CHM210 Class 11/17

The E2 elimination reaction is a bimolecular process involving simultaneous bond breaking and bond making.
Involves special stereoelectronic requirements similar to the S_N2 reaction.

To predict E2 products, one must:
- Identify the leaving group.
- Identify the alpha carbon and the beta carbon.
- Identify beta hydrogens.
- Indicate where double bonds will form.

The reaction mechanism is concerted, meaning all bond-making and bond-breaking events occur simultaneously.
Requires a periplanar arrangement of atoms in the transition state:
- All reacting atoms/groups should lie in one plane.

The leaving group (e.g., bromine) must be positioned to allow for a proper reaction trajectory.
Two arrangements may allow for a periplanar state:
1. Eclipsed Conformation:
- Dihedral angle between the leaving group and the beta hydrogen is zero degrees.
- Higher energy state and thus less favorable for the reaction.
1. Staggered Conformation:
- Dihedral angle is 180 degrees (antiperiplanar).
- Most favorable for E2 reactions.

E2 reactions can be stereospecific especially in cases where starting materials are chiral.
Key steps for chiral substrates to follow to ensure formation of the correct stereoisomers:
1. Identify the leaving group, alpha carbon, beta carbon, beta hydrogens.
2. Ensure proper orientation for the periplanar transition state.

Example discussed in class reflects substrates with chirality that require careful analysis:
- Use of phenyl groups to simplify analysis as they lack beta hydrogens, thus preventing unnecessary complications.
- The second bromine's presence in substrates complicates analysis but is present for examination of leaving groups during prediction.

Upon successful elimination via the E2 mechanism, double bonds are formed between the alpha carbon and beta carbon. Results may lead to:
- Formation of products with stereochemical considerations (E/Z nomenclature).
Example includes predicting stereochemical outcome based on comparative groups, leading to the identification of Z-isomers.

In cyclohexane systems, it is critical to have the leaving group in an axial position to allow for beta hydrogens to be accessible in the same plane for E2 reactions:
- Eclipsed conformations are unfavorable due to steric strain.
- A higher energy conformational state (axial) may be necessary for the reactions to proceed.

E2 reactions are instrumental in synthesizing alkenes from vicinal dihalides through two consecutive eliminations promoting formation of alkenes.
Vicinal and geminal dihalides serve as important substrates in reactions leading to alkynes or alkenes:
- Reaction of vicinal dihalides can yield alkynes via a series of E2 eliminations.
- Strong bases such as alkoxides or hydroxides facilitate these eliminations.

The stability and preferences of particular products may depend heavily on the structure of the starting materials:
- Employ alkoxide (strong base) to yield high yield products from E2 reactions.
- Consideration of substituents on double bonds dictates major product formation according to regioselectivity:
- More substituted alkenes (Zaitsev's rule) typically favor formation over less substituted alkenes.
- Use of bulky bases can lead to non-Zaitsev products.

E1 reactions (unimolecular elimination) and E2 share similarities but also critical differences:
- E1 reactions compete with S_N1 reactions, leading to mixtures of substitution and elimination products.
- Mechanistically, E1 starts with ionization to form a carbocation, with subsequent loss of a proton leading to double bond formation.
- General conditions for predominance lean towards favoring elimination at higher temperatures.

Stereoelectronic requirements drive both E2 elimination reactions and result in nuanced product outcomes.
Shift towards bulky bases can shift expected product distributions away from Zaitsev's rule.
Understanding conformational dynamics, orient