Indicate the products of the following reactions and point...

Indicate the products of the following reactions and point out the mechanism as $\mathrm{S}_{\mathrm{N}} 1, \mathrm{~S}_{\mathrm{N}} 2$, E1 or E2. (a) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Br}+\mathrm{LiAlH}_4$ (source of : $\mathrm{H}^{-}$) (d) $\mathrm{BrCH}_2 \mathrm{CH}_2 \mathrm{Br}+\mathrm{Mg}$ (ether) (b) $\left(\mathrm{CH}_3\right)_3 \mathrm{CBr}+\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}$, heat at $60^{\circ} \mathrm{C}$ (e) $\mathrm{BrCH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Br}+\mathrm{Mg}$ (ether) (c) $\mathrm{CH}_3 \mathrm{CH}=\mathrm{CHCl}+\mathrm{NaNH}_2$ (f) $\mathrm{CH}_3 \mathrm{CHBrCH}_3+\mathrm{NaOCH}_3$ in $\mathrm{CH}_3 \mathrm{OH}$ (a) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_3$; an $\mathrm{S}_{\mathrm{N}} 2$ reaction, : $\mathrm{H}^{-}$of $\mathrm{AlH}_4^{-}$replaces $\mathrm{Br}^{-}$. (b) (c) $\mathrm{CH}_3 \mathrm{CH}=\mathrm{CHCl}+\mathrm{NaNH}_2 \longrightarrow \mathrm{CH}_3 \mathrm{C} \equiv \mathrm{CH}+\mathrm{NH}_3+\mathrm{NaCl}$ Vinyl halides are quite inert toward $\mathrm{S}_{\mathrm{N}} 2$ reactions. (d) $\mathrm{BrCH}_2 \mathrm{CH}_2 \mathrm{Br}+\mathrm{Mg} \longrightarrow \mathrm{H}_2 \mathrm{C}=\mathrm{CH}_2+\mathrm{MgBr}_2$ This is an $\mathrm{E} 2$ type of $\beta$-elimination via an alkyl magnesium iodide. $$ \mathrm{Mg}+\mathrm{BrCH}_2 \mathrm{CH}_2 \mathrm{Br} \longrightarrow \mathrm{BrMg}^{-} \overparen{\mathrm{CH}_2}-\mathrm{CH}_2-\bigodot_{\mathrm{Br}} \longrightarrow \mathrm{MgBr}_2+\mathrm{H}_2 \mathrm{C}=\mathrm{CH}_2 $$ (e) This reaction resembles that in $(d)$ and is an internal $\mathrm{S}_{\mathrm{N}} 2$ reaction. (f) This $2^{\circ} \mathrm{RBr}$ undergoes both $\mathrm{E} 2$ and $\mathrm{S}_{\mathrm{N}} 2$ reactions to form propylene and isopropyl methyl ether. $$ \mathrm{CH}_3 \mathrm{CHBrCH}_3+\stackrel{+}{\mathrm{NaO}}-\overline{C H} \mathrm{CH}_3\left(\mathrm{CH}_3 \mathrm{OH}\right) \longrightarrow \underset{\mathrm{OCH}_3}{\mathrm{CH}_3 \mathrm{CH}=\mathrm{CH}_2}+\underset{\mathrm{CH}_3 \mathrm{CHCH}_3}{\mathrm{CH}_3} $$

Indicate the products of the following reactions and point out the mechanism as $\mathrm{S}_{\mathrm{N}} 1, \mathrm{~S}_{\mathrm{N}} 2$, E1 or E2.
(a) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Br}+\mathrm{LiAlH}_4$ (source of : $\mathrm{H}^{-}$)
(d) $\mathrm{BrCH}_2 \mathrm{CH}_2 \mathrm{Br}+\mathrm{Mg}$ (ether)
(b) $\left(\mathrm{CH}_3\right)_3 \mathrm{CBr}+\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}$, heat at $60^{\circ} \mathrm{C}$
(e) $\mathrm{BrCH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Br}+\mathrm{Mg}$ (ether)
(c) $\mathrm{CH}_3 \mathrm{CH}=\mathrm{CHCl}+\mathrm{NaNH}_2$
(f) $\mathrm{CH}_3 \mathrm{CHBrCH}_3+\mathrm{NaOCH}_3$ in $\mathrm{CH}_3 \mathrm{OH}$
(a) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_3$; an $\mathrm{S}_{\mathrm{N}} 2$ reaction, : $\mathrm{H}^{-}$of $\mathrm{AlH}_4^{-}$replaces $\mathrm{Br}^{-}$.
(b)
(c) $\mathrm{CH}_3 \mathrm{CH}=\mathrm{CHCl}+\mathrm{NaNH}_2 \longrightarrow \mathrm{CH}_3 \mathrm{C} \equiv \mathrm{CH}+\mathrm{NH}_3+\mathrm{NaCl}$
Vinyl halides are quite inert toward $\mathrm{S}_{\mathrm{N}} 2$ reactions.
(d) $\mathrm{BrCH}_2 \mathrm{CH}_2 \mathrm{Br}+\mathrm{Mg} \longrightarrow \mathrm{H}_2 \mathrm{C}=\mathrm{CH}_2+\mathrm{MgBr}_2$
This is an $\mathrm{E} 2$ type of $\beta$-elimination via an alkyl magnesium iodide.
$$
\mathrm{Mg}+\mathrm{BrCH}_2 \mathrm{CH}_2 \mathrm{Br} \longrightarrow \mathrm{BrMg}^{-} \overparen{\mathrm{CH}_2}-\mathrm{CH}_2-\bigodot_{\mathrm{Br}} \longrightarrow \mathrm{MgBr}_2+\mathrm{H}_2 \mathrm{C}=\mathrm{CH}_2
$$
(e) This reaction resembles that in $(d)$ and is an internal $\mathrm{S}_{\mathrm{N}} 2$ reaction.
(f) This $2^{\circ} \mathrm{RBr}$ undergoes both $\mathrm{E} 2$ and $\mathrm{S}_{\mathrm{N}} 2$ reactions to form propylene and isopropyl methyl ether.
$$
\mathrm{CH}_3 \mathrm{CHBrCH}_3+\stackrel{+}{\mathrm{NaO}}-\overline{C H} \mathrm{CH}_3\left(\mathrm{CH}_3 \mathrm{OH}\right) \longrightarrow \underset{\mathrm{OCH}_3}{\mathrm{CH}_3 \mathrm{CH}=\mathrm{CH}_2}+\underset{\mathrm{CH}_3 \mathrm{CHCH}_3}{\mathrm{CH}_3}
$$

Key Concepts

E2 Reaction Mechanism

The E2 mechanism is a concerted, bimolecular elimination process in which a strong base abstracts a ?-hydrogen while the leaving group simultaneously departs. The reaction leads to the formation of a double bond and requires an anti-periplanar arrangement of the leaving group and hydrogen, being common with secondary and primary substrates in the presence of strong bases.

SN2 Reaction Mechanism

This mechanism involves a concerted backside attack by a nucleophile on an electrophilic carbon bearing a leaving group, resulting in inversion of configuration. It typically occurs in one step and is favored by primary substrates with minimal steric hindrance and strong nucleophiles.

SN1 Reaction Mechanism

This mechanism takes place in two stages: first, the leaving group dissociates to form a carbocation intermediate, and then the nucleophile attacks the planar carbocation. It is favored by tertiary substrates that can stabilize the positive charge and often competes with elimination reactions under certain conditions.

E1 Reaction Mechanism

This elimination pathway involves two distinct steps: formation of a carbocation intermediate after the leaving group departs, followed by deprotonation to form a double bond. E1 reactions are favored by tertiary substrates and are commonly observed under conditions of heat or in polar solvents that stabilize the carbocation intermediate.

Substrate Structure and Reaction Pathways

The structure of the alkyl halide—whether it is primary, secondary, or tertiary—greatly influences the preferred reaction mechanism. Primary substrates typically undergo SN2 or E2 reactions due to low steric hindrance, while tertiary substrates favor SN1 or E1 reactions due to the stability of the resulting carbocations.

Role of Reagents and Reaction Conditions

Reagents such as strong nucleophiles (e.g., hydride donors) and strong bases can direct reactions toward substitution or elimination. Reaction conditions like solvent, temperature, and the presence of metals (for example, magnesium in ether to form organometallic species) are crucial in determining whether substitution or elimination pathways predominate.

Reactivity of Vinyl Halides

Vinyl halides, in which the halogen is attached to an sp2-hybridized carbon, are largely unreactive toward SN2 reactions due to the partial double-bond character and the resulting electron density. Instead, they often undergo elimination reactions under strongly basic conditions to form alkynes or other unsaturated compounds.

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