NaOH is a strong nucleophile and strong base. The substrate in this case is primary. Therefore, we expect SN2 (giving the major product) and E2 (giving the minor product). b) NaSH is a strong nucleophile and weak base. The substrate in this case is primary. Therefore, we expect only SN2. c) When a primary alkyl halide is treated with t-BuOK, the predominant pathway is expected to be E2. d) DBN is a weak nucleophile and a strong base. Therefore, we expect only E2. e) NaOMe is a strong nucleophile and strong base. The substrate in this case is primary. Therefore, we expect SN2 (giving the major product) and E2 (giving the minor product).
The correct answer and explanation is:
Your analysis of substitution and elimination reactions involving primary alkyl halides and different reagents is largely on track. Let’s go through each case and then summarize the logic behind the expected mechanisms:
a) NaOH (Strong Nucleophile and Strong Base) + Primary Substrate
Correct Prediction: Major SN2, Minor E2
Explanation: Primary substrates are sterically unhindered, favoring bimolecular substitution (SN2) with strong nucleophiles like NaOH. However, because NaOH is also a strong base, E2 can still occur as a minor pathway, especially under heat.
b) NaSH (Strong Nucleophile, Weak Base) + Primary Substrate
Correct Prediction: Only SN2
Explanation: NaSH is a good nucleophile but a poor base. It prefers substitution over elimination. Given the primary substrate’s low steric hindrance, SN2 is highly favored, with negligible E2.
c) t-BuOK (Strong Base, Bulky) + Primary Substrate
Correct Prediction: Predominantly E2
Explanation: Despite the substrate being primary, t-BuOK is a bulky base, disfavoring backside attack (required for SN2). As a result, elimination via the E2 mechanism dominates, yielding the less substituted (Hofmann) alkene due to steric hindrance.
d) DBN (Strong Base, Poor Nucleophile) + Primary Substrate
Correct Prediction: Only E2
Explanation: DBN is a non-nucleophilic base, favoring elimination. Even with a primary substrate, E2 occurs efficiently, especially under heat or in aprotic solvents.
e) NaOMe (Strong Nucleophile and Base) + Primary Substrate
Correct Prediction: Major SN2, Minor E2
Explanation: Similar to NaOH, NaOMe is both a strong base and nucleophile. The primary substrate allows SN2 to proceed rapidly, but E2 remains possible under suitable conditions.
Summary:
For primary substrates:
- Strong nucleophile + weak base → SN2 only (e.g., NaSH)
- Strong nucleophile + strong base → SN2 major, E2 minor (e.g., NaOH, NaOMe)
- Bulky or non-nucleophilic strong base → E2 favored (e.g., t-BuOK, DBN)
This approach ensures accurate prediction of mechanisms based on the substrate and reagent nature.