High School

In the Grignard reaction, why was it important to use dry ether during the synthesis of the Grignard reagent?

A. The ether did not need to be dry during this step.
B. To avoid a potential fire.
C. The acidic proton in H₂O destroys the Grignard reagent.
D. To avoid the excess production of biphenyl and benzene.

Answer :

In the Grignard reaction, it is crucial to use dry ether as the solvent during the synthesis of the Grignard reagent. The correct answer to the question is option (C) "The acidic proton in H₂O destroys the Grignard reagent."

Explanation:

  1. Grignard Reagents:

    • Grignard reagents are organomagnesium compounds typically represented as R-Mg-X, where R is an organic group and X is a halide.
    • They are highly reactive and used to form carbon-carbon bonds in organic synthesis.

  2. Role of Dry Ether:

    • The ether solvent, such as diethyl ether, serves as a non-polar, inert medium that stabilizes the Grignard reagent by coordination with the magnesium atom.
    • Ether molecules with lone pairs can effectively coordinate with magnesium, enhancing the reagent's stability.

  3. Importance of 'Dry' Condition:

    • Grignard reagents react violently with water because they are highly sensitive to moisture. The presence of water leads to the protonation of the carbon-magnesium bond, which destroys the Grignard reagent.

    [tex]\text{R-Mg-X + H}_2\text{O} \rightarrow \text{R-H} + \text{MgX(OH)}[/tex]

    • This reaction with water results in the formation of alkanes and magnesium hydroxide salts, thus consuming the Grignard reagent and reducing its effectiveness for the intended organic synthesis.

  4. Why Avoiding Water is Crucial:

    • Water contains acidic protons, and since the Grignard reagent acts as a strong base, its reaction with water leads to its decomposition.
    • Therefore, using an anhydrous or 'dry' ether solvent is essential to prevent this decomposition and ensure the success of the Grignard reaction.

Dry conditions are maintained by careful preparation, usually involving drying glassware and using distilled solvents, to ensure the Grignard reagent is not destroyed by unwanted reactions with water.

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