Will the following reactions be primarily displacement or...

Will the following reactions be primarily displacement or elimination? (a) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Cl}+\mathrm{I}^{-} \longrightarrow$ (b) $\left(\mathrm{CH}_3\right)_3 \mathrm{CBr}+\mathrm{CN}^{-}$(ethanol) $\longrightarrow$ (c) $\mathrm{CH}_3 \mathrm{CHBrCH}_3+\mathrm{OH}^{-}\left(\mathrm{H}_2 \mathrm{O}\right) \longrightarrow$ (d) $\mathrm{CH}_3 \mathrm{CHBrCH}_3+\mathrm{OH}^{-}$(ethanol) $\longrightarrow$ (e) $\left(\mathrm{CH}_3\right)_3 \mathrm{CBr}+\mathrm{H}_2 \mathrm{O} \longrightarrow$ (a) $\mathrm{S}_{\mathrm{N}} 2$ displacement. $\mathrm{I}^{-}$is a good nucleophile, and a poor base. (b) E2 elimination. A $3^{\circ}$ halide and a fairly strong base. (c) Mainly $\mathrm{S}_{\mathrm{N}} 2$ displacement. (d) Mainly E2 elimination. A less polar solvent than that in (c) favors E2. (e) $\mathrm{S}_{\mathrm{N}} \mathrm{I}$ displacement. $\mathrm{H}_2 \mathrm{O}$ is not basic enough to remove a proton to give elimination.

Will the following reactions be primarily displacement or elimination?
(a) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Cl}+\mathrm{I}^{-} \longrightarrow$
(b) $\left(\mathrm{CH}_3\right)_3 \mathrm{CBr}+\mathrm{CN}^{-}$(ethanol) $\longrightarrow$
(c) $\mathrm{CH}_3 \mathrm{CHBrCH}_3+\mathrm{OH}^{-}\left(\mathrm{H}_2 \mathrm{O}\right) \longrightarrow$
(d) $\mathrm{CH}_3 \mathrm{CHBrCH}_3+\mathrm{OH}^{-}$(ethanol) $\longrightarrow$
(e) $\left(\mathrm{CH}_3\right)_3 \mathrm{CBr}+\mathrm{H}_2 \mathrm{O} \longrightarrow$
(a) $\mathrm{S}_{\mathrm{N}} 2$ displacement. $\mathrm{I}^{-}$is a good nucleophile, and a poor base.
(b) E2 elimination. A $3^{\circ}$ halide and a fairly strong base.
(c) Mainly $\mathrm{S}_{\mathrm{N}} 2$ displacement.
(d) Mainly E2 elimination. A less polar solvent than that in (c) favors E2.
(e) $\mathrm{S}_{\mathrm{N}} \mathrm{I}$ displacement. $\mathrm{H}_2 \mathrm{O}$ is not basic enough to remove a proton to give elimination.

Key Concepts

Substitution Reactions (SN)

Substitution reactions involve the replacement of a leaving group by a nucleophile. In an SN2 mechanism, the nucleophile attacks the substrate from the backside, leading to a concerted reaction where bond-making and bond-breaking occur simultaneously. This mechanism is generally favored by unhindered, primary substrates and strong nucleophiles in polar aprotic solvents.

Elimination Reactions (E)

Elimination reactions result in the formation of a double bond through the removal of a leaving group and a proton. The E2 mechanism is a concerted process where bond-breaking and proton abstraction occur simultaneously, often favored by strong bases and sterically hindered, typically secondary or tertiary, substrates. The choice between elimination and substitution is often determined by reaction conditions and substrate structure.

Substrate Structure

The structure of the organic substrate (whether it is primary, secondary, or tertiary) plays a critical role in determining the reaction pathway. Primary substrates, with less steric hindrance, are more likely to undergo SN2 reactions, while tertiary substrates, which are hindered, are more prone to elimination reactions such as E2 due to the difficulty for a nucleophile to access the electrophilic carbon.

Nucleophilicity vs. Basicity

Nucleophilicity refers to the ability of a species to donate a pair of electrons to an electrophilic center, while basicity is the ability to accept a proton. Many species can act as nucleophiles or bases, but their behavior in a particular reaction is dictated by factors such as solvent and substrate structure. Strong nucleophiles in the absence of steric hindrance favor substitution, whereas strong bases, especially in the presence of sterically hindered substrates, tend to promote elimination.

Solvent Effects

The choice of solvent dramatically affects the reaction mechanism. Polar aprotic solvents enhance the reactivity of nucleophiles by not solvating anions too strongly, thus favoring SN2 mechanisms. In contrast, polar protic solvents can stabilize ionic intermediates and the leaving group, which may sway the balance toward elimination reactions or SN1 mechanisms depending on other factors, including substrate structure and nucleophile/base strength.

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