Design syntheses of the following. (5 points each). Allowed starting materials include cyclopentanol or any acyclic alcohol or alkene wth ≤4
Answer
Synthesis Plans for Compounds a and b
🧪 Compound a: 1-Cyclopentylbutan-1-one
Target: A ketone with a cyclopentyl group on one side and a butyl group on the other.
Key Strategy: Perform a Friedel-Crafts acylation equivalent via Grignard reaction followed by oxidation or acyl chloride addition.
- Start from cyclopentanol → oxidize with PCC to form cyclopentanone.
- Prepare butyl magnesium bromide via reaction of 1-bromobutane (from 1-butanol and PBr3) with Mg in dry ether → forms Grignard reagent.
- React Grignard (butyl-MgBr) with cyclopentanone → gives tertiary alcohol.
- Oxidize tertiary alcohol using PCC or Dess–Martin periodinane to form the target ketone.
🧪 Compound b: 2-Methylhex-2-ene
Target: A six-carbon chain with a methyl-substituted double bond at the 2-position.
Key Strategy: Construct via Wittig reaction or acid-catalyzed dehydration after alcohol synthesis.
- React butanal (4-carbon aldehyde, derived from 1-butanol) with methyl magnesium bromide (from CH3Br + Mg) → gives 2-pentanol.
- React 2-pentanol with ethyl magnesium bromide (from 1-bromoethane + Mg) to add the ethyl group → gives 2-methylhexanol.
- Dehydrate the alcohol with acid (H2SO4, heat) → elimination (E1) gives mixture of alkenes, major product: 2-methylhex-2-ene.