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SC CHEM 333 - E/Z, Substitution vs. Elimination, Alkoxides

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Good nucleophileConjugate acidpKa < 11Good baseConjugate acidpKa > 11Bulky baseLecture 17Outline of Last Lecture I. Final summary of SN1 and SN2II. Elimination Reactionsa. E1 Mechanismb. E2 MechanismCHEM 333 1st EditionIII. Regioselectivity of E1 and E2a. Zaitser’s Ruleb. Alkene StabilityIV. Carbocation Rearrangementa. StereoselectivityV. AlkenesOutline of Current LectureI. E/Z SystemII. Elimination on Cyclohexane RingsIII. Substitution vs. Eliminationa. SN1 vs. E1b. SN2 vs. E2c. SummaryIV. Alkoxide FormationV. Acid Catalyzed Dehydration of AlcoholsCurrent LectureI. E/Z System: uses priority rules of R/S system to assign priorities to each substituent. Z E together oppositea. Use cis/trans or E/Z for simple alkenesb. E/Z must be used for more complicated alkenesc. Example: Zd. Example: Ee. Example: Ef. Be able to predict E or Z product geometry in E2 reactionsi. Example:II. Elimination on Cyclohexane Ringsf. H and leaving group need to both be axial to have them both “anti”g. Example: III. Substitution vs. Eliminationa. SN1 vs. E1i. Polar protic solventsii. Many reactions both give SN1 and E1 productsiii. Use weak bases, therefore SN1 > E1b. SN2 vs. E2i. Methyl halides are SN2 onlyii. β and α branching retards SN2 (sterics), but increase E2 (increases alkenestability)iii. Good nucleophiles: SN2 is faster than E2iv. Good base: E2 is faster than SN2c. SummaryH2OMeOHEtOHGood nucleophileConjugate acidpKa < 11Good baseConjugate acidpKa > 11BulkybaseMethyl NRSN2 SN2 SN2NRSN2 SN2 E2SN1/E1(SN1 is majorproduct)SN2 E2/SN2(E2 is major product)E2SN1/E1(SN1 is majorproduct)SN1/E1(SN1 is majorproduct)E2 E2d.IV. Alkoxide Formation: Alcohols react with metals to generate alkoxides (Li/Na/K)a. Example:b. Example:c. Can also use strong basesV. Acid Catalyzed Dehydration of Alcohols (E1 Reaction)a. Ease of dehydration: 1° < 2° < 3°b. Need a strong acid: H2SO4 or H3PO4c. Need heat (Δ)d.


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SC CHEM 333 - E/Z, Substitution vs. Elimination, Alkoxides

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