Grignard reagent synthesis is always carried out in dry ether because

Grignard reagent forms hydrocarbon with water

The correct answer is option 4. Grignard reagent forms hydrocarbon with water.

A Grignard reagent is an organomagnesium halide with the general formula \( \text{R-MgX} \), where \( \text{R} \) is an alkyl or aryl group, and \( \text{X} \) is a halogen (usually chlorine, bromine, or iodine). Grignard reagents are highly reactive and are used in a wide range of organic syntheses to form carbon-carbon bonds.

Role of Dry Ether in Grignard Reagent Synthesis

Reactivity of Grignard Reagents:

Grignard reagents are very reactive, especially towards water. This high reactivity is due to the strong nucleophilic nature of the carbon-magnesium bond (\(\text{R-Mg}\)). Here’s what happens if water is present:

Reaction with Water: Grignard reagents react with water to produce hydrocarbons and magnesium hydroxide:

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

In this reaction, the Grignard reagent is destroyed, and the desired Grignard reagent cannot be synthesized.

Implications: Because water destroys the Grignard reagent, it is crucial to exclude water from the reaction environment to ensure that the Grignard reagent is successfully formed and preserved.

Why Dry Ether?

Ether is used as the solvent for Grignard reagent synthesis for several reasons:

Solubility: Grignard reagents are soluble in ether. Ether provides a suitable medium for dissolving both the alkyl or aryl halide and the magnesium metal, allowing the reaction to proceed efficiently.

Non-reactivity: Ether itself is relatively inert and does not react with the Grignard reagent under the conditions used. This inertness makes ether an ideal solvent.

Drying Requirement: The term “dry ether” refers to ether that is free from moisture. It is essential because any presence of water can lead to the hydrolysis of the Grignard reagent, as described earlier. Thus, dry ether ensures that no water is present to interfere with the synthesis

Explanation of the Correct Answer

Reaction with Water: When a Grignard reagent comes into contact with water, it undergoes a reaction that destroys the reagent:

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

Hydrocarbon Formation: The Grignard reagent reacts with water to form the corresponding hydrocarbon (alkane or arene) and magnesium hydroxide. This reaction effectively consumes the Grignard reagent.

Importance of Dry Conditions: To prevent this unwanted reaction, it is critical to ensure that the reaction environment (including the solvent) is free of water. This is why dry ether is used—it provides a water-free environment that allows the Grignard reagent to form and be used in further reactions.

Summary

In summary, the use of dry ether in Grignard reagent synthesis is essential because:

Grignard reagents react violently with water, leading to their decomposition.

Dry ether provides an appropriate and inert solvent for the reaction, ensuring that the Grignard reagent is not destroyed by the presence of moisture.

Thus, option 4 (Grignard reagent forms hydrocarbon with water) accurately describes why it is crucial to use dry ether for the synthesis of Grignard reagents.