Question

The major product formed in the reaction of (28,3R)-2-chloro-3-phenylbutane with NaOEt in EtOH is

• A. (E)-2-phenyl-but-2-ene
• B. (Z)-2-phenyl-but-2-ene
• C. 3-phenyl-but-1-ene
• D. (2R,3R)-2-ethoxy-3-phenylbutane

Answer 1

The Correct Option is B

The reaction involves the elimination of hydrogen and chlorine from (2S,3R)-2-chloro-3-phenylbutane using sodium ethoxide in ethanol (NaOEt in EtOH). This reagent combination is typical for an E2 elimination reaction, which tends to form the most stable alkene. Here's the step-by-step reasoning for finding the major product:

1. **Identify the Substrate and Reaction Type:**
   • The substrate is (2S,3R)-2-chloro-3-phenylbutane.
   • NaOEt is a strong base that can promote an E2 elimination.
2. **Determine the Elimination Mechanism:**
   • In E2 elimination, the base attacks a β-hydrogen (adjacent carbon), forming a double bond and eliminating the leaving group (Cl in this case).
3. **Analyze the Stereochemistry:**
   • Considering the stereochemistry of the compound (2S,3R), H and Cl are on the adjacent carbon atoms, allowing for anti-periplanar geometry required for E2 elimination.
   • The β-hydrogen elimination from the adjacent carbon can lead to the formation of either an E or Z isomer of 2-phenyl-but-2-ene, depending on the stereochemistry.
4. **Predict the Major Product:**
   • (Z)-2-phenyl-but-2-ene is typically the major product as it results from the more stable stereochemistry alignment.
   • The Z configuration is typically more stable due to lesser steric hindrance between substituents.

Therefore, the major product formed is (Z)-2-phenyl-but-2-ene, which aligns with option 2. The other options can be ruled out based on this detailed mechanism analysis. (E)-2-phenyl-but-2-ene would require different starting stereochemistry, while 3-phenyl-but-1-ene and (2R,3R)-2-ethoxy-3-phenylbutane are not favored under these reaction conditions.