Physical and chemical properties of ethers

PHYSICAL PROPERTIES:

  1. Dimethyl ether and ethyl methyl ether are gases. All others are colourless liquids with pleasant odours.
  2. Lower ether are highly colatile and very flammable.
  3. Boiling points of ethers show a gradual increase with the increase in molecular weight.

  4. Ethers have lower boiling points than isomeric alcohols. This is because ether molecules cannot form hydrogen bonds with each other as they have no –OH groups. The boiling points of ethers are close to the boiling points of alkanes. For example, diethyl ether (MW=74) boils at 35 C; n-pentane (MW=72) boils at 36 C.
  5. Ethers are slightly soluble in water. This is because they can form hydrogen ties with standard water.
  6. IR Spectrum. Ethers show a strong, board band due to C-O stretching in the 1060-1300 cm range.
GUIDELINES: Comparison of Boiling Points and Solubilities of Alkanes, Ethers, and Alcohols


The boiling points and solubility characteristic of ethers may be understood from their structures. Ethers, unlike water or alcohols, have no hydrogens directly attached to the oxygen atom. As a result there is no hydrogen bonding between one ether molecule and another, as there is between alcohol molecules. We can therefore predict that the boiling points of ethers will be considerably lower than those of alcohols, and about the same as those of alkanes, of comparable molecular weights.
With regard solubility in water, we may expect ethers to behave much like alcohols, is capable of hydrogen bonding with water molecules. Alkanes, of course, are insoluble because no hydrogen bonding with water is possible.

Chemical Properties:

Ethers are quite stable compounds. They do not react with bases, reducing agents, oxidising agents, and active metals.

Reaction Involving Alkyl Groups:

Halogenation. When ethers are treated with Cl2 or Br2 in the dark, substitution products are obtained. The levelof replacement relies on the response conditions. Halogenation preferentially take place at carbon atoms.

 Reaction at Ethereal Oxygen:

Formation of Oxonium oxygen salts. Ethers react with cold concentrated H2SO4 or HCl to give oxonium salts. Ethers donate a pair of electron to a hydrogen ion to form these salts.The oxonium salt once established are disolveable in the acid solution. The ethers can be regenerated from the oxonium salts by treatment with standard water.

This reaction can be used to distinguish between ethers and alkanes since alkanes do not react with concentrated acids.
Formation of Peroxides (Autooxidation). Ethers combine with atmospheric oxygen to form peroxides.

This is a very undesirable reaction. All ethers that have been exposed to the atmosphere for any length of time invariably contain peroxides. Peroxides are hazardous because they decompose violently at high temperature. When ether that contains peroxides is distilled, the peroxides concentrated in the residue. At the end of distillation, the temperature increases, and the residual peroxides may expoled. For this reason, ethers should never be evaporated to dryness.


Reaction Involving C-O Bonds:

Reaction with Dilute H2SO4(Hydrolysis). When ethers are heated with dilute sulphuric  acid, alcohols formed.
Reaction With PCl5. Ethers react with phosphorus pentachloride to give alkyl chlorides.


      Cleavage by Acids. Ethers react with hot concentrated Hl or HBr to give an alcohol and an alkyl halide.

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