Physical Properties of Ether

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Jan 23, 2023
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- Properties of Ethers
- Reactivity of Ethers

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Dimethylether and ethyl methyl ether are gases at ordinary temperature. The other lower homologues are colorless, pleasant smelling, volatile liquids with typical ether smell.

Boiling points

The C - O bonds in ether are polar and thus ethers have a net dipole moment. The weak polarity of ethers do not appreciably affect their boiling points which are comparable to those of the alkenes of comparable molecular mass. Ethers have much lower boiling points as compared to isomeric alcohols. This is because alcohols molecules are associated by hydrogen bonds while ether molecules are not.

Solubility

Ethers containing up to 3 carbon atoms are soluble in water, due to their hydrogen bond formation with water molecules.

The solubility decreases with increase in the number of carbon atoms. The relative increase in the hydrocarbon portion of the molecule decreases the tendency of H-bond formation. Ethers are appreciably soluble in organic solvents like alcohol, benzene, acetone etc.

Structure of Ethers

Ethers are a class of organic compounds that contain an sp³ hybridized oxygen between two alkyl groups and have the formula R-O-R'. these compounds are used in dyes, perfumes, oils, waxes and other industrial uses. Aliphatic ethers have no aryl groups directly attached to the ether oxygen.

Examples of Aliphatic Ethers

Aromatic ethers have at least one aryl ring directly attached to the ether oxygen. In aryl ethers, the lone pair elections on oxygen are conjugated with the aromatic ring which significantly changes the properties of the ether.

Aryl Ethers.svg

Example of Aromatic Ethers

The sp³ hybridization of oxygen gives ethers roughly the same geometry as alcohols and water. The R-O-R' bond angle is close to what is expected in a tetrahedral geometry. The bond angle of dimethyl ether is 112^o which is larger than the H-O-H bond angle in water (104.5^o) due to the steric repulsion of the methyl groups.

Ether Bond Angle.svg

The presence of an electronegative oxygen atom gives ethers a small dipole moment.

Comparisons of Physical Properties of Alcohols and Ethers

Ethers, unlike alcohols, have no hydrogen atom on the oxygen atom (that is, no OH group). Therefore, there is no intermolecular hydrogen bonding between ether molecules, which makes their boiling points much lower than an alcohol with similar mass. Despite the presence of a small dipole moment, ethers have boiling points that about the same alkanes of comparable molar mass. (Table 18.1.2).

Table 18.1.2 Comparison of Boiling Points of Alkanes, Alcohols, and Ethers
Condensed Structural Formula	Name	Molar Mass	Boiling Point (°C)	Intermolecular Hydrogen Bonding in Pure Liquid?
CH₃CH₂CH₃	propane	44	–42	no
CH₃OCH₃	dime thyl ether	46	–25	no
CH₃CH₂OH	ethyl alcohol	46	78	yes
CH₃CH₂CH₂CH₂CH₃	pentane	72	36	no
CH₃CH₂OCH₂CH₃	diethyl ether	74	35	no
CH₃CH₂CH₂CH₂OH	butyl alcohol	74	117	yes

Ether molecules do have an oxygen atom, however, and engage in hydrogen bonding with water molecules. Consequently, an ether has about the same solubility in water as the alcohol that is isomeric with it. For example, dimethyl ether and ethanol (both having the molecular formula C₂H₆O) are completely soluble in water, whereas diethyl ether and 1-butanol (both C₄H₁₀O) are barely soluble in water (8 g/100 mL of water).

Peroxide Formation

Many ethers can react with oxygen to form explosive peroxide compounds n a free radical process called autoxidation. For this reason ethers should not be stored for long periods of time and should not be stored in glass bottles. The danger is particularly acute when ether solutions are distilled to near dryness. The hydroperoxides can become more concentrated during a distillation because they tend to have a slightly higher boiling point than the corresponding ether. Before performing an ether distillation great care should be taken to test for the presence of peroxides.

Contributors

Binod Shrestha (University of Lorraine)

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