Give structures of all alkenes formed and underline the...

Give structures of all alkenes formed and underline the major product expected from E2 elimination of: (a) 1-chloropentane, (b) 2-chloropentane. (a) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Cl} \longrightarrow \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}=\mathrm{CH}_2$ A $1^{\circ}$ alkyl halide, therefore one alkene. (b) A $2^{\circ}$ alkyl halide flanked by two R's; therefore two alkenes are formed. The more substituted alkene is the major product because of its greater stability.

Give structures of all alkenes formed and underline the major product expected from E2 elimination of: (a) 1-chloropentane, (b) 2-chloropentane.
(a) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Cl} \longrightarrow \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}=\mathrm{CH}_2$
A $1^{\circ}$ alkyl halide, therefore one alkene.
(b)
A $2^{\circ}$ alkyl halide flanked by two R's; therefore two alkenes are formed. The more substituted alkene is the major product because of its greater stability.

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Key Concepts

E2 Elimination Reaction

The E2 elimination reaction is a one-step mechanism where a base abstracts a proton from a carbon adjacent to the one bearing the leaving group, resulting in the formation of a double bond and expulsion of the leaving group. This process occurs in a concerted, bimolecular fashion and typically requires an anti-periplanar arrangement between the hydrogen and the leaving group.

Alkyl Halide Classification

Alkyl halides are classified based on the nature of the carbon attached to the halogen as primary, secondary, or tertiary. This classification influences their reactivity in elimination reactions, with primary halides generally favoring fewer elimination pathways compared to secondary halides, which can lead to the formation of multiple alkenes.

Zaitsev's Rule

Zaitsev's rule predicts that in elimination reactions, the formation of the more substituted alkene is favored because it is thermodynamically more stable. This rule explains why, in reactions where multiple alkenes can form, the product with the greater number of alkyl substituents on the double bond is typically the major product.

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