《有机化学》课程教学课件（McMurry’s Organic Chemistry, 6th edition）Chapter 05 An Overview of Organic Reactions

An Overview of Organic Reactions Based on McMurry's Organic Chemistry,6th edition,Chapter 5

1 An Overview of Organic Reactions Based on McMurry’s Organic Chemistry, 6th edition, Chapter 5

Kinds of Organic Reactions Addition reactions -two molecules combine to form a single new product with no atoms"left over" H H H Br These two C=C+H-Br→H-C-C-H add to give reactants... this product. H H HH Ethylene Bromoethane (an alkene) (an alkyl halide) ■ Elimination reactions-one molecule splits into two H Br H H This one ...gives these reactant... H-C-C-H Base +H一Br two products. HH H H Bromoethane Ethylene (an alkyl halide) (an alkene) 2004 Thomson-Brooks/Cole

Kinds of Organic Reactions „ Addition reactions – two molecules combine to form a single new product with no atoms “left over” „ Elimination reactions – one molecule splits into two

Substitution-two reactants exchange parts to give two H H These two H一C-H+CI一C Light H一C一CI+H一C ..give these reactants... two products H H Methane Chloromethane (an alkane) (an alkyl halide) @2004 Thomson-Brooks/Cole Rearrangement reactions-a single reactant undergcHCH H HC H id atoms Acid catalyst H H CHa 1-Butene 2-Butene

„ Substitution – two reactants exchange parts to give two new products „ Rearrangement reactions – a single reactant undergoes a reorganization of bonds and atoms

How Organic Reactions Occur:Mechanisms A mechanism describes in detail exactly what takes place in each stage of a chemical transformation: -which bonds are broken(formed)and in what order relative rates of each step of the reaction A complete mechanism must account for all reactants used,all products formed and the amount of each Each step involves either the formation or breaking of a covalent bond -steps can occur in individually or in combination with other steps when several steps occur at the same time they are said to be concerted

How Organic Reactions Occur: Mechanisms „ A mechanism describes in detail exactly what takes place in each stage of a chemical transformation: - which bonds are broken (formed) and in what order - relative rates of each step of the reaction „ A complete mechanism must account for all reactants used, all products formed and the amount of each „ Each step involves either the formation or breaking of a covalent bond - steps can occur in individually or in combination with other steps - when several steps occur at the same time they are said to be concerted

Bond Cleavage A bond can break in an electronically symmetrical way so that one electron remains with each product fragment (homolytic cleavage)or it can break in an electronically unsymmetrica/manner such that both bonding electrons remain with one fragment,leaving the other fragment with a vacant orbital (heterolytic cleavage) A·+B Homolytic bond breaking(radical) (one electron stays with each fragment) A:B →A++B Heterolytic bond breaking(polar) (two electrons stay with one fragment) C2004 Thomson-Brooks/Cole

Bond Cleavage „ A bond can break in an electronically symmetrical way so that one electron remains with each product fragment (homolytic cleavage) or it can break in an electronically unsymmetrical manner such that both bonding electrons remain with one fragment, leaving the other fragment with a vacant orbital (heterolytic cleavage)

Indicating Steps in Mechanisms Curved arrows indicate breaking and forming of bonds ■Arrowheads with a“half'head(“fish- hook")indicate one electron movement (called 'radical processes') A radical is a neutral chemical species that contains an odd number of electrons and thus has a single, unpaired electron in one of its orbitals Arrowheads with a complete head indicate two electrons movement (called 'polar processes')

Indicating Steps in Mechanisms „ Curved arrows indicate breaking and forming of bonds „ Arrowheads with a “half” head (“fish￾hook”) indicate one electron movement (called ‘radical processes’) A radical is a neutral chemical species that contains an odd number of electrons and thus has a single, unpaired electron in one of its orbitals „ Arrowheads with a complete head indicate two electrons movement (called ‘polar processes’)

Bond Formation A bond can be formed in an electronically symmetrical homogenic way where one electron is donated to the new bond by each reactant 人人 A A·+·B A:B A bond can also be formed in an electronically unsymmetrical heterogenic way where both electrons are donated to the new bond by one reactant 只K→人 A++:B → A:B

Bond Formation „ A bond can be formed in an electronically symmetrical homogenic way where one electron is donated to the new bond by each reactant „ A bond can also be formed in an electronically unsymmetrical heterogenic way where both electrons are donated to the new bond by one reactant

Radical Reactions and How They Occur Radicals react to complete electron octet of valence shell A radical can abstract an atom and one bonding electron from another molecule,leaving behind a new radical Unpaired electron /Unpaired electron Rad.+A:B Rad:A B Reactant Substitution Product radical product radical A radical can add to an alkene to give a new radical (radical addition reaction) Unpaired electron Unpaired electron Rad Rad. Reactant Alkene Addition product radical radical

Radical Reactions and How They Occur „ Radicals react to complete electron octet of valence shell „ A radical can abstract an atom and one bonding electron from another molecule, leaving behind a new radical „ A radical can add to an alkene to give a new radical (radical addition reaction)

Steps in Radical Substitution ■ Three types of steps Initiation-homolytic formation of two reactive species with unpaired electrons … :Cl:Cl: Light 2:C1 …” Propagation-reaction with molecule to generate radical (a):Cl +H:CHg →H:Cl:+·CH3 (b)·CH3+:Cl:Cl: → :Cl:CH+:Cl. “G e2o04Thomson-BrocksCcle

Steps in Radical Substitution „ Three types of steps „ Initiation – homolytic formation of two reactive species with unpaired electrons „ Propagation – reaction with molecule to generate radical

Termination-combination of two radicals to form a stable product → :Cl:C1: :.cH → :Cl:CHg Possible termination steps H.C. H C:CH In a radical reaction,all bonds are broken and formed by reaction of species that have an odd number of electrons

„ Termination – combination of two radicals to form a stable product In a radical reaction, all bonds are broken and formed by reaction of species that have an odd number of electrons