A. What products (including stereoisomers if applicable)...

a. What products (including stereoisomers if applicable) would be formed from the reaction of 3 -bromo-2-methylpentane with $\mathrm{HO}^{-}$ under $\mathrm{S}_{\mathrm{N}} 2 / \mathrm{E} 2$ conditions and under $\mathrm{S}_{\mathrm{N}} 1 / \mathrm{E} 1$ conditions? b. Answer the same question for 3 -bromo- 3 -methylpentane.

a. What products (including stereoisomers if applicable) would be formed from the reaction of 3 -bromo-2-methylpentane with $\mathrm{HO}^{-}$ under $\mathrm{S}_{\mathrm{N}} 2 / \mathrm{E} 2$ conditions and under $\mathrm{S}_{\mathrm{N}} 1 / \mathrm{E} 1$ conditions?
b. Answer the same question for 3 -bromo- 3 -methylpentane.

Key Concepts

Substrate Structure Effects

The structure of the substrate, including the degree of steric hindrance and the ability to stabilize carbocations, plays a vital role in determining whether substitution or elimination reactions will dominate. More substituted carbons often favor carbocation formation, promoting SN1/E1 pathways, while less hindered substrates are more susceptible to direct backside attack by nucleophiles in SN2/E2 pathways.

E1 Mechanism

The E1 mechanism is a unimolecular elimination reaction that begins with the departure of the leaving group to form a carbocation intermediate, followed by deprotonation to form an alkene. This pathway can be accompanied by carbocation rearrangements and typically results in mixtures of alkene products which are governed by the stability of the intermediate and follow trends such as Zaitsev's rule.

Stereochemistry in Substitution and Elimination Reactions

Stereochemical outcomes are key to understanding these mechanisms. In SN2 reactions, the configuration at the reaction center inverts due to backside attack, while SN1 reactions often yield racemic mixtures due to the planar nature of the intermediate carbocation. In elimination reactions, the E2 mechanism frequently shows stereospecific behavior requiring an anti-periplanar alignment, whereas E1 eliminations may produce a mix of geometric isomers influenced by subsequent deprotonation steps.

SN1 Mechanism

The SN1 mechanism is a unimolecular nucleophilic substitution that occurs in two steps. Initially, the leaving group departs forming a planar carbocation intermediate, which can then be attacked by the nucleophile from either side, often leading to a racemic mixture. This pathway is preferred in substrates that can stabilize carbocations and in polar protic solvents.

SN2 Mechanism

The SN2 mechanism is a bimolecular nucleophilic substitution process in which the nucleophile attacks the electrophilic carbon from the opposite side of the leaving group, leading to a concerted reaction and inversion of configuration. This pathway is favored in less sterically hindered substrates, strong nucleophiles, and typically polar aprotic solvents.

E2 Mechanism

The E2 mechanism is a bimolecular elimination reaction that occurs in a single concerted step. In this process, a strong base abstracts a proton from a carbon adjacent to the one bearing the leaving group, while the leaving group departs simultaneously, resulting in the formation of a double bond. The reaction generally requires an anti-periplanar geometry between the leaving group and the abstracted proton.

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