Methods of Preparation of Ether: Preparation, Formation

Preparation Methods of Ethers: Ethers are organic compounds with an oxygen atom bonded to two alkyl groups. Ethers are classified as symmetrical and unsymmetrical ethers depending upon whether they have the same or different alkyl groups attached on either side of the oxygen atom. In this article, we will discuss everything about the preparation methods of Ether in detail.

Classification of Ethers

Depending on whether the alkyl groups on either side of the oxygen atom are the same or different, ethers are categorised as symmetrical or unsymmetrical. They can be written as:

While diethyl ether is symmetrical with an ethyl group on both sides of oxygen, Ethyl methyl ether is unsymmetrical with one ethyl and one methyl group on two sides of oxygen.

Ethers are considered derivatives of water, wherein both hydrogen atoms of water are replaced by alkyl groups. On the other hand, they can also be considered derivatives of alcohol with a hydroxyl group replaced by the alkyl group.

Preparation of Ether

Ethers can be prepared in the laboratory from alcohol and alkyl halides through Williamson synthesis. Both dehydration of alcohol and Williamson synthesis are popular methods of preparation of ethers. However, other ways of laboratory preparation include:

1. Passing alcohol vapours over \({\rm{A}}{{\rm{l}}_2}{{\rm{O}}_3}\)
2. Heating alkyl halides with Silver Oxide
3. The reaction of diazomethane with alcohol

We will look at the preparation of ethers by dehydration of alcohols and by Williamson synthesis.

Preparation of Ethers by Dehydration of Alcohols

Symmetrical ethers can be prepared by heating an excess of alcohol in the presence of sulphuric acid at \({140^ \circ }{\rm{C}}\) or \(413\,{\rm{K}}\).  The reaction is a dehydration reaction and usually takes place in the presence of protic acids, such as sulphuric acid, phosphoric acid, etc. The condition at which the reaction is carried out is extremely significant, as the dehydration of alcohol can result in either alkene or Ether.

Example: Dehydration of ethanol by using sulphuric acid at \(413\,{\rm{K}}\) or \({140^ \circ }{\rm{C}}\) gives diethyl Ether, as shown:

\({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}} + {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\left( {{{140}^ \circ }{\rm{C}}} \right) \to {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2} – {\rm{O – C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_3}\)

However, the same ethanol, at \(443\,{\rm{K}}\) or \({170^ \circ }{\rm{C}}\) (and also with concentrated sulphuric acid), can result in ethene, an alkene.

\({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}} + {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\left( {{{170}^ \circ }{\rm{C}}} \right) \to {\rm{C}}{{\rm{H}}_2} = {\rm{C}}{{\rm{H}}_2}\)

Preparation of Ether by dehydration of alcohol is a nucleophilic bimolecular substitution reaction \(\left( {{{\rm{S}}_{\rm{N}}}2} \right)\), and it involves the attack of one alcohol molecule on the other, protonated alcohol. The steps involved in the reaction are as follows:

The method of dehydration of alcohol to give ethers is used for preparing those ethers which have the primary alkyl groups in them. Some of the cares taken to ensure the reaction proceeds correctly include:

1. The alkyl groups present in the alcohols used should be unhindered by the presence of other groups
2. The temperature must be kept at an optimum level.

Taking care of these aspects will ensure ethers are formed in the reaction and not alkene. With the secondary or tertiary alcohol, the steps follow the \({{\rm{S}}_{\rm{N}}}1\) mechanism instead of the \({{\rm{S}}_{\rm{N}}}2\). However, the dehydration reaction for secondary and tertiary is not as successful as that of a primary alcohol because instead of substitution reaction, elimination takes place, thereby forming alkenes.

Also, the dehydration of alcohol to produce ethers is only preferred for the formation of symmetrical ethers. For unsymmetrical ethers like ethyl methyl ether, the preferable mode of preparation is from suitable sodium alkoxides and an alkyl halide, as seen in the case of Williamson synthesis.

Preparations of Ethers by Williamson Synthesis

Both symmetrical and unsymmetrical ethers can be prepared using Williamson synthesis, named after Alexander William Williamson, in the year \(1850\). It is an \({{\rm{S}}_{\rm{N}}}2\) reaction.  An alkyl halide is made to react with sodium alkoxides to yield Ether. The reaction is as follows:

Williamson synthesis can be used to also prepare ethers with substituted alkyl groups, both secondary and tertiary.

a. Preparation of unsymmetrical Ether:
Ethyl methyl ether can be prepared by reaction between sodium methoxide and ethyl bromide.

b. Preparation of symmetrical Ether:
Symmetrical ethers like diethyl ethers can be prepared from Williamson synthesis by the reaction of Sodium alkoxides and alkyl bromide, with similar alkyl groups.

Mechanism:

Ethers with substituted alkyl groups can also be prepared by Williamson synthesis. The substitution reaction \(\left( {{{\rm{S}}_{\rm{N}}}2} \right)\) in Williamson synthesis takes place as below:

The yield of Ether is better with primary alkyl halides. However, if the reaction is carried out in the presence of secondary or tertiary alkyl halides, the elimination reaction takes precedence over substitution, and alkenes are obtained as products instead of ethers. So, if the alkyl groups in the above reaction are reversed with tertiary halide and sodium methoxide, the resultant product would be \(2\)-methylpropene.

The reason behind this is that the alkoxides reagents act as strong bases too (even though they are nucleophiles) and therefore, react with alkyl halides in an elimination reaction.

Other methods of preparation, such as the reaction of alkyl halides with silver oxide and diazomethane on alcohols are also employed in laboratories for the production of ethers.

Reactions of Ethers

Ethers are quite stable in nature and do not react with bases, oxidizing agents, reducing agents, or active metals.  Hence, reactions involving their functional groups are very limited. Some examples include:

a. Hydrolysis reaction: Ethers on hydrolysis give alcohols. Ethers are heated in the presence of dilute sulphuric acid to form alcohols.
\({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2} – {\rm{O}} – {\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_3} + {{\rm{H}}_2}{\rm{O}}2{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}}\,{\rm{Ethyl Alcohol}}\)

b. With \({\rm{PC}}{{\rm{l}}_5}\):  Ethers react with Phosphorus pentachloride to form alkyl chlorides. Diethyl ether reacts to form ethyl chloride in the below example.
\({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2} – {\rm{O}} – {\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_3} + {\rm{PC}}{{\rm{l}}_5}2{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{Cl}} + {\rm{POC}}{{\rm{l}}_3}\)

c. Cleavage with Acids – Ethers, on reaction with hot and concentrated hydrogen iodide or hydrogen bromide gives an alcohol and an alkyl halide. Diethyl ether on heating with hydrogen iodide yields ethyl iodide and ethanol.
\({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2} – {\rm{O}} – {\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_3} + {\rm{HI}}{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{l}} + {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}}\)

Summary

Ethers are prepared by dehydration of alcohols and from Williamson synthesis, using alkyl halides and sodium alkoxides. Both reactions are substitution reactions and follow \({{\rm{S}}_{\rm{N}}}2\) mechanism. Ethers are quite stable and do not undergo reactions with bases, oxidizing and reducing agents, and active metals.

FAQs on Preparation Methods of Ether

Q.1. Which is the best method of preparing an unsymmetrical ether, and why?
Ans: Williamson synthesis can be used to prepare unsymmetrical ethers because, in this method of preparation, alkyl halide and sodium alkoxides react to form Ether. The alkyl groups in both reactants can be chosen depending upon the type of unsymmetrical Ether is to be prepared.
\({\rm{C}}{{\rm{H}}_3} – {\rm{ONa}} + {{\rm{C}}_2}{{\rm{H}}_5}{\rm{Br}} \to {\rm{C}}{{\rm{H}}_3} – {\rm{O}} – {{\rm{C}}_2}{{\rm{H}}_5} + {\rm{NaBr}}\)

Q.2. How do you make Ether from alcohol?
Ans: Ethers are prepared from alcohols by dehydrating them by heating in the presence of sulphuric acid at \({140^ \circ }{\rm{C}}\) or \(413\,{\rm{K}}\). Only symmetrical ethers are prepared from this method. The reaction is as follows:
\({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}} + {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}} \to {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2} – {\rm{O}} – {\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_3} + {{\rm{H}}_2}{\rm{O}}\)

Q.3. How is diethyl ether prepared?
Ans: Diethyl ether is symmetrical Ether. It can be prepared by dehydration of alcohols in the presence of sulphuric acid and by the Williamson method with ethyl halide and sodium ethoxide.

Q.4. What is unsymmetrical Ether?
Ans: Ethers that have different alkyl groups on both sides of the oxygen are called unsymmetrical ethers.
Example: Ethyl methyl ether

Q.5. What happens when diethyl ether reacts with dil. H₂SO₄?
Ans: Diethyl ether reacts with dilute sulphuric acid to hydrolyze into alcohol.
\({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2} – {\rm{O}} – {\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_3} + {{\rm{H}}_2}{\rm{O}}2{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}}\)