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Substitution & elimination reactions.pdf

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Department
Chemistry
Course
CHEM 267
Professor
Steve Forsey
Semester
Winter

Description
SubstitutionandEliminationReactionsThisdocumenthasbeenwrittentoprovideyouwithanoverviewofsubstitutionandeliminationreactionsThisisacondensedoverviewandprovidesyouwiththeessentialknowledgeneededtoanswersubstitutionandeliminationreactionsquestionsinvolvingalkylhalidesFormoredetaileddescriptionsconsultyourtextbookAnycommentsorquestionspleaseemailDrStevenForseyspforseyuwaterlooca10IntroductiontoSubstitutionReactions211TheS2mechanism6N12TheS1mechanism7N2andS1reactions813TheeffectofthesubstrateontherateofSNN14SolventEffectsinS1andS2reactions10NN15Nucleophilicity1316SterichindranceofnucleophilesinSN2reactions17andEMechanisms1920EliminationReactionsofHaloalkanesE1221E1Mechanism1922E2Mechanism2130HowtodeterminingifanS2E2S1orE1reactionwilloccur22NNwithPrimaryandSecondaryAlkylHalides40RegiochemistryandStereochemistry2441E2ReactionswithUnsymmetricalAlkylHalides241E1reactionsandUnsymmetricalAlkylHalides2842SN50CarbocationRearrangementandS1E1reactions29N60StericallyHinderednucleophilebase3070Overview3180Vinylarylallylicandbenzyliccompounds37 Dr Steven Forsey 1SubstitutionandEliminationReactions10IntroductiontoSubstitutionReactionsSubstitutionreactionsreplaceoneatomwithanotherForexampleinthesubstitutionreactionofmethylbromidewithsodiumhydroxidereplacestheBratombyahydroxideatomtogivemethanolBrNaOHCHOHClNaCH33ThistypeofchemicalreactioniscalledanucleophilicsubstitutionreactionbecausethehydroxideionisanucleophileareactantthatseekscentersofpositivechargeinamoleculeThenucleophileiselectronrichanditistheattackingspeciesinthereactionthatdonatesapairofelectronstoanothercompoundWelearnedthatthecarbontochlorinebondispolarsotheCatomisaregionofslightpositivechargeandissaidtobeelectrophilicelectronattractingTheelectrophilealsoknownasthesubstrateisthespeciesthatisbeingattackedbythenucleophileandacceptsapairofelectronsElectrophilesarecompoundsoratomsthatareelectrondeficientInthissubstitutionreactionthehydroxideionattacksattheslightlypositiveCatomdisplacingpushingoutthechlorideionwhichiscalledtheleavinggroupWhenconsideringthefeasibilityofasubstitutionreactionwemustconsidernotonlythenucleophilicityofthenucleophilebutalsotheleavingabilityoftheleavinggroupAnimportantruleinorganicchemistryisasfollowsTheweakerthebasethebettertheleavinggroupNucleophilesoftendenotedbytheabbreviationNumaybenegativelychargedorneutralbuteverynucleophilecontainsatleastonepairofunsharedelectronsNucleophilesareinfactLewisbasesThenucleophilicsubstitutionofahaloalkanesisdescribedbyeitheroftwogeneralequationsAsanexampleconsiderthefollowingreactionThenucleophileisthemethylacetylideaniontheelectrophileisbromoethaneandtheBranionistheleavinggroupArrowsareusedtoshowbondformationandbreakingTheredarrowshowsthemovementofthelonepairofelectronsfromthenegativelychargedcarbontothedeltapositivecarbonbondedtothehalogenlabeledcarbonThisarrowsshowthatthelonepairofelectronsonthecarbonDr Steven Forsey 2areusedtoformanewbondwithcarbonNowsincethecarboncannothave5bondsabondmustbebrokenThebluearrowshowstheelectronsfromthecarbonbrominesigmabondmovingontothebromineatomtoformthebromideionIfyouusearrowstoshowbondformationandbreakingyouknowthestructureoftheproductsThereactionwillproceedtotherightbecausethebromideionisamuchweakerbasethanthemethylacetylideionNomenclatureThecarbontowhichthefunctiongroupisbondedtoiscalledthealphacarboninthiscaseahalogenThecarbonbondedtothealphacarboniscalledthebetacarbonthenextiscalledgamaandsoforthThisnomenclatureisimportantwhendiscussingreactionsInthisreactionthenucleophileattacksthe carbontodisplacetheleavinggroupItdoesnotattackthecarbonfunctionalgroupBrOOExamplesCHCHHCCOHCHCHHCCCHCH32232223functionalfunctionalgroupgroupSimilarlyinthereactionbelowbetweensodiumethoxideandiodomethanethenucleophileistheanionaweakbaseethoxideaniontheelectrophileisiodomethaneandtheleavinggroupistheIFinallyinthefollowingreactionthenucleophileistheammoniamoleculetheelectrophileisbromoethaneandtheleavinggroupisthebromideionBrInstudyingthesereactionsyoumayhavenoticedthesimilarityofthereactionTotheBronstedLowryacidbasereactionHBHXBXIntheBronstedLowryacidbasereactionthestrongerbaseandthestrongeracidreacttoformtheweakeracidbasepairSimilarlyinanucleophilicsubstitutionreactionboththenucleophileandtheleavinggrouparebasesInalloftheabovereactionstheweakeracidbasepairsareformedandtheequilibriumsareshiftedtotherightThisconceptcanbeusedtodeterminethelikelihoodofareactionoccurringThusforexamplewewouldnotexpectthefollowingreactionstooccurCHHCOHHHOH33Dr Steven Forsey 3
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