[理学]加州大学伯克利分校有机化学第七章.ppt

? University of California ? Univesity of California Chapter 7: SN1, E1, E2, notjust SN2 Mechanism Incomplete Racemization The Strong Effect of Polar Solvents on the SN1 Reaction Hyperconjugation X-Ray Structure of the 1,1-Dimethylethyl Cation(Laube, 1993) Summary Mechanism of the E1 Reaction Bimolecular Elimination E2 Mechanism of the E2 Reaction E2 in Cyclic Systems Hindered Base Ensures E2 Factors that Affect the Competition between SN and E An alkene 1. Mechanism: SN2 SN1 E2 E1? 2. At lower temperatures one of the following ratios will increase: SN2 / SN1 SN1 / E1 E2 / E1 SN2 / E2 Problem: No (or exceedingly slow) reaction with poor nucleophiles Reactivity of Prim Haloalkanes R-X with Nucleophiles (Bases) CH3CH2CH2Br Br - CN - CH3CH2CH2CN Acetone + + CH3CH2CH2Br Br - CH3O - CH3CH2CH2OCH3 + + CH3OH SN2 with good nucleophiles that are not strongly basic SN2 with good nucleophiles that are also strong bases But E2 with strong, hindered bases CH3CH2CH2Br + CH3CO CH3 CH3 - (CH3)3COH HBr CH3CH CH2 For unhindered primary R X : (CH3OH) * Relative “rates” of 4 arrows differ C H C L B Nu : - : - E1, E2 SN2, SN1, E1, E2 E1, E2 SN2, SN1 “Lingo”: α position, ? position; further away: γ, δ, and so on. SolventMeOH SolventDMF CH3Br + H2O But (CH3)3CBr (CH3)2CHBr or react! + (CH3)3C Br H OH (CH3)3C OH Acetone H Br + Weak Nu Despite being α-branched Hydrolysis + (CH3)2CH Br H OCH3 (CH3)2CH OCH3 DMF H Br + Methanolysis Generally: Solvolysis Recall: SN2 slows with weak Nu (and branching). For example: Mechanism: 1. Rate = k[R-L], 1st order unimolecular, only R-L in rate-determining TS: “bottleneck”. 2. Stereochemistry: Racemization (extensive, although often not complete). Both observations inconsistent with SN2 mechanism 3. Accelerates with polar (best with protic, in contrast to SN2) solvents: Hexane CHCl3 CH3CCH3 CH3OH O 4. Accelerates with better L L: Cl Br I OSO2R 5. Product determining steps