Ethers are the organic compounds in which an oxygen atom is connected to two alkyl or aryl groups. These are represented by the general formula R-O-R' where R and R' may be alkyl or aryl groups.
Classification of Ethers
On the basis of similarity or dissimilarity of alkyl or aryl groups attached to the central oxygen atom, ethers are classified into the two following categories:
Simple or Symmetrical ethers:
If R = R', then the ether is called simple or symmetrical ether. For example:
Mixed or Unsymmetrical ethers:
If R ≠ R’, then ether is called mixed or unsymmetrical ether. For example:
Nomenclature of Ethers
Common naming system:
First identify the alkyl or aryl groups attached to the central oxygen atom.
Then arrange the alkyl or aryl groups alphabetically and write a suffix ether
IUPAC naming system:
In the JUPAC system, ethers are regarded as ‘alkoxy alkanes’ in which the ethereal oxygen (to which the two alkyl or aryl groups are attached) is taken along with smaller alkyl group while the bigger alkyl group is regarded as a part of the alkane.
The oxygen atom is sp3 hybridised and the two bond pairs and two lone pairs of electrons on oxygen are arranged approximately in a tetrahedral arrangement. The R−O−R’ bond angle is more than the typical tetrahedral angle of 109°. This is due to the interaction between the two bulky (–R) groups. The C–O bond length (141 pm) is almost the same as in alcohols.
Preparation of Ethers
Ethers by dehydration of alcohols
Alcohols undergo dehydration in the presence of protic acids (H2SO4, H3PO4). The formation of the reaction product, alkene or ether depends on the reaction conditions. For example, ethanol is dehydrated to ethene in the presence of sulphuric acid at 443 K. At 413 K, ethoxyethane is the main product.
The formation of ether is a nucleophilic bimolecular reaction (Sn2) involving the attack of alcohol molecule on a protonated alcohol, as indicated below:
Acidic dehydration of alcohols, to give an alkene is also associated with substitution reaction to give an ether.
It is an important laboratory method for the preparation of symmetrical and unsymmetrical ethers. In this method, an alkyl halide is allowed to react with sodium alkoxide.
Ethers containing substituted alkyl groups (secondary or tertiary) may also be prepared by this method. The reaction involves Sn2 attack of an alkoxide ion on primary alkyl halide.
Ethers containing substituted alkyl groups (2° or 3°) may also be prepared by this method.
On the other hand alkyl halides (RX) yield only alkenes as main product.
Phenols are also converted to ethers by this method. In this, phenol is used as the phenoxide moiety.
Physical Properties of Ethers
· Physical state, colour and odour: Dimethyl ether and ethyl methyl ether is gas at ordinary temperature while the other lower homologues of ethers are colourless liquid with characteristic 'ether smell'.
· Dipole nature: Ethers have a tetrahedral geometry i.e., oxygen is sp3 hybridized. The C— O—C angle in ethers is 110°. Because of the greater electronegativity of oxygen than carbon, the C—O bonds are slightly polar and are inclined to each other at an angle of 110°, resulting in a net dipole moment.
Bond angle of ether is greater than that of tetrahedral bond angle of 109°28'.
· Solubility and boiling point: Due to the formation of less degree of hydrogen bonding, ethers have lower boiling point than their corresponding isomeric alcohols and are slightly soluble in water.
Chemical Reactions of ether
Cleavage of C–O bond in ethers
Reaction with hydrogen halides (HX):
For symmetrical ethers:
R ‒ O ‒ R + HX → 2RX + H2O
For asymmetrical ethers:
R ‒ O ‒ R’ + HX → RX + R’‒OH
For a given ether, the reactivity of hydrogen halides follows the order:
HCl < HBr < HI
For aromatic ethers:
In case of aromatic ether, reaction with HI results in the cleavage of R−O bond due to the stability of aryl-oxygen bond.
When one of the alkyl group is a tertiary group, the halide formed is a tertiary halide. This is due to the formation of more stable 3° carbonium ion.
The reaction of an ether with concentrated HI starts with protonation of ether molecule.
The reaction takes place with HBr or HI because these reagents are sufficiently acidic.
Iodide is a good nucleophile. It attacks the least substituted carbon of the oxonium ion formed in step 1 and displaces an alcohol molecule by Sn2 mechanism.
Thus, in the cleavage of mixed ethers with two different alkyl groups, the alcohol and alkyl iodide formed, depend on the nature of alkyl groups. When primary or secondary alkyl groups are present, it is the lower alkyl group that forms alkyl iodide (Sn2 reaction).
When HI is in excess and the reaction is carried out at high temperature, ethanol reacts with another molecule of HI and is converted to ethyl iodide.
Electrophilic substitution of ethers
The alkoxy group (-OR) on benzene ring is o- and p-directing and activates the aromatic ring towards electrophilic substitution.
Halogenation of ethers
Phenylalkyl ethers undergo usual halogenation in the benzene ring, e.g., anisole undergoes bromination with bromine in ethanoic acid even in the absence of iron (III) bromide catalyst. It is due to the activation of benzene ring by the methoxy group. Para isomer is obtained in 90% yield.
Anisole undergoes Friedel-Crafts reaction, i.e., the alkyl and acyl groups are introduced at ortho and para positions by reaction with alkyl halide and acyl halide in the presence of anhydrous aluminium chloride (a Lewis acid) as catalyst.
Nitration of ethers
Anisole reacts with a mixture of concentrated sulphuric and nitric acids to yield a mixture of ortho and para-nitroanisole.