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E2 Reactions

E2 Reactions. E2 = elimination, bimolecular Reaction is concerted (one step) Rate = k[substrate][base] (a second order process) E2 can occur with S N 2. Occurs by abstraction of H + from a C adjacent to the C with the LG. Products follow Zaitsev’s Rule.

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E2 Reactions

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  1. E2 Reactions • E2 = elimination, bimolecular • Reaction is concerted(one step) • Rate = k[substrate][base] (a second order process) • E2 can occur with SN2. • Occurs by abstraction of H+ from a C adjacent to the C with the LG. • Products follow Zaitsev’s Rule. • If the base is strong, E2 will occur instead of E1 for 3° alkyl halides.

  2. E2 Reaction Mechanism • One-step and requires a strong base. • Best transition state is anti-coplanar. • Example: t-butyl bromide + methoxide • Why don’t SN2, SN1, or E1 occur?

  3. E2 Reaction Mechanism • Example: t-butyl bromide + methoxide

  4. E2 Reaction Mechanism • Anti-periplanar alignment allows pi bond to form from sp3 orbitals.

  5. E2 Reaction Profile rate = ? k = ?

  6. Factors Affecting E2 Reactions • Structure of the substrate • Strength of the base • Nature of the leaving group • The solvent in which the reaction is run.

  7. Factors Affecting E2 Reactions - Structure of the Substrate • Structure of the alkyl halide: 3° > 2° >1° • This is due to the stability of the more highly substituted alkene (Zaitsev’s Rule).

  8. Factors Affecting E2 Reactions - Structure of the Substrate Decide whether the following substrates could react by E1 or E2 (and by SN1 or SN2).

  9. Factors Affecting E2 Reactions - Strength of the Base • The base must be strong. Which of the following bases favor E2 and which favor E1 reactions?

  10. Factors Affecting E2 Reactions - the Leaving Group • The LG should be good. Which of the following substrates have good LGs?

  11. Factors Affecting E2 Reactions - Solvent Effects • Polarity is not so important because negative charge is spread over the transition state.

  12. Zaitsev’s Rule • When two or more elimination products are possible, the more stable alkene will predominate. This is usually the product with the more substituted double bond.

  13. Zaitsev’s Rule • Show the E2 elimination products that could come from: • Which E2 reaction would occur more quickly?

  14. E2 Reactions - Summary • The structure of the substrate affects the rate. • Relative rates for E2: 3°>2°>1°. • The base must be strong. • The LG should be good. • The solvent should be polar. • Coplanar (usually anti) transition state is required. • Products will follow Zaitsev’s Rule. • Can occur with SN2 for 2° alkyl halides.

  15. E2 Reactions - Summary • Predict the products

  16. E2 Competes with SN2 for 2° Alkyl Halides

  17. E1 and E2 Reactions

  18. E1 and E2 Reactions

  19. How Do You Decide Which Reaction(s) Happen? • The strength and structure of the nucleophile/base is paramount. • Strong ones always give a second order reaction (SN2 or E2). • If the base is bulky, E2 will occur instead of SN2.

  20. How Do You Decide Which Reaction(s) Happen? • Now, look at the structure of the substrate. • 1° alkyl halides will undergo SN2 and perhaps E2 unless rearrangement is possible. • 3° alkyl halides will NOT undergo SN2. • 2° alkyl halides are the toughest to predict.

  21. How Do You Decide Which Reaction(s) Happen? • Evaluate, in order, • Nucleophile, strength and structure • -or- Base, strength and structure • Structure of the alkyl halide/carbocation • Leaving group • Solvent • YOU MUST BE ABLE TO DRAW THE MECHANISM!

  22. Elimination Reactions • Predict the products

  23. Did this product come from the reaction of 1-chlorobutane with a) sodium acetate or b) sodium t-butoxide?

  24. This product came from the reaction of 1-chlorobutane with sodium acetate. The C=O and C-O peaks show ester.

  25. Did this product come from the reaction of 1-chloropentane with a) sodium acetate or b) sodium t-butoxide?

  26. This product came from the reaction of 1-chloropentane with sodium t-butoxide. The sp2C-H and C=C peaks show alkene.

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