CH 6-9: E1 Elimination Mechanism - E1: “Elimination….Unimolecular” - Rate = k [substrate] (1 st order reaction) Shares many (if not most) of the same mechanistic.

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Presentation transcript:

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

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

The SN1 Mechanism

The E1 Mechanism

Distinguishing Between SN1, SN2, E1 and E2 Reactions (1) Poor nucleophiles, weak bases (H 2 O, ROH): Mixture of SN1 and E1 products with 2 o or 3 o 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 (H 2 O, ROH; solvolysis). (2) Good nucleophile, weak base ( - CN, X -, NH 3, HS -, RCOO - ): SN2 products only, with methyl, 1 o or 2 o alkyl halides; SN2: Inversion of configuration always; 3 o alkyl halide gives SN1/E1.

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

Keys for Solving Substitution/Elimination Reactions For each reaction consider: Type of alkyl halide (1 o, 2 o, 3 o ) 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.