Question

Which of the following compounds would make the best solvent in a Grignard synthesis with an alkylbromide? A. \(\mathrm{H}_2 \mathrm{O}\) B. \(\left(\mathrm{C}_2 \mathrm{H}_5\right)_2 \mathrm{O}\) C. \(\mathrm{C}_2 \mathrm{H}_3 \mathrm{O}_2 \mathrm{Na}\) D. \(\mathrm{C}_2 \mathrm{H}_3 \mathrm{OH}\)

Short answer

Option B: \left( \mathrm{C}_2 \mathrm{H}_5 \right)_2 \mathrm{O} is the best solvent.

Step-by-step solution

- Grignard Reagent Characteristics

Understand that a Grignard reagent is highly reactive and can react with water and alcohols. It must be prepared in an anhydrous (without water) environment to avoid decomposition.

- Eliminate Water and Alcohol

Water \(\text{Option A: } \mathrm{H}_2 \mathrm{O}\) and ethanol \(\text{Option D: } \mathrm{C}_2 \mathrm{H}_5\mathrm{OH}\) contain \(\text{-OH}\) groups, which would react with the Grignard reagent, making them unsuitable solvents.

- Eliminate Sodium Acetate

Sodium acetate \(\text{Option C: } \mathrm{C}_2 \mathrm{H}_3 \mathrm{O}_2 \mathrm{Na}\) is an ionic compound and would not be suitable as it could disrupt the formation of the Grignard reagent.

- Select Ether

Diethyl ether \(\text{Option B: } \left(\text{C}_2 \text{H}_5\right)_2 \text{O}\) is a non-reactive solvent that is commonly used in Grignard syntheses because it does not contain reactive \(\text{-OH}\) groups and provides a suitable environment for the reaction.

Key concepts

Grignard Reagent Preparation

Grignard reagents are a type of organomagnesium compound used in various organic synthesis reactions. They are made by reacting an alkyl or aryl halide with magnesium metal in an anhydrous environment. This reaction forms an alkyl or aryl magnesium halide, known as the Grignard reagent. For instance, when bromobutane and magnesium react in an anhydrous ether solution, they form butylmagnesium bromide, a typical Grignard reagent.

These reagents are very reactive and must be prepared carefully. Water or any source of protons can destroy the Grignard reagent by reacting with it to form a hydrocarbon and magnesium halide. This is why it is essential to keep the reaction environment completely free of moisture. To emphasize, even trace amounts of water can spoil the whole batch, so the reaction vessels and solvents must be dried thoroughly before use.

Anhydrous Conditions

Maintaining anhydrous conditions is critical for the successful preparation and use of Grignard reagents. Water (H₂O) or any moisture can quench the Grignard reagent, rendering it useless. Here are some key points to ensure anhydrous conditions:

• Drying Apparatus: Use drying tubes, typically containing a drying agent like calcium chloride, to prevent atmospheric moisture from entering the reaction apparatus.
• Drying Solvents: Diethyl ether and tetrahydrofuran (THF) are commonly used solvents that must be dried to remove any traces of water. Drying can be done using desiccants such as sodium/benzophenone or molecular sieves.
• Sealed Systems: Grignard reagents should be prepared in a sealed system to avoid moisture from the air. A septum can be used to withdraw reagents with a syringe under an inert atmosphere (usually nitrogen or argon).

Maintaining these anhydrous conditions is vital because the reactivity of Grignard reagents makes them highly sensitive to even the tiniest amounts of water.

Solvent Selection for Grignard Reaction

Choosing the right solvent is crucial for the success of a Grignard synthesis. The solvent must not contain any functional groups that would react with the Grignard reagent. Let's dig deeper into why diethyl ether is an excellent choice:

• No Reactive Groups: Diethyl ether \(\text{Option B: }(\text{C}_2 \text{H}_5)_2 \text{O}\) lacks \text{-OH} groups or other reactive functionalities that would otherwise react with the Grignard reagent.
• Stabilization: Diethyl ether stabilizes the Grignard reagent by forming a complex with the magnesium atom, thus helping the reaction proceed smoothly.
• Low Reactivity: Ethers, in general, have low chemical reactivity and high boiling points, which is ideal for maintaining an anhydrous environment during the reaction.

In contrast, water (H₂O), ethanol (\text{C}_2 \text{H}_5 \text{OH}) and sodium acetate (\text{C}_2 \text{H}_3 \text{O}_2 \text{Na}) are poor choices. Water and ethanol contain \text{-OH} groups that would react with the Grignard reagent, and sodium acetate, being ionic, could also interfere with the sensitive Grignard formation process. Therefore, diethyl ether offers the needed stability and inertness, making it the best solvent choice for Grignard syntheses.