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Here is a complete overview of non-aqueous solvents:
Non-Aqueous Solvents
Definition
A non-aqueous solvent is any solvent other than water used as a medium for chemical reactions or dissolution. Water is the most common solvent in everyday chemistry, but many reactions cannot be carried out in aqueous solution - either because:
- Reactants react with water
- Water's properties (polarity, boiling point) are unsuitable
- Certain compounds are insoluble in water
- Special chemical environments are required
Classification
Solvents are most conveniently classified by their electrolytic (ionising) characteristics:
1. Non-Ionising Solvents (Aprotic / Nonpolar)
- Do not ionise or dissociate
- Examples: benzene, carbon tetrachloride (CCl₄)
- Used for non-polar compounds, organic synthesis
2. Ionising Solvents
These can dissociate into ions and are further divided:
A. Protonic Solvents (contain hydrogen, can donate or accept protons):
| Type | Description | Examples |
|---|---|---|
| Protogenic (Acidic) | Donate protons | H₂SO₄, HF, CH₃COOH |
| Protophilic (Basic) | Accept protons | Liquid NH₃, pyridine, ethylenediamine |
| Amphiprotic | Both donate and accept | H₂O, alcohols |
B. Aprotic Ionising Solvents (no H, but still ionise via other mechanisms):
- Examples: liquid SO₂, N₂O₄, POCl₃, ionic liquids
Important Physical Properties
Key properties that determine a solvent's suitability:
| Property | Significance |
|---|---|
| Dielectric constant (ε) | Higher value = better ability to dissolve ionic compounds |
| Dipole moment | Indicates polarity |
| Boiling / melting point | Determines working temperature range |
| Viscosity | Affects reaction rates and diffusion |
| Heat of vaporization | Affects energy requirements |
A comparison of common solvents:
| Property | H₂O | Liquid NH₃ | Liquid SO₂ |
|---|---|---|---|
| Density (g/cc) | 0.96 | 0.68 | 1.46 |
| Critical temp (°C) | 374 | 132.4 | 157.5 |
| Equivalent conductance | 6×10⁻⁸ | 5×10⁻⁸ | 1×10⁻⁷ |
Common Inorganic Non-Aqueous Solvents
These are solvents other than water that are also not organic compounds:
- Liquid Ammonia (NH₃) - the most studied; dissolves alkali metals to form electrically conductive solutions containing solvated electrons. Chemistry largely parallels aqueous reactions.
- Liquid Sulfur Dioxide (SO₂) - versatile inert solvent for dissolving highly oxidising salts
- Anhydrous Hydrofluoric Acid (HF) - extremely strong protogenic solvent
- Dinitrogen Tetroxide (N₂O₄) - used in special oxidising environments
- Phosphoryl Chloride (POCl₃), Sulfuryl Chloride (SO₂Cl₂)
- Pure Sulfuric Acid (H₂SO₄) - superacid solvent
- Ionic Liquids - salts that are liquid at room temperature; tunable properties
Common Organic Non-Aqueous Solvents
| Solvent | Type | Uses |
|---|---|---|
| Ethanol / methanol | Protic polar | General dissolution, reactions |
| Acetone | Aprotic polar | Wide dissolving ability |
| Diethyl ether | Aprotic nonpolar | Extraction, Grignard reactions |
| DMSO (dimethyl sulfoxide) | Aprotic polar | SN2 reactions, biology |
| DMF (dimethylformamide) | Aprotic polar | Amide synthesis, polymers |
| Benzene / toluene | Nonpolar | Nonpolar compound reactions |
| Acetonitrile (MeCN) | Aprotic polar | Electrochemistry |
Types of Chemical Reactions in Non-Aqueous Solvents
- Metathetical (Precipitation) Reactions - two solutions mixed to form a precipitate (double decomposition)
- Acid-Base Reactions - defined relative to the solvent system (solvent-oriented definition)
- Redox Reactions - special reactions possible due to the solvent's unique properties (e.g., dissolving metals in liquid NH₃)
- Solvolysis Reactions - analogous to hydrolysis but occurring in non-aqueous media
Why Use Non-Aqueous Solvents?
- Reactions that are incompatible with water (e.g., Grignard reagents)
- Extending the temperature range of reactions (liquid NH₃ works at -33°C)
- Achieving different acid-base chemistry
- Dissolving compounds insoluble in water
- Industrial and pharmaceutical processing where water causes degradation
- Modern green chemistry using ionic liquids and supercritical fluids as tunable, sustainable media
The Chemistry LibreTexts chapter on non-aqueous media provides detailed coverage of individual solvents including liquid ammonia, liquid SO₂, HF, ionic liquids, and supercritical fluids.
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