Lecture 16 Outline of Last Lecture I Solvent role in nucleophilicity a Polar aprotic solvent b Polar protic solvent I Good vs poor nucleophiles II Other rules of nucleophilicity CHEM 333 1st Edition III Alkyl halide effect IV Allylic halides allylic carbocations V Carbocation stability VI Summary of SN1 and SN2 VII Leaving groups Outline of Current Lecture I Final summary of SN1 and SN2 II Elimination Reactions a E1 Mechanism b E2 Mechanism III Regioselectivity of E1 and E2 a Zaitser s Rule b Alkene Stability IV Carbocation Rearrangement a Stereoselectivity V Alkenes Current Lecture I Final summary of SN1 and SN2 SN1 SN2 methyl Yes No 1 alkyl halide Yes No 2 alkyl halide Yes aprotic solvent good nucleophile Yes protic solvent poor nucleophile 3 alkyl halide No Yes chiral center substitution Inversion Racemic Mixture II Elimination Reactions reactions that synthesize alkenes Also known as B beta elimination Dehydrohalogenation takes place Alkyl halide reacts with a strong base Substitution and elimination reactions are competing Strong base examples a E1 Mechanism unimolecular rate d RX dt b E2 Mechanism bimolecular concerted reactions Leaving group leaves at the same timea s the base deprotonates and double bond forms rate d RX k RX base dt III Regioselectivity of E1 and E2 a Zaitser s Rule B ellimination s major product will be more subtituted alkene therefore more stable alkene i Example ii Exception will always give less substituted product Too sterically hindered b Alkene stability IV Carbocation Rearrangement SN1 and E1 possible rearrangement SN2 and E2 no rearrangement Sometimes H and C can move forms a more stable carbocation It moves to an adjacent carbon 1 2 shift Example Example a Stereoselectivity i E1 goes through carbocation no selectivity 1 Slectivity cis vs trans ii E2 must have leaving group be antiplanar for elimination to occur V Alkenes Carbons connected to double bond are sp2 hybridized Double bond is made up of sigma bond with sp2 sp2 orbtitals and pi bond with p p orbitals a Double bond as a substituent b Cycloalkenes C7 or less double bond found in ring bond is cis
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