Ethers

Definition of ethers and their Classification

Definition
Ethers are a class of organic compounds which contain an ether group — a bivalent oxygen atom directly bonded to two carbon atoms of alkyl or aryl groups (C-O-C). Ethers are alkoxy (RO–)-substituted alkanes, alkenes, and alkynes. (As with alcohols, only saturated carbon atoms may be substituted in alkenes and alkynes).

Functional Group of Ether
Ether linkage (-O-) is the functional group of ether.

Type Formula of ethers
R–O–R'

General Formula of ethers

Ethers form the homologous series with following general formulae:

1.         C_nH_2n+1----- OR           (n = 1,2,3,4, ------)

2.         C_nH_2n+2O                     (n = 2,3,4, ------)

Derivation from Alcohols
They may be regarded as alkyl oxide or anhydride of alcohols obtained by the removal of water from two molecules of alcohols.

  

Derivation from Alkanes
Ethers may be regarded as the derivative of alkanes that are obtained by replacing its one hydrogen atom by alkoxy group.

Functional Isomeric with

Alcohols

Types of Isomerism shown

Metamerism

Positional Isomerism

Functional isomerism

Classification of ethers

Depending on the groups at R and R', ethers are classified into two types:

1) Simple ethers or symmetrical ethers

2) Mixed ethers or asymmetrical ethers

Physical Properties of diethyl ether

1. It is a colourless, low boiling and highly inflammable and volatile liquid.

2. It has sweet, sickly characteristic ethereal smell.

3. It is a good solvent for fats, oils, gums and a large number of organic compounds mainly due to its chemical inactivity.

4. It is highly and readily soluble in concentrated H2SO4, a characteristic of oxygen containing compounds. [This property is used as a test to distinguish between ethers and saturated hydrocarbons.

Uses of diethyl ether

1. It is used as general anesthetic (because it depresses the activity of CNS).

2. It is used as refrigerant.

3. It is used as a solvent for fats, oils, gums, etc.

4. It is used in making artificial silk and smokeless gun powder.

5. It is used as a medium (solvent) in synthesis of Grignard’s reagent & Wurtz reaction.

General Methods of Preparation of diethyl ether

1. By Williamson’s Synthesis/ (Nucleophilic displacement of alkyl halides by alkoxides)

2. By condensation of excess of alcohols

3. By heating alkyl halides with dry silver oxide

4. By the action of alcoholic silver nitrate on alkyl halides

1. By Williamson’s Synthesis/Williamson ether synthesis

Ethers can be indirectly prepared from alcohols by first converting them to sodium alkoxide (alcoholate) and then reacting it with alkyl halide. Alkoxide (RO^-) being a strong nucleophilic reacts with alkyl halides in S_N2 manner forming ether.

This method usually does not work well for aryl halides (e.g. bromobenzene 

Likewise, this method only gives the best yields for primary halides. Secondary and tertiary halides are prone to undergo E₂ elimination on exposure to the basic alkoxide anion used in the reaction due to steric hindrance from the large alkyl groups.

2. By condensation of excess of alcohols

Simple ethers can be prepared by heating excess of alcohols with conc. H₂SO₄ at 140°C. One molecule water is removed from two molecules of alcohols producing ether.

The reaction completes in two steps:

3. By heating alkyl halides with dry silver oxide

Alkyl halides on heating with dry silver oxide produces ether.

4.  By the action of alcoholic silver nitrate on alkyl halides 

Chemical Properties of diethyl ether

 

Action of halogen acid (Ether Cleavage)

Ethers split to form one molecule of alcohol and alkyl halide when treated with halogen acids in cold. At high temperature, simple ethers give an alkyl halide and water with excess of halogen acids.

Although ethers resist hydrolysis, they are cleaved by mineral acids such as hydrobromic acid and hydroiodic acid. Hydrogen chloride cleaves ethers only slowly. Methyl ethers typically afford methyl halides:

These reactions proceed via onium intermediates, i.e. [RO(H)CH₃]⁺Br^-.

Some ethers rapidly cleave with boron tribromide (even aluminium chloride is used in some cases) to give the alkyl bromide.^[Depending on the substituents, some ethers can be cleaved with a variety of reagents, e.g. strong base.