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Ester also reacts with Grignard's reagent in a similar fashion to acid chloride. Write the mechanism of the reaction of Grignard's reagent with Ester.

Answer :

The reaction between an ester and a Grignard reagent is a key reaction in organic chemistry that allows for the formation of tertiary alcohols. Here's the step-by-step mechanism:

  1. Formation of the Grignard Reagent: Grignard reagents have the formula [tex]R-MgX[/tex], where [tex]R[/tex] is an alkyl or aryl group and [tex]X[/tex] is a halogen. They are typically prepared by reacting an organic halide with magnesium metal in anhydrous ether.

  2. Nucleophilic Attack: The Grignard reagent acts as a strong nucleophile. Once formed, it attacks the electrophilic carbon atom of the carbonyl group in the ester, forming a tetrahedral intermediate. The [tex]R^-[/tex] group from the Grignard reagent adds to the carbonyl carbon, and the carbonyl oxygen pulls electrons towards itself, becoming negatively charged:

    [tex]\text{R'}-\overset{\displaystyle{||}}{C}-O-\text{R} + \text{R-MgX} \rightarrow \text{R'}-\overset{\displaystyle{\dagger}}{^{\text{R}}_\text{O-}}\text{C}-O-\text{R}[/tex]

  3. Elimination of the Leaving Group: The alkoxy group from the ester leaves, resulting in the formation of a ketone intermediate:

    [tex]\text{R'}-\overset{\displaystyle{||}}{C}-O-\text{MgX} + \text{ROH}[/tex]

    Here, one mole of the Grignard reagent is consumed in transforming the ester to a ketone.

  4. Second Nucleophilic Attack: Another equivalent of the Grignard reagent attacks the newly formed ketone, again forming a tetrahedral intermediate:

    [tex]\text{R'}-\overset{\displaystyle{||}}{C}-\text{O-MgX} + \text{R-MgX} \rightarrow \text{R'}-\overset{\displaystyle{\dagger}}{^{\text{R}}_\text{O-}}\text{C}-\text{R}[/tex]

  5. Formation of Tertiary Alcohol: Upon hydrolysis of the complex with water or dilute acid, a tertiary alcohol is formed, along with products like [tex]Mg(OH)X[/tex]:

    [tex]\text{R'}-\text{C}(-\text{R}_2)-\text{OH}[/tex]

This reaction is valuable in organic synthesis as it allows the construction of complex alcohols from simple starting materials. It's essential to conduct this reaction in an anhydrous environment to preserve the reactivity of the Grignard reagent.

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