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Reactions of Alkanes, Alkenes, and Alkynes

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University of Alberta
Hubert Taube

Chemistry 164 Reactions of Interest Introduction SubstrateReagentProductsSubstrate Must be positive for reaction to happen electrophile have positive dipolecenter electron sink low energy empty MOReagent Must be negative Electron Donor BaseALKANESSN2 The reaction rate depends on rkSNu SN2 speed is based from Nu and S There is no reaction intermediate and only 1 TS The nucleophile attacks the antibonding MO of the C attached to the LG This pushes out the LG and takes the bond with the C An inversion of stereochemistry occurs R to SS to R if reaction occurs at stereogenic center If substrate is enantiometically pure product will remain so no racemization remains optically inactiveFactors that influence SN2 reactions Substrate structure Methyl123 SN only works on sp3 C no vinylic aromatic etcNucleophilicity of Reagent Strong baseWeak baseNeg NuNeutral NuMethoxideHydroxideAcetateSoft NuHard NuWeak baseGood LGPolar aprotic solvent good for this reaction because is stabilizes TS faster reactionPolar Protic Solvents are capable of H bonding generally have OH NH bondsPolar Aprotic solvents are polar but do not H bond acteonitrile DMSO HMPA dimethylformamideSN1 The reaction is only dependent on the S It is a 2 step mechanism with 1 intermediate present The rate limiting step is in which the carbocation is created Product is racemic because a front or backside attack can happen to the carbocation Factors that influence SN1 reactionsStability of carbocation 321methyl due to hyperconjugation effectAllylic and Benzyllic substrates work well in SN1 because the carbocation is stabilized by resonance Elimination rxns at an sp3 C will only occur if a betaH is available These reactions will compete with SN reactions If more than betaH it can happen on either Elimination is favored over substitutionif B is a strong base or a poor Nuif a chemical is bulky or sterricaly hindered it cannot get to the backside attack of C and therefore will approach the betaH insteadRegioselectivityFrom which C is the H removed Ans Produce the alkene that is most substituted Tetrasubstituted will be most stable and therefore preferred
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