Organic chemistry mechanisms - IB Chemistry revision notes：有機化學機制- IB化學復習筆記

Organicreactiontypesandmechanisms

Organiccompoundsarepredominantlycovalent.Thisisaconsequenceofthesimilarelectronegativityoftheatomsinvolvedinthestructures.Thebondenergiestendtobelargeandreactionsslow.Thismakesfollowingthereactioneasierintermsoftheactualprocessesinvolvedduringthecourseofareaction.Thepathreactionfollowsfromreactanttoproductsiscalledthemechanism.Paradoxically,althoughthemajorityoforganiccompoundsarecovalent,theirreactionsoftenproceedviaionicintermediates,whichgovernthespeedofthereaction(kinetics)andnatureoftheproducts.

Note:Tounderstandreactionmechanismsitisimportanttounderstandthedefinitionsof“reactant”and“reagent”.Areactantisanysubstanceonthelefthandsideofthereactionequation.Thereagentisthesubstance(ormixtureofsubstances)addedtobringaboutthechemicalchangeofthereactantthatweareconsidering,i.e.theorganiccomponentinthiscase.

Namingmechanisms

Mechanismsarenamedaccordingtotwofactors:

Theoverallchangeoccurringfromreactantstoproducts:

Name

Description

Addition

Asexpected,amoleculeorgroupofatomsisaddedtotheoriginalorganicstructure

Elimination

Amoleculeorgroupofatomsisremovedfromtheoriginalorganicstructure

Substitution

Anatomoratomsarereplacesbyothersintheoriginalstructure

Addition-elimination(condensation)

Acombinationofthefirsttwoabove

Rearrangement

Theatomsofthestructurearethesamebutnowinadifferentarrangement.

Theinitiatingfactorfromthepointofviewofthereagentmolecule/ion(notfromthepointofviewoftheorganicmoleculeitself).

Thereagentiseitherattractedtotheorganicmoleculebyelectrostaticattractionorthereisasimplecollisionofparticles(itshouldberememberedthattheparticlesinanymixturearecollidingmillionsoftimespersecondundernormalconditionsduetokineticactivity).

Forelectrostaticattractionthereagentmaycarryapartial(orwhole)positivechargeandbeattractedtoaregionofnegativechargeontheorganicstructure-inthiscasethereagentissaidtobeelectrophilic(negativeseeking).

Iftheoppositeistrueandthereagenthasalonepairactingasaregionofnegativechargeandisattractedtoaregionofpositivechargeontheorganicmolecule,thenthereagentissaidtobenucleophilic(positiveseeking).

Processesinvolvingfreeradicals(neutral,highlyreactivespecies)areclassifiedsimplyasjustthat-freeradicalprocesses.

Name

Description

Electrophilicattack

Thereagenthasaregionofpartialpositivechargethatisattractedtoapairofelectronsontheorganicmolecule

Nucleophilicattack

Thereagenthasapairofelectronsthatisattractedtoapartially(orwholly)positiveregionontheorganicmolecule

Freeradicalprocess

Thereagentisafreeradicalofbreaksdowntogivefreeradicalswhichthenattack

Acid/basecatalysed

Thereactionisinitiatedbyprotonation(additionofahydrogenion)ordeprotonation(removalofahydrogenion)byanacidorbaserespectively.

Table:Summaryofinitiatingprocesses

Examples:

Inthereactionofanalkenewithhydrogenbromide:

CH2=CH2+HBrCH3-CH2Br

Thereagenthasadipolewithapartialpositivechargeonthehydrogen.Thisisattractedtothe“pi”electronsofthealkenedoublebondinitiatingthereaction,whichissaidtobeelectrophilic(fromthepointofviewofthereagent).

Theoverallchangefromorganicreactanttoproductisanadditionoftwoatomsthereforeaddition

Mechanismname-Electrophilicaddition

Drawingrepresentationsofreactionmechanisms

Theprocessesoccurringarerepresentedbyshowingthemovementoftheelectronsascurlyarrows.Itmustbestressedthatonlyelectronpairsareshowntomove(orsingleelectronsinthecaseoffreeradicalmechanisms)andneverpositivecharges,evenifwearedealingwithattackbyapartially(orwholly)positivereagent(electrophile).

Electrophilicadditionofalkenes

CH2=CH2H-BrCH3-CH2+-BrCH3-CH2Br

Summary:

Reactionmechanismsmaybedividedintothefollowingcategories:

Electrophilicaddition

Electrophilicsubstitution

Elimination

Nucleophilicsubstitution

Nucleophilicaddition

Rearrangement-(notusuallyconsideredatIBlevel)

Freeradicalprocesses

Additionelimination(condensation)

Electrophilicaddition

ReactionbetweenalkenesandsomesuitablypolarmoleculesegHBr,HOBr(Br2solutioninwater),H2SO4etc.Itisalsopossibleforhalogenmolecules(Cl2,Br2,I2)toundergothisreaction;theyarethoughttodevelopaninduceddipoleonapproachingthealkene,whichpromotesthereaction.

Thepolarmoleculeistheelectrophile,whichisattractedtothepairofelectronsinthepiorbitalofthedoublebond.Thereactiontakesplaceintwostageswithanintermediatecarbocation(positivelychargedionalsocalledacarboniumion).

Thepairofpielectronsreachesouttothepartialpositiveregionofthereagentweakeningthebondinthereagentandleavingoneofthecarbonsofthedoublebondsystemwithoutitsshareofthepielectronsgivingitapositivecharge.

Electrophilicattack

Intermediatecarbocation

Finaladditionproduct

CH2=CH2H-Br

CH3-CH2+Br-

CH3-CH2Br

Markovnikovaddition

Ifthealkeneisasymmetricalthereisthepossibilityoftwodifferentproducts.IntheadditionofHBrthehydrogen(andofcoursethebromine)hastwodistinctcarbonstoattachto,eachgivingadifferentfinalproduct.Themajorproductwillbethatwhoseintermediatewasthemoststableenergetically(andthereforethemosteasilyformed).

Astheintermediatecarriesaformalpositivecharge,anyatomorgroupofatomsthattendstospreadthischargeoutwillalsostabilisetheintermediate.Ifwecompareanalkyl