(14)Ethers, Epoxides, and Thioethers Notes
Ethers, Epoxides, and Thioethers
Ethers
- Ethers have the general formula R—O—R', where R and R' can be alkyl or aryl groups.
- Ethers can be symmetrical (R = R') or unsymmetrical (R ≠ R').
Structure and Polarity
- Oxygen in ethers is sp3 hybridized.
- Ethers have a bent molecular geometry.
- The typical C—O—C bond angle is around 110°, close to the tetrahedral angle.
- Ethers have polar C—O bonds due to the electronegativity difference between carbon and oxygen.
Physical Properties of Ethers
- Ethers cannot hydrogen bond with each other, resulting in lower boiling points compared to alcohols of similar molecular weight.
- Ethers can act as hydrogen bond acceptors, forming hydrogen bonds with water and alcohol molecules.
- Ethers are useful solvents because they dissolve both polar and nonpolar substances and are generally unreactive towards strong bases.
Table 14-1. Comparison of the Boiling Points of Ethers, Alkanes, and Alcohols of Similar Molecular Weights
| Compound | Formula | MW | bp (°C) | Dipole Moment (D) |
|---|
| Water | H2O | 18 | 100 | 1.9 |
| Ethanol | CH<em>3CH</em>2–OH | 46 | 78 | 1.7 |
| Dimethyl Ether | CH<em>3–O–CH</em>3 | 46 | –25 | 1.3 |
| Propane | CH<em>3CH</em>2CH3 | 44 | –42 | 0.1 |
| n-Butanol | CH<em>3CH</em>2CH<em>2CH</em>2–OH | 74 | 118 | 1.7 |
| Tetrahydrofuran | C<em>4H</em>8O (cyclic) | 72 | 66 | 1.6 |
| Diethyl Ether | CH<em>3CH</em>2–O–CH<em>2CH</em>3 | 74 | 35 | 1.2 |
| Pentane | CH<em>3CH</em>2CH<em>2CH</em>2CH3 | 72 | 36 | 0.1 |
- Alcohols exhibit hydrogen bonding, leading to significantly higher boiling points.
- Ethers have boiling points similar to alkanes with comparable molecular weights.
Table 14-2. Physical Properties of Ether
| Name | Structure | mp (°C) | bp (°C) | Density (g/mL) |
|---|
| Dimethyl Ether | CH<em>3−O−CH</em>3 | -140 | -25 | 0.66 |
| Ethyl Methyl Ether | CH<em>3CH</em>2−O−CH3 | | 8 | 0.72 |
| Diethyl Ether | CH<em>3CH</em>2−O−CH<em>2CH</em>3 | -116 | 35 | 0.71 |
| Di-n-propyl Ether | CH<em>3CH</em>2CH<em>2−O−CH</em>2CH<em>2CH</em>3 | -122 | 91 | 0.74 |
| Diisopropyl Ether | (CH<em>3)</em>2CH−O−CH(CH<em>3)</em>2 | -86 | 68 | 0.74 |
| 1,2-Dimethoxyethane (DME) | CH<em>3−O−CH</em>2CH<em>2−O−CH</em>3 | -58 | 83 | 0.86 |
| Methyl Phenyl Ether (Anisole) | CH3−O | -37 | 154 | 0.99 |
| Diphenyl Ether | | 27 | 259 | 1.07 |
| Furan | | -86 | 32 | 0.94 |
| Tetrahydrofuran (THF) | | -108 | 65 | 0.89 |
| 1,4-Dioxane | | 11 | 101 | 1.03 |
Solvation of Ions with Ether
- Ionic substances like lithium iodide (LiI) can dissolve in ethers because the ether's lone pairs strongly solvate the small lithium cation (Li+).
- Ethers are poor at solvating small anions because they cannot act as hydrogen bond donors.
Ether Complexes
- Grignard reagents: Ethers stabilize Grignard reagents by complexing with the magnesium ion, helping to keep the reagent in solution.
- Electrophiles: The nonbonding electrons of ethers can stabilize electrophiles like borane (BH3).
Crown Ether Complexes
- Crown ethers can complex with metal cations in the center of their ring structure.
- The size of the crown ether ring determines its selectivity for different cation sizes.
- Complexation with crown ethers can enable polar inorganic salts to dissolve in nonpolar organic solvents.
Nomenclature of Ethers
- Common Names: Name the two alkyl groups attached to the oxygen atom, listing them in alphabetical order, and add the word