For each of the following reactions, draw the major...

For each of the following reactions, draw the major elimination product; if the product can exist as stereoisomers, indicate which stereoisomer is obtained greater yield. a. $(R)$ -2-bromohexane $+$ high concentration of $\mathrm{CH}_{3} \mathrm{O}^{-}$ b. $(R)-3$ -bromo- 3 -methylhexane $+\mathrm{CH}_{3} \mathrm{OH}$ c. trans-1-chloro-2-methylcyclohexane + high concentration of $\mathrm{CH}_{3} \mathrm{O}^{-}$ d. trans-1-chloro-3-methylcyclohexane + high concentration of $\mathrm{CH}_{3} \mathrm{O}^{-}$ e. 3 -bromo- 3 -methylpentane $+$ high concentration of $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}$ f. 3 -bromo- 3 -methylpentane $+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}$

For each of the following reactions, draw the major elimination product; if the product can exist as stereoisomers, indicate which stereoisomer is obtained greater yield.
a. $(R)$ -2-bromohexane $+$ high concentration of $\mathrm{CH}_{3} \mathrm{O}^{-}$
b. $(R)-3$ -bromo- 3 -methylhexane $+\mathrm{CH}_{3} \mathrm{OH}$
c. trans-1-chloro-2-methylcyclohexane + high concentration of $\mathrm{CH}_{3} \mathrm{O}^{-}$
d. trans-1-chloro-3-methylcyclohexane + high concentration of $\mathrm{CH}_{3} \mathrm{O}^{-}$
e. 3 -bromo- 3 -methylpentane $+$ high concentration of $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}$
f. 3 -bromo- 3 -methylpentane $+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}$

Key Concepts

E2 Mechanism

This reaction pathway involves a bimolecular elimination in which the base abstracts a proton while the leaving group departs simultaneously. It occurs in one concerted step and avoids formation of intermediates. Because of its concerted nature, the reaction requires a specific geometric orientation (usually anti?periplanar) between the hydrogen that is removed and the leaving group, making stereochemistry an important factor in determining the major product.

Zaitsev's Rule (Regioselectivity in Elimination Reactions)

Zaitsev's rule states that in an elimination reaction, the most substituted alkene, which is generally more stable, is usually the predominant product. This concept helps predict which hydrogen is abstracted by the base when multiple ?-hydrogens are available, leading to the formation of the most thermodynamically favorable double bond.

Anti?Periplanar Geometry

For an E2 reaction to occur efficiently, the hydrogen that is being abstracted and the leaving group must be positioned in an anti?periplanar (opposite sides in the same plane) arrangement. This geometric requirement is critical for the effective orbital overlap during the concerted process, and it heavily influences both the reaction rate and the stereochemical outcome of the product.

Stereoselectivity in Cyclohexane Systems

In cyclic systems like substituted cyclohexanes, the conformational preferences (chair forms) play a crucial role in determining the stereochemistry of the elimination product. When the leaving group and the abstracted hydrogen are arranged in an anti?periplanar fashion, typically only one stereoisomer of the alkene is formed in major amounts. The reaction is therefore stereospecific, with the chair conformation dictating the anti elimination pathway.

Role of Strong Bases and Reaction Conditions

The use of strong bases (such as methoxide or ethoxide) under high concentrations and specific solvent conditions favors elimination over substitution reactions. The nature of the base and the reaction conditions help drive the reaction toward the E2 mechanism, influencing the rate of proton abstraction and the overall elimination process, while also impacting the regiochemical and stereochemical outcomes.

Transcript

Please give Ace some feedback