Draw the products of each of the following S_N2/E2 reactions. If the products can exist as stereoisomers, show which stereoisomers are formed.
a. (3S,4S)-3-bromo-4-methylhexane + CH₃O⁻

Verified step by step guidance

1. Step 1: Identify the reaction type. The problem specifies SN2/E2 reactions. SN2 (bimolecular nucleophilic substitution) involves a backside attack by the nucleophile, leading to inversion of configuration at the carbon center. E2 (bimolecular elimination) involves the removal of a proton and a leaving group, forming a double bond.
2. Step 2: Analyze the substrate. The substrate is (3S,4S)-3-bromo-4-methylhexane. The bromine atom is the leaving group, and the stereochemistry at carbon-3 is S. Carbon-4 has a methyl group and is also stereogenic with S configuration.
3. Step 3: Consider the nucleophile/base. CH3O⁻ (methoxide ion) is both a strong nucleophile and a strong base. This means it can participate in both SN2 and E2 mechanisms. The reaction conditions (e.g., solvent, temperature) will influence which pathway dominates, but both products should be considered.
4. Step 4: For the SN2 pathway, the methoxide ion will attack the carbon-3 (where the bromine is attached) from the opposite side of the leaving group. This backside attack will result in inversion of configuration at carbon-3, changing it from S to R. The product will retain the stereochemistry at carbon-4 (S).
5. Step 5: For the E2 pathway, the methoxide ion will abstract a β-hydrogen (a hydrogen on a carbon adjacent to the carbon bearing the leaving group). The most likely β-hydrogen to be removed is on carbon-2, leading to the formation of a double bond between carbon-2 and carbon-3. The stereochemistry of the double bond will depend on the anti-periplanar arrangement of the β-hydrogen and the leaving group during the elimination process.

Key Concepts

SN2 Reaction Mechanism

The SN2 (substitution nucleophilic bimolecular) reaction is a one-step mechanism where a nucleophile attacks an electrophile, resulting in the simultaneous displacement of a leaving group. This reaction is characterized by a backside attack, leading to inversion of configuration at the carbon center. The rate of the reaction depends on the concentration of both the nucleophile and the substrate.

E2 Reaction Mechanism

The E2 (elimination bimolecular) reaction is a concerted mechanism where a base abstracts a proton while a leaving group departs, resulting in the formation of a double bond. This reaction typically requires a strong base and occurs in a single step, leading to the formation of alkenes. The stereochemistry of the reactants can influence the product's geometry, often favoring the formation of the more stable trans isomer.

Stereoisomerism

Stereoisomerism refers to the phenomenon where compounds have the same molecular formula and connectivity but differ in the spatial arrangement of atoms. In the context of SN2 and E2 reactions, stereoisomers can arise from the different orientations of substituents around a chiral center or double bond. Understanding stereoisomerism is crucial for predicting the specific products formed in these reactions, including their potential enantiomers and diastereomers.