REACTIONS OF ETHERS

Reactions of Ethers

Although ethers are relatively inert toward reaction, they usually show good solvent properties for many nonpolar organic compounds. This strong dissolving power coupled with low reactivity makes ethers good solvents in which to run reactions.

An acid‐catalyzed cleavage that occurs when hydriodic acid (HI) mixes with ethers is the most significant reaction that ethers experience. This reaction proceeds via a nucleophilic substitution mechanism. Primary and secondary alkyl ethers react by an S _N2 mechanism, while tertiary, benzylic, and alcylic ethers cleave by an S _N1 mechanism. A typical S _N2 reaction would be the reaction of ethylisopropyl ether with HI. The mechanism for this reaction is:

Notice that for S _N2 substitution, the alkyl halide came from the less sterically hindered group. For S _N1 type reactions, the alkyl halide forms from the fragment of the original molecule that forms the more stable cation. Thus, the reaction of t‐butyl ethyl ether with HI gives t‐butyl iodide and ethyl alcohol. The following mechanism occurs:

Notice that if the original ionization of the t‐butyl ethyl ether formed a t‐butoxide ion and an ethyl carbocation, this would be a less stable arrangement. (Remember, the order of stability of carbocations is 3° > 2° > 1°.)

PHYSICAL AND CHEMICAL PROPERTIES OF ETHERS

Physical And Chemical Properties Of Ether

Ether is an organic compound containing an oxygen atom bonded to two same or different alkyl or aryl groups. The general formula for ethers can be R-O-R, R-O-Ar or Ar-O-Ar, where R represents an alkyl group and Ar represents an aryl group.

Ethers are generally classified into two categories on the basis of substituent groups attached: symmetrical ether (when two identical groups are attached to the oxygen atom) and asymmetrical ether (when two different groups are attached to the oxygen atom). Ethers exhibit a wide range of physical and chemical properties.

Physical Properties of Ethers

1. An ether molecule has a net dipole moment due to the polarity of C-O bonds.
2. The boiling point of ethers is comparable to the alkanes but much lower than that of alcohols of comparable molecular mass despite the polarity of the C-O bond. The miscibility of ethers with water resembles those of alcohols.
3. Ether molecules are miscible in water. This is attributed to the fact that like alcohol, the oxygen atom of ether can also form hydrogen bonds with a water molecule.
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Chemical Properties of Ethers

Ethers generally undergo chemical reactions in two ways:

1. Cleavage of C-O bond

Ethers are generally very unreactive in nature. When an excess of hydrogen halide is added to the ether, cleavage of C-O bond takes place leading to the formation of alkyl halides. The order of reactivity is given as HI>HBr>HCl

R-O-R + HX → RX + R-OH

2. Electrophilic Substitution

The alkoxy group in ether activates the aromatic ring at ortho and para positions for electrophilic substitution. Common electrophilic substitution reactions are halogenation, Friedel Craft’s reaction etc.

3. Halogenation of Ethers

Aromatic ethers undergo halogenation, for example, bromination, upon the addition halogen in the presence or absence of a catalyst.

4. Friedel Craft’s Reaction of Ethers

Aromatic ethers undergo Friedel Craft’s reaction for example addition of alkyl or acyl group upon the reaction with alkyl or acyl halide in the presence of a Lewis acid as catalyst.

PREPARATION METHODS OF ETHERS

Preparation of Ethers

Preparation of Ether – Different Methods

There are various methods for the preparation of Ethers. Ethers are the organic compounds containing an oxygen atom bonded to two same or different alkyl or aryl groups. The general formula for ethers can be R-O-R, R-O-Ar or Ar-O-Ar, where R represents an alkyl group and Ar represents an aryl group.

Ethers are generally classified into two categories on the basis of substituent group attached: symmetrical ethers (when two identical groups are attached to the oxygen atom) and asymmetrical ethers (when two different groups are attached to the oxygen atom). With advancements in technologies, others are synthesized in industries in many ways.

1. Preparation of Ethers by Dehydration of Alcohols

In the presence of protic acids (sulphuric acid), alcohols undergo dehydration to produce alkenes and ethers under different conditions. For example: in the presence of sulphuric acid, dehydration of ethanol at 443 K yields ethene whereas it yields ethoxyethane at 413 K. This is an ideal method of preparation through primary alcohols.

The preparation of ethers by dehydration of alcohol is a nucleophilic substitution reaction. The alcohol involved in reaction plays two roles: one alcohol molecule acts as a substrate while the other acts as a nucleophile. It can follow either an SN1 or SN2 mechanism. 

The choice of the mechanism depends on whether the protonated alcohol loses water before or simultaneously upon the attack of a second alcohol molecule. Generally, the secondary and tertiary alcohols follow the SN1 mechanism while the primary alcohols follow the SN2 mechanism.

2. Preparations of Ethers by Williamson Synthesis

Williamson synthesis is an important method for the preparation of symmetrical and asymmetrical ethers in laboratories. In this method, an alkyl halide is reacted with sodium alkoxide which leads to the formation of ether. The reaction generally follows the SN2 mechanism for primary alcohol.

As we know alkoxides are strong bases and they can react with alkyl halides leading to elimination reactions. Williamson synthesis exhibits higher productivity in the case of primary alkyl halides. In the case of secondary alkyl halides, elimination competes with substitution whereas, we observe the formation of elimination products only in the case of tertiary alkyl halides.