E-2 Reactions

Overview

  • Warm weather update: snow is melting, discussing snowman, dog interaction with snowman.

Class Logistics

  • Weekly plan:

    • Continue with E1 and E2 mechanisms.

    • Finish E1 comparisons towards the end of the chapter.

    • Major topics this week: E1 and E2 content.

    • Next Monday: Practice slides to review comparisons across Sn1, Sn2, E1, and E2.

    • Next Wednesday: Review session before exam on the following Friday.

  • Exam 3:

    • Date: Next Friday in class.

    • Materials covered: Chapters 6, 10, and 7A (Sm1, Sm2) and 7B (E1, E2).

  • Practice exam will be posted, timing is on the agenda.

Exam Important Notes

  • The fourth exam and final are scheduled at the same time during finals block; each exam will take around 50 minutes.

  • Timings are added to course notice to clarify concerns from students.

Mechanism Overview

E2 Mechanism Overview

  • Similarity to Sn2 mechanism but critical differences discussed.

  • E2 is a concerted mechanism; all actions occur in one step.

  • Comparison to E1:

    • E1 follows a leaving group leaving driven reaction.

    • E2 is driven by a strong base removing a hydrogen.

    • Requires specific arrangement termed "anticoplanar".

Recap of E1

  • E1 preferences:

    • Tertiary over primary leaving groups.

    • Polar protic solvents preferred.

    • Conditions for weak bases/nucleophiles are acceptable.

    • Product stability is determined by stability of carbocation formed during the first step.

Detailed Look into E2 Mechanism

Reaction and Transition State

  • E2 reaction requires:

    • Hydrogens adjacent to leaving group.

    • Base deprotonates hydrogen while the leaving group departs.

Reaction Coordinate Diagram

  • E2 is a one-step process; rate depends on:

    • Concentration of halide and base.

  • No carbocation, hence no rearrangement needed.

Reaction Type Characteristics

  • Comparison of

    • E1 favoured by good leaving groups and weak bases.

    • E2 favoured by strong bases.

    • E2 is concerted; sterically hindered conditions need to be addressed.

Trends in Substrate

  • Preferences for various alkyl halides:

    • No E2 elimination on methyl alkyl halide.

    • Tertiary > Secondary > Primary for E2.

Leaving Groups

  • Iodine > Bromine > Chlorine for best leaving group in E2 mechanism.

Bases

  • E2 requires strong bases.

  • Bulky bases can prevent competing nucleophilic reactions.

Concentration Effects

  • Increase in base concentration will increase reaction rate in E2, opposite for E1 because base is not involved in rate-determining step.

Solvent Effects

  • Polar aprotic solvents preferred since they stabilize bases.

Regioselectivity in E2 Eliminations

Stability of Products

  • Double bond stability and regioselectivity:

    • Zaitsev’s rule applies: preferring more substituted internal double bonds when possible.

Selectivity Factors

  • Position of base influences the outcome; steric bulk can lead to less stable products in some cases.

  • More substitution on alkyl halide increases elimination likelihood.

Stereochemistry of Products

Major and Minor Products

  • Stability of cis vs. trans products discussed.

    • Trans is more stable due to reduced steric hindrance.

    • Use of Newman projections to visually depict relationships.

Conclusion

  • Discussed importance of specific geometric arrangements (anticoplanar) in E2 mechanisms.

  • Emphasized the significance of hybridization and orbital interactions.

  • Encouragement to come prepared for practical examples in the next class.