Types Of Organic ReactionsTypes Of Organic Reactions

Types of Organic Reactions1. Addition Reactions: A B COOOHHO H 2OOHOHHOOOfumaratemalate2. Elimination Reactions: D E FOOOHHOOHHOO H2Ofumaratesuccinate3. Substitution Reactions: G-H I G-I HOOHOOOHOa-ketoglutarate SCoAOHOSCoA OHOsuccinyl CoA1

Types of Organic Reactions4. Rearrangements: J KOHOOHOCO2HOOHcitrateiCO2HOHOOHOHi iisocitrateHow do these reactions occur?Why do these reactions occur?We must explain how electrons are exchanged during the formation ofnew bonds and breaking of old bonds.bonds2

How Reactions OccurSymmetrical Bond Making/Breaking (homolytic process)vsUnsymmetrical Bond Making/Breaking (heterolytic process) - POLAR Reactions3

Indicating Electron Movement inReactioni Mechanismsh i Curved arrows indicate breaking and formingof bonds Arrowheads with a “half” head (“fish-hook”)indicate homolytic and homogenic steps(called ‘radical processes’) Arrowheads with a complete head indicateheterolytic and heterogenic steps (called‘polar processes’)4

Polar ReactionsOpposite charges attract Nucleophiles (electron rich sites, seek nucleus) - have lone pair ofelectrons or C C bondHOHO HClONH2 Electrophiles (electron poor sites, seek electrons) - have formalpositive charge or partial positive charge via bond dipoleNucleophiles are synonymous with Lewis Bases,Bases Electrophilesare synonymous with Lewis Acids, Polar reactions are Lewis AcidBase reactions.5

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Nucleophiles and ElectrophilesNote that compounds can be nucleophilic and electrophilic. Youmust look at specific atoms and what the compound is reacting withto determine how it will react:O OHO HOHO HJudging relative 'strengths' of Nuc and E Strong Nucless stable anionmore stable anionlone pair less EN atomlone pair more EN atomC CWeak NucStrong E less stable cationmore stable cationneutral atom incomplete octetlarge partial positive atomsmall partial positive atomWeak E 7

Drawing Reaction MechanismsUse of curved arrows to show electron movement in bond makingand bond breaking process. For polar reactions, electrons move as apair. Note that charges must also balance. TheTh arrow goes fromftheth nucleophilicl hili reactionti siteit tto ththe electrophilicl t hilireaction site The nucleophilic site can be neutral or negatively charged8

Drawing Reaction Mechanisms The electrophilic site can be neutral or positively charged Theh octet rulel must beb followedf lld (never have 2 or -2 charge on a single atom)9

Drawing Reaction Mechanisms O O Br10

Drawing Reaction Mechanisms OO OO Br Br11

An Example of a Polar Reaction:Additionddi i off HBr to Ethyleneh l HBr adds to the ppart of C-C double bond The bond is electron-rich, allowing it to function asa nucleophile H-Br is electron deficient at the H since Br is muchmore electronegative, making HBr an electrophile12

Mechanism of Addition of HBr toEthyleneh l13

Why Reactions OccurAll organicgreactions we will studyy will reach equilibriumqthat favors themore stable side. In most cases, these will be reaction products as writtenfrom left to right (note, however, sometimes you must predict which side isfavored as in acid-base reactions). Gº - RT ln Keq H - T SIf Keq 1, energy is released to the surroundings(exergonic reaction, negative value of Gº, reaction favored)If Keqeq 1,, energye e gy iss absorbedabso bed fromo thee susurroundingsou d gs(endergonic reaction, positive value of Gº, reaction not favored)Energy changes in a reaction are illustrated by Energy Diagrams14

Reaction Energy DiagramsOne Stepp Reactions:Exergonic (- G)(th(thermofavored)fd)Endergonic ( G)(th(thermonott favored)fd)15

Reaction Energy DiagramsMultiplep Stepp Reactions:16

Reaction Energy Diagrams The highestgenergygy pointpin a reaction step is calledthe transition state TheTh energy neededd d tot gofrom reactant to transitionstate is the activationenergy ( G‡) BrHHC CHHH17

Reaction Energy Diagrams If a reaction occurs in morethanh one step, iti must involveilspecies that are neither thereactant nor the final product TheseThare calledll d reactiontiintermediates or simply“intermediates” EachE h step hash itsi own freefenergy of activation The complete diagram for thereactionti showshtheth freef energychanges associated with anintermediate18

Estimating G from H H (energy needed to break bonds) - (energy released when bonds form)Bonds BreakingC C 611 kJ/molH B 366 kJ/H-BrkJ/mollBonds FormingC-C 355 kJ/molC B 285 kJ/C-BrkJ/mollC-H 420 kJ/mol H (977 kJ/kJ/mol)l) - (1060 kJ/mol)kJ/ l) - 83 kJ/kJ/moll (exothermic)( thi )19

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Are the following reactionsthermodynamicallyhdi ll favored?fd OO OO Br Br23

Types of Organic ReactionsTypes of Organic Reactions 1. Addition Reactions: A1. Addition Reactions: A B C HO OH O O H 2O HO OH O O OH fumarate malate 2. Elimination Reactions: D E F HO OH O O H 2 HO OH O O succinate fumarate 3. Substitution Reactions: G-H I G-I