Question

Mg reacts with alkyl bromide best in (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OCH}_{3}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}\left(\mathrm{CH}_{3}\right)_{2}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\) (d) equally in all the three

Short answer

(c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\).

Step-by-step solution

Understanding the Grignard Reagent Formation

Magnesium (Mg) reacts with alkyl bromide to form a Grignard reagent. This reaction needs a non-protic, non-polar solvent to proceed efficiently. Grignard reagents are very sensitive to moisture; hence, anhydrous conditions are essential.

Assessing Solvent Polarity and Protic Nature

Evaluate the given options for their suitability as solvents in Grignard reactions: - (a) \(\text{C}_{6} \text{H}_{5} \text{OCH}_{3}\) (anisole) is an ether and a common non-polar solvent.- (b) \(\text{C}_{6} \text{H}_{5} \text{~N(CH}_{3})_{2}\) is a tertiary amine which is polar.- (c) \(\text{C}_{2} \text{H}_{5} \text{OC}_{2} \text{H}_{5}\) (diethyl ether) is a prototypical solvent for Grignard reactions and is very common due to its ether linkage.

Evaluating Reactivity in Different Solvents

- (a) Anisole, being non-polar and an ether, can support Grignard reagent formation but isn't the most inert solvent due to benzene's stable resonance structure but is usable. - (b) Dimethyl aniline as a solvent may interfere due to its polarity and amine nature, binding with Mg, decreasing reactivity. - (c) Diethyl ether is non-polar, an excellent solvent for Grignard reactions and offers no interference with reagent formation.

Conclusion Based on Solvent Properties

Thus, \(\text{C}_{2} \text{H}_{5} \text{OC}_{2} \text{H}_{5}\) (diethyl ether) is the best solvent to facilitate the reaction due to its stability, non-polarity, and neutrality to the reaction.

Key concepts

Magnesium-Alkyl Bromide Reaction

The reaction between magnesium and alkyl bromide is crucial for forming a Grignard reagent. In this process, the magnesium atom inserts itself into the carbon-bromine bond of the alkyl bromide molecule. This results in the creation of a new compound, known broadly as a Grignard reagent, represented as RMgBr, where **R** symbolizes the organic part of the molecule. The insertion of magnesium into this bond is facilitated when certain environmental conditions are right, such as the choice of solvent, which is pivotal.
Grignard reagents are essential in organic chemistry due to their role in forming carbon-carbon bonds through various chemical reactions. These reagents act as valuable intermediates in synthesizing alcohols, acids, and other organic compounds. However, producing Grignard reagents needs carefully selected conditions, especially considering the solvent choice, to ensure successful and efficient reactions.

Non-Polar Solvents for Grignard Reactions

The choice of solvent is critical when forming Grignard reagents. Non-polar solvents, particularly ethers, are favored for these reactions because they can dissolve both reactants and support the formation of the Grignard reagent without interfering. Ethers like diethyl ether and tetrahydrofuran (THF) are the preferred choices due to their ability to stabilize the magnesium ion through coordination with their lone pair of electrons. This coordination helps in smooth insertion of magnesium into the carbon-bromide bond.
Ethers also provide a non-polar environment, which is beneficial as it doesn't introduce competing reactions or bonding that can deactivate or degrade the Grignard reagent. The non-polar nature ensures that the solvent does not participate in protic or polar interactions that could compromise the reaction efficiency.

Solvent Polarity and Protic Nature

Understanding the polarity and protic nature of solvents is crucial in Grignard reactions. Solvents can generally be categorized based on their polarity (polar or non-polar) and whether they are protic or aprotic. Protic solvents, like water and alcohols, contain a hydrogen atom bonded to an electronegative atom like oxygen, making them unsuitable for Grignard reactions as they can donate a proton, which would react unfavorably with the Grignard reagent.

In contrast, non-polar aprotic solvents do not favor these hydrogen-transfer reactions, making them ideal for Grignard reagent formation. For example, diethyl ether is non-polar and aprotic, meaning it lacks a hydrogen atom capable of forming such bonds. This characteristic ensures the stability and integrity of the Grignard reagent, allowing it to react predictively and effectively in subsequent synthesis steps.

Anhydrous Conditions in Grignard Reactions

Achieving and maintaining anhydrous conditions—conditions devoid of water—are vital in Grignard reactions. Water and other sources of moisture act as contaminants that can destroy the Grignard reagent by reacting it away. Since water is protic (capable of donating a proton), even minimal exposure can lead to the degradation of the Grignard compound into a hydrocarbon and magnesium hydroxide, rendering the reaction useless for further synthetic transformations.

• Utilizing anhydrous solvents, which are devoid of water, and ensuring that all glassware and materials are dry, aids in sustaining these conditions.
• In the lab, it is common to use dry argon or nitrogen gas to blanket the reaction mixture, keeping moisture out.

Ensuring the absence of water throughout the process isn't just recommended, it's absolutely necessary, as any inadvertent introduction of moisture compromises the entire reaction, making the Grignard reagent formation impossible.