加州大学伯克利分校有机化学第七章课件.ppt

Relative “rates” of 4 arrows differ;CH3Br;Mechanism:;3. Accelerates with polar (best with protic, in contrast to SN2) solvents:;Mechanism;2.;3.;Unimolecular nucleophilic substitution: ;SN1PE;Incomplete Racemization;The Strong Effect of Polar Solvents on the SN1 Reaction;C;Hyperconjugation;X-Ray Structure of the 1,1-Dimethylethyl Cation(Laube, 1993);Summary;Reactivity of R-X;Problem:;(-);Mechanism of the E1 Reaction;Ratios of SN1 to E1 products are independent of L;E1 Gives Mixtures ;E1/SN1 ratios are difficult to predict;Bimolecular Elimination E2;One diastereomer of RX (e.g. R,R/S,S below) gives only one stereoisomer of alkene product:;Or the S,R-R,S pair:;Mechanism of the E2 Reaction;E2 in Cyclic Systems;Hindered Base Ensures E2;Summary;Factor 1: Base strength of the nucleophile;Factor 3: Steric hindrance in nucleophile (strong base); An alkene;No (or exceedingly slow) reaction with poor nucleophiles ;Br;Reactivity of Sec Haloalkanes R-X with Nucleophiles (Bases);E2 with high concentrations of strong base (for example, HO or RO in alcohol solvent);Reactivity of Tert Haloalkanes R-X with Nucleophiles (Bases)