11.2 : エーテルの物性

Ethers are colorless liquids with a pleasant smell, except for dimethyl ether and ethyl methyl ether which are gases at room temperature.

Ethers are polar molecules because of the difference in electronegativity between oxygen and carbon atoms.

Moreover, the bent geometry of the molecule results in a net dipole moment, implying that weak intermolecular dipole–dipole interactions exist between them.

Consequently, the boiling points of ethers are significantly lower than the isomeric alcohols but often close to those of hydrocarbons of comparable molecular weight.

For example, dimethyl ether, ethanol, and propane have similar molecular weights but considerably different boiling points.

The difference in the boiling points of ethers and alcohols arises from the fact that ethers, unlike alcohols, fail to engage in intermolecular hydrogen bonding. They lack a hydrogen atom bonded to the oxygen and thus cannot act as hydrogen bond donors.

The slightly higher boiling points of ethers than the comparable hydrocarbons can be attributed to the net dipole moment of ethers.

Additionally, ethers with larger alkyl groups have higher boiling points due to the intermolecular London dispersion forces, which become more assertive with increasing molecular mass and larger surface area.

The solubility of ethers is dictated by their ability to act as hydrogen bond acceptors. Ethers can form hydrogen bonds with water due to the electronegative oxygen atom, making them water-soluble.

The solubility of ethers, however, decreases with an increase in the number of carbon atoms. This is because the relative increase in hydrocarbon portions decreases the propensity of hydrogen bonding with water molecules.

Overall, ethers are relatively unreactive and make excellent solvents for a variety of organic reactions. Their low boiling points make them highly volatile, therefore they evaporate quickly after a reaction is complete.