Lesson eLearning

Reduce methyl vinyl ketone () with sodium borohydride in ethanol at room temperature. Identify the major product, indicating the stereochemistry formed.

hard 5 marks Class 12 · Chemistry · Aldehydes Ketones and Carboxylic Acids
Answer
(E)-2‑(3‑hydroxypropyl)butan‑3‑one with syn addition to give the 3‑R,4‑S diastereomer

The correct answer is (E)-2‑(3‑hydroxypropyl)butan‑3‑one with syn addition to give the 3‑R,4‑S diastereomer — 7. Hence, the major product is (E)-3‑hydroxy‑4‑methyl‑butan‑3‑one (3‑R,4‑S).

Step-by-step solution

  1. 1. donates a hydride ion to the electrophilic carbonyl carbon of methyl vinyl ketone.
  2. 2. Hydride addition gives a alkoxide intermediate .
  3. 3. Protonation by ethanol converts the alkoxide to the corresponding alcohol .
  4. 4. The new chiral center at carbon‑3 (former carbonyl) adopts the 3‑R configuration due to the hydride attack from the less hindered side.
  5. 5. The double bond remains intact as (E)-CH2=CHCH2; thus the product is (E)-3‑hydroxy‑4‑methyl‑butan‑3‑one.
  6. 6. Stereochemistry at C‑4 is determined by the geometry of the starting alkene, giving the 4‑S diastereomer.
  7. 7. Hence, the major product is (E)-3‑hydroxy‑4‑methyl‑butan‑3‑one (3‑R,4‑S).
Common mistake: Assuming complete saturation of the double bond instead of hydride addition to the carbonyl.

Concept tested: Reduction of α,β‑unsaturated ketones

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