature. trying to memorize a couple of dozen reactions and making sense of mecha- nisms at the same time.However,these topics are the basics of the subject and once they have been grasped,the overall p icture hecomes clear Understanding the me hanism of how a reaction takes place is particularly crucial in this.It brings a logic to the reactions of the different functional gr This in turn transfor ms a list of ar which makes remembering the aetyuneatedacsiaioaStebl5ep reactions a 'piece of cake'(well,nearly). Once this ha state of affairs has been reached.the relevance of organic etics and biochemistry r suddenly lea off the p organic chemistry leads to a better unders tandi of life chemistry and how the ody works at the molecular level.It also helps in the nding of the molecular mecha volved in dise n to ar ug an be designed o those dis edicinal che A that's not all An unde ic che the ndus- trial che or che ith P chemical nd the cientist trying proces and syn the d fu eco frie

This textbook aims to provide a comprehensive set of basic notes in organic chemistry, which will be suitable for undergraduate students taking chemistry, chemistry-related courses, or courses which involve organic chemistry as an ancillary subject. The book concentrates on core topics which are most likely to be common to those organic chemistry courses which follow on from a foun￾dation or introductory general chemistry course. Organic chemistry is a subject which can lead some students to the heights of ecstasy, yet drive others up the wall. Some students ‘switch on’ to it imme￾diately, while others can make neither head nor tail of it, no matter how hard they try. Certainly, one of the major problems in studying the subject is the vast amount of material which often has to be covered. Many students blanche at the prospect of having to learn a seemingly endless number of reactions, and when it comes to drawing mechanisms and curly arrows, they see only a confusing maze of squiggly lines going everywhere yet nowhere. The concepts of organic reaction mechanisms are often the most difficult to master. These difficulties are often compounded by the fact that current textbooks in organic chemistry are typically over 1200 pages long and can be quite expen￾sive to buy. This book attempts to condense the essentials of organic chemistry into a manageable text of 310 pages which is student friendly and which does not cost an arm and a leg. It does this by concentrating purely on the basics of the subject without going into exhaustive detail or repetitive examples. Furthermore, key notes at the start of each topic summarize the essential facts covered and help focus the mind on the essentials. Organic chemistry is a peculiar subject in that it becomes easier as you go along! This might seem an outrageous statement to make, especially to a first￾year student who is struggling to come to terms with the rules of nomenclature, trying to memorize a couple of dozen reactions and making sense of mecha￾nisms at the same time. However, these topics are the basics of the subject and once they have been grasped, the overall picture becomes clear. Understanding the mechanism of how a reaction takes place is particularly crucial in this. It brings a logic to the reactions of the different functional groups. This in turn transforms a list of apparently unrelated facts into a sensible theme which makes remembering the reactions a ‘piece of cake’ (well, nearly). Once this happy state of affairs has been reached, the relevance of organic chemistry to other subjects such as genetics and biochemistry suddenly leaps off the page. Understanding organic chemistry leads to a better understanding of life chemistry and how the body works at the molecular level. It also helps in the understanding of the molecular mechanisms involved in disease and bodily malfunction, leading in turn to an understanding of how drugs can be designed to cure these disease states – the science of medicinal chemistry. And that’s not all. An understanding of organic chemistry will help the indus￾trial chemist or chemical engineer faced with unexpected side-reactions in a chemical process, and the agro-scientist trying to understand the molecular processes taking place within plants and crops; and it will assist in the design and synthesis of new herbicides and fungicides which will be eco-friendly. It PREFACE

Preface will aid the forensic scientist wishing to analyze a nondescript white powder is it heroin or flour? The list of scientific subject areas involving or its doval d go ing new mole on and on.Ore chemistr ting su bject since it leads to an esse tial nderstandingo The order in which the arly topics of this book are sented is i The first tw lust why doe structure and bor The third s ctional grou cr are to be capable ing e apparent actio whic 0r8a4 pounds on D nd E in by secti y, e b asic th eory on ms nucleophil ean e r ns can be u order.These look at the react the common functional groups which are important ir that studen ill find this textbook useful in their studies and ey have graspe what organic stry is all about they will read more widely and enter a truly exciting world of molecular science

will aid the forensic scientist wishing to analyze a nondescript white powder – is it heroin or flour? The list of scientific subject areas involving organic chemistry is endless – designing spacesuits, developing new photographic dyes, inventing new mole￾cular technology in microelectronics – one could go on and on. Organic chemistry is an exciting subject since it leads to an essential understanding of molecules and their properties. The order in which the early topics of this book are presented is important. The first two sections cover structure and bonding, which are crucial to later sections. Just why does carbon form four bonds? What is hybridization? The third section on functional groups is equally crucial if students are to be capable of categorizing the apparent maze of reactions which organic compounds can undergo. It is followed by section D on stereochemistry, then sections E and F, in which the basic theory of reactions and mechanisms is covered. What are nucleophiles and electrophiles? What does a mechanism represent? What does a curly arrow mean? The remaining sections can be used in any order. These look at the reactions and mechanisms of the common functional groups which are important in chemistry and biochemistry. It is hoped that students will find this textbook useful in their studies and that once they have grasped what organic chemistry is all about they will read more widely and enter a truly exciting world of molecular science. x Preface

Section A-Structure and bonding A1 ATOMIC STRUCTURE OF CARBON Key Notes Atomic orbitals ,thesorbital shel orbitals are 2公金aee马 hape and can be assigned 2p2p,or 2p.depend- ing on the axis along which they are aligned. Energy levels The 1s orbital has a lower energy than the 2s orbital which has a lower energy than the 2p orbitals.The 2p orbitals have equal energy (i.e.they are degenerate). Electronic Carbon is in the second row of the periodic table and has six electrons which configuration will fill up lower energy atomic orbitals before entering higher energy orbitals(aufbau principle).Each orbital is allowed a maximum of two elec trons of opposite spin (Pauli exclusion principle).When orbitals of equal energy are available,electrons will occupy separate orbitals before pairing 2器2m.n，感he Related topic Covalent bonding and hybridization(A2) Atomic orbitals Carbon has six electrons and is in row 2 of the periodic table.This means that there are two shells of atomic orbitals available for these electrons The first shell closest to the nucleus has a single s orbital-the 1s orbital.The second shell has a single s orbital (the 2s orbital)and three p orbitals (3 x 2p).Therefore,there are a total of five atomic orbitals into which these six electrons can fit thes orbitals an in shape with the 2s orbital being much large are dumbbell-shaped and are aligned along the x,yand z axes.Therefore,they are ssigned the and patomic orbitals (Fig.1). 4是@ Fig.1.Atomic orbitals

Section A – Structure and bonding A1 ATOMIC STRUCTURE OF CARBON Atomic orbitals Carbon has six electrons and is in row 2 of the periodic table. This means that there are two shells of atomic orbitals available for these electrons. The first shell closest to the nucleus has a single s orbital – the 1s orbital. The second shell has a single s orbital (the 2s orbital) and three p orbitals (3
2p). Therefore, there are a total of five atomic orbitals into which these six electrons can fit. The s orbitals are spherical in shape with the 2s orbital being much larger then the 1s orbital. The p orbitals are dumbbell-shaped and are aligned along the x, y and z axes. Therefore, they are assigned the 2px, 2py and 2pz atomic orbitals (Fig. 1). Key Notes The atomic orbitals available for the six electrons of carbon are the s orbital in the first shell, the s orbital in the second shell and the three p orbitals in the second shell. The 1s and 2s orbitals are spherical in shape. The 2p orbitals are dumbbell in shape and can be assigned 2px, 2py or 2pz depend￾ing on the axis along which they are aligned. The 1s orbital has a lower energy than the 2s orbital which has a lower energy than the 2p orbitals. The 2p orbitals have equal energy (i.e. they are degenerate). Carbon is in the second row of the periodic table and has six electrons which will fill up lower energy atomic orbitals before entering higher energy orbitals (aufbau principle). Each orbital is allowed a maximum of two elec￾trons of opposite spin (Pauli exclusion principle). When orbitals of equal energy are available, electrons will occupy separate orbitals before pairing up (Hund’s rule). Thus, the electronic configuration of a carbon atom is 1s 2 2s 2 2px 1 2py 1 . Related topic Covalent bonding and hybridization (A2) Electronic configuration Atomic orbitals Energy levels 1s 2s 2px 2py 2pz y z x y z x y z x y z x y z x Fig. 1. Atomic orbitals

2 Section A-Structure and bonding Energy levels The atomic orbitals described above are not of equal energy (Fig.2).The 1s orbital has the lowest energy.The 2s orbital is next in energy and the 2p orbitals have the highest energies.The three 2p orbitals have the same energy,meaning that they are degenerate. Energy 20 Fig.2.Energy levels of atomic orbitals. Electronic Carbon is in the second row of the periodic table and has six electrons which will configuration fill up the lower energy atomic orbitals first.This is known as the aufbau princi- ple.The 1s orbital is filled up before the 2s orbital,which is filled up before the 2p orbitals.The Pauli exclusion principle states that each orbital is allowed a maxi- mum of two electrons and that these electrons must have fore the first four electrons fill un the 1s and 2s orbitals The electone rons in each and this i oppdown.There are tw nted in Fig.3 by drawing the a ns left to fit into the in: 2 e are two half-filled orbitals and one orbitals c 1 up once degenerate o wn Energy x↓2 ，—2p 1s Fig.3.Electronic configuration for carbon. The electronic configuration for carbon is 1s2s22p,2p,.The numbers in superscript refer to the numbers of electrons in each orbital.The letters refer to the types of atomic orbital involved and the numbers in front refer to which shell the orbital belongs

2 Section A – Structure and bonding Energy levels The atomic orbitals described above are not of equal energy (Fig. 2). The 1s orbital has the lowest energy. The 2s orbital is next in energy and the 2p orbitals have the highest energies. The three 2p orbitals have the same energy, meaning that they are degenerate. Electronic Carbon is in the second row of the periodic table and has six electrons which will configuration fill up the lower energy atomic orbitals first. This is known as the aufbau princi￾ple. The 1s orbital is filled up before the 2s orbital, which is filled up before the 2p orbitals. The Pauli exclusion principle states that each orbital is allowed a maxi￾mum of two electrons and that these electrons must have opposite spins. There￾fore, the first four electrons fill up the 1s and 2s orbitals. The electrons in each orbital have opposite spins and this is represented in Fig. 3 by drawing the arrows pointing up or down. There are two electrons left to fit into the remaining 2p orbitals. These go into separate orbitals such that there are two half-filled orbitals and one empty orbital. Whenever there are orbitals of equal energy, electrons will only start to pair up once all the degenerate orbitals are half filled. This is known as Hund’s rule. The electronic configuration for carbon is 1s 2 2s 2 2px 1 2py 1 . The numbers in superscript refer to the numbers of electrons in each orbital. The letters refer to the types of atomic orbital involved and the numbers in front refer to which shell the orbital belongs. Energy 1s 2s 2px 2py 2pz Fig. 2. Energy levels of atomic orbitals. Energy 1s 2s 2px 2py 2pz Fig. 3. Electronic configuration for carbon

Section A-Structure and bonding A2 COVALENT BONDING AND HYBRIDIZATION Key Notes Covalent When two hydrogen atoms approach each other,their Is atomic orbitals bonding interact to form a bonding and an antibonding molecular orbital(MO).A stable covalent bond is formed when the bonding MO is filled with a pair of electrons and the antibonding MO is empty. Sigma bonds Sigma (o)bonds are strong bonds with a circular cross-section formed by the head-on overlap of two atomic orbitals. Hybridization The electronic configuration of atomic carbon implies that carbon should form two bonds.However,it is known that carbon forms four bonds.When mix'the 2s and 2p orbitals of s hvbridiza ion.Ther Posibteypesofnbridaiom-ppadphybrid2atioh are three Related topics A) Covalent bonding Acovalent bond binds two atoms together in a molecula nic orbitals caularorbi one spe A atoms. om has a hal n two hydrogen the atoms the number of resulting MOs must equal the number of original atomic orbitals,Fig.1). Energy &ampgdngmoecularobtadl ⊙+ meamc = H:H ondingmolecular orbital Fig.1.Molecular orbitals for hydrogen (Ha). The MOsare of different enere One is more stable than the orinal atomic MO he c er is le an antibonding MO.The bondingM is shaped likea ruby ball and results from

Section A – Structure and bonding A2 COVALENT BONDING AND HYBRIDIZATION Covalent bonding A covalent bond binds two atoms together in a molecular structure and is formed when atomic orbitals overlap to produce a molecular orbital – so called because the orbital belongs to the molecule as a whole rather than to one specific atom. A simple example is the formation of a hydrogen molecule (H2) from two hydrogen atoms. Each hydrogen atom has a half-filled 1s atomic orbital and when the atoms approach each other, the atomic orbitals interact to produce two MOs (the number of resulting MOs must equal the number of original atomic orbitals, Fig. 1). The MOs are of different energies. One is more stable than the original atomic orbitals and is called the bonding MO. The other is less stable and is called the antibonding MO. The bonding MO is shaped like a rugby ball and results from Key Notes When two hydrogen atoms approach each other, their 1s atomic orbitals interact to form a bonding and an antibonding molecular orbital (MO). A stable covalent bond is formed when the bonding MO is filled with a pair of electrons and the antibonding MO is empty. Sigma (σ) bonds are strong bonds with a circular cross-section formed by the head-on overlap of two atomic orbitals. The electronic configuration of atomic carbon implies that carbon should form two bonds. However, it is known that carbon forms four bonds. When carbon is part of an organic structure, it can ‘mix’ the 2s and 2p orbitals of the valence shell in a process known as hybridization. There are three possible types of hybridization – sp3 , sp2 and sp hybridization. Related topics Atomic structure of carbon (A1) sp3 Hybridization (A3) sp2 Hybridization (A4) sp Hybridization (A5) Covalent bonding Sigma bonds Hybridization + 1s atomic orbital 1s atomic orbital = Bonding molecular orbital (full) Antibonding molecular orbital Energy (empty) H H H H HH HH Fig. 1. Molecular orbitals for hydrogen (H2 )