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SolutionsAddition of Organometallic Reagents

1. Products

Which alcohol is obtained in each case, after acid hydrolysis?

  1. CH3MgBr + formaldehyde
  2. CH3CH2MgBr + ethanal
  3. C6H5MgBr + acetone
  4. CH3Li + cyclohexanone
  1. Ethanol (primary).
  2. 2-Butanol (secondary).
  3. 2-Phenyl-2-propanol (tertiary).
  4. 1-Methylcyclohexanol (tertiary).

2. Two ways

Propose two combinations of a Grignard reagent and a carbonyl compound to prepare 2-pentanol.

CH3CH2CH2MgBr + ethanal, or CH3MgBr + butanal. In both cases the carbon of the organometallic bonds to the carbonyl carbon.

3. Conditions

Why are Grignard reagents prepared in dry ether under an inert atmosphere?

Because the carbon of the organometallic is a very strong base (the corresponding alkane has a pKa above 50): water destroys it (R–MgX + H2O → R–H) and oxygen oxidizes it. Dry ether also stabilizes the magnesium by coordinating to it.

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