Question

(a) Give four examples of non-aqueous solvents commonly used in organic chemistry, and give one example of a reaction that is carried out in each solvent. (b) Assess the relative importance of the use of aqueous and nonaqueous media in organic and inorganic general synthesis.

Short answer

Non-aqueous solvents like diethyl ether, DCM, THF, and DMSO are crucial in specific reactions where water is incompatible. Aqueous media provide environmental benefits and simplicity, while non-aqueous media are indispensable for certain reaction compatibilities.

Step-by-step solution

Identify Non-Aqueous Solvents

Non-aqueous solvents are solvents that do not contain water. They are typically organic and often used where water would disrupt or be incompatible with a reaction. Four common examples of non-aqueous solvents used in organic chemistry are: 1. Diethyl ether 2. Dichloromethane (DCM) 3. Tetrahydrofuran (THF) 4. Dimethyl sulfoxide (DMSO)

Provide Examples of Reactions

Now, let's provide an example for each of these solvents: 1. **Diethyl Ether**: Used in Grignard reactions. Grignard reagents, which are organomagnesium compounds, react with carbonyl compounds to form alcohols. Diethyl ether is crucial here because it stabilizes the Grignard reagent. 2. **Dichloromethane (DCM)**: Used in the extraction and purification steps in organic synthesis. An example is the chlorination of acetyl chloride, where DCM is used as a solvent due to its stability and inertness under reaction conditions. 3. **Tetrahydrofuran (THF)**: Used in the polymerization of epoxides, where THF serves as both a solvent and a reactant. An example is the polymerization of ethylene oxide to form polyethylene glycol. 4. **Dimethyl Sulfoxide (DMSO)**: Used in SN2 reactions, such as the reaction between bromoethane and sodium cyanide to form ethyl cyanide, due to its ability to dissolve a wide range of polar compounds.

Assess Importance of Solvent Types

For organic and inorganic synthesis, the choice of aqueous vs non-aqueous solvents depends on various factors such as solubility, reactivity, and stability of reactants and products. - **Aqueous Media**: Often preferred for reactions where water can participate, stabilize ionic species, or where solvents are required to be environmentally benign. Water is non-toxic and inexpensive, making it ideal for many large-scale reactions. - **Non-Aqueous Media**: Essential for reactions involving compounds that are sensitive to water or where water could interfere or negatively impact yields. Organic solvents often dissolve a wider range of organic compounds and can help facilitate reactions that require non-polar environments. In summary, non-aqueous solvents are crucial in situations where water is incompatible with the reagents or might cause side reactions, while aqueous solvents are chosen for eco-friendliness and cases where water can aid the reaction process.

Key concepts

Grignard reactions

Grignard reactions are a cornerstone of organic chemistry, especially when it comes to forming carbon-carbon bonds. These reactions involve Grignard reagents, which are organomagnesium compounds represented typically by the formula RMgX, where R is an organic group and X is a halogen. The uniqueness of Grignard reactions stems from the highly reactive nature of these organomagnesium compounds.

Grignard reactions are most commonly performed in non-aqueous solvents such as diethyl ether or tetrahydrofuran because water can deactivate the Grignard reagent by converting it into a hydrocarbon through acid-base reaction. In these reactions, the Grignard reagent is combined with carbonyl compounds such as ketones or aldehydes to produce alcohols. This capability to form alcohols makes Grignard reactions invaluable in synthetic organic chemistry.

• Non-polar environment: Essential for maintaining the activity of the Grignard reagent.
• Examples: Formation of primary, secondary, or tertiary alcohols depending on the carbonyl compound used.
• Reactive species: Grignard reagent and carbonyl compound.

Dichloromethane

Dichloromethane, often abbreviated as DCM, is a versatile non-aqueous solvent widely used in organic chemistry. It is known for its effectiveness in both extraction and purification processes due to its properties such as low boiling point, moderate polarity, and its ability to dissolve a wide array of organic compounds.

One typical application of DCM is in the chlorination reaction of organic compounds, such as in the preparation of acetyl chloride. In such processes, DCM serves as a stable and inert medium that allows the reaction to proceed without unwanted side reactions. Its inertness is key, as DCM does not interact with the reactants or products during the reaction.

• Stability: DCM remains unimpaired during most chemical reactions.
• Use in extractions: Efficiently separates organic compounds from aqueous phases.
• Solubility: Dissolves various organic substances, facilitating numerous reaction types.

Tetrahydrofuran

Tetrahydrofuran (THF) is a cyclic ether that’s prized for its solvation capabilities, particularly in polymerization reactions. Its polar nature and ability to both dissolve and participate in chemical reactions mark it as a dual-function solvent in many synthetic pathways.

THF is often chosen for the polymerization of epoxides such as ethylene oxide, leading to the creation of polyethylene glycol. In such polymerization, THF acts not just as a medium but also as a participant, sometimes forming part of the polymer chain itself. The solvent's effectiveness stems from its ability to stabilize intermediates, enabling a controlled reaction environment.

• Polar solvent: Provides a suitable environment for ionic and polar reactions.
• Versatility: Bridges roles between solvent and reactant.
• Application: Used in diverse reactions like polymerizations, Grignard reactions, and other organometallic syntheses.

Dimethyl sulfoxide (DMSO)

Dimethyl sulfoxide (DMSO) is a highly polar non-aqueous solvent appreciated for its remarkable ability to dissolve a wide variety of polar organic compounds. Its unique boiling point and miscibility with water and organic solvents make it indispensable in organic synthesis, especially in nucleophilic substitution reactions.

One common application of DMSO is in the SN2 reaction, such as the reaction that transforms bromoethane into ethyl cyanide using sodium cyanide. DMSO's polarity and solvating power can stabilize the transition state of the reaction, thus increasing the reaction rate and yield. Its utility is enhanced by its compatibility with a broad range of compounds.

• High polarity: Facilitates various chemical reactions, particularly those involving ionic species.
• Versatility: Solvent for reactions where a wide range of solutes need to be dissolved.
• Reactivity: Stabilizes reaction intermediates, improving reaction outcomes.