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CH 6-9: E1 Elimination Mechanism

CH 6-9: E1 Elimination Mechanism. Shares many (if not most) of the same mechanistic characteristics with SN1 reactions:. - E1: “Elimination….Unimolecular”. - Rate = k [substrate] (1 st order reaction). - mixture of SN1 and E1 products are always formed. - poor nucleophile/weak base.

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CH 6-9: E1 Elimination Mechanism

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  1. CH 6-9: E1 Elimination Mechanism • Shares many (if not most) of the same mechanistic characteristics with SN1 reactions: - E1: “Elimination….Unimolecular” - Rate = k [substrate] (1st order reaction) - mixture of SN1 and E1 products are always formed - poor nucleophile/weak base - carbocation intermediate - rearrangements possible - 3o > 2o >>>> 1o …..(1o reacts by SN2/E2 mechanism) - most stable alkene is always formed - stereochemistry is irrelevant (not stereospecific)

  2. SN1/E1 Substitution/Elimination Mechanisms • What type of products will the partial reaction above give us? In other words, what mechanisms are most likely? • ANSWER: Combination of SN1 & E1 mechanisms and products.

  3. The SN1 Mechanism

  4. The E1 Mechanism

  5. Distinguishing Between SN1, SN2, E1 and E2 Reactions (1) Poor nucleophiles, weak bases (H2O, ROH): • Mixture of SN1 and E1 products with 2o or 3o alkyl halides; • SN1: Racemic mixture of stereoisomers; • E1: Most stable alkene is always major product; • Carbocation intermediates: beware of rearrangements; • Nucleophile is usually the solvent (H2O, ROH; solvolysis). (2) Good nucleophile, weak base (-CN, X-, NH3, HS-, RCOO-): • SN2 products only, with methyl, 1o or 2o alkyl halides; • SN2: Inversion of configuration always; • 3o alkyl halide gives SN1/E1.

  6. Distinguishing Between SN1, SN2, E1 and E2 Reactions (3) Good nucleophile, strong base: • Mixture of SN2 and E2 with 1o or 2o alkyl halides; • SN2: Inversion of configuration always; • E2/small base (HO-, RO-, NH2-) produces most stable alkene; • E2 and large, bulky base (tert-butoxide salt): SN2/E2 with methyl and 1o; E2 only with 2o and 3o; produces less stable alkene; • 3o alkyl halide gives E2 products only ; • Best solvents are acetone, DMSO, DMF.

  7. Keys for Solving Substitution/Elimination Reactions For each reaction consider: • Type of alkyl halide (1o, 2o, 3o) • Classify the nucleophile (poor/good, weak base/strong base, small/bulky) • ID Mechanism(s)/Reaction(s) (SN1, SN2, E1, E2) • For substitution reactions: inversion or racemization? • For eliminations: most or least stable alkene? • Draw structure(s) of all major product(s) • I will provide a flow chart in class and on the web site.

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