Ethers

Organic compounds having an R-O-R functional group are known as ethers. They are anhydride of alcohol.

\underset{alcohol}{R-OH}+H-O-R \rightarrow \underset{ether}{R-O-R} + H_{2}O

Nomenclature

Aromatic Ether

Classification of ethers

On the basis of the nature of groups attached on either side of functional groups, ethers are classified as:

1. Simple or symmetrical ether: Two groups present on either side of the functional group are the same.
   eg: CH₃OCH₃, C₂H₅OC₂H₅, C₃H₇OC₃H₇, etc.

2. Mixed or unsymmetrical ether: Two groups present on either side of the functional group are different.
   eg. CH₃OC₂H₅, C₃H₇OC₆H₅, etc.

Isomerism

1. Chain isomerism:

\underset{Ethoxy\ ethane}{CH_{3}CH_{2}-O-CH_{2}CH_{3}}\\ \underset{1-Methoxy\ propane}{CH_{3}CH_{2}CH_{2}-O-CH_{3}}

2. Positional isomerism:

3. Functional isomerism:

\underset{Methoxy\ methane}{CH_{3}-O-CH_{3}}\ \ \ \ \underset{Ethanol}{CH_{3}CH_{2}OH}

4. Metamerism:

Preparation of aliphatic ether:

1. Williamson’s synthesis: Haloalkane is reacted with sodium or potassium alkoxide to give ether. This reaction is called Williamson’s synthesis. It is used to prepare both symmetrical and unsymmetrical ether.

*Haloalkane should only be primary.

Physical properties of ether:

1. Methoxy methane and methoxy ethane are colorless gases. Other members are colorless volatile liquid with pleasant odour.
2. Ethers are insoluble in water.
3. They have lower boiling point than corresponding alcohols due to absence of intermolecular hydrogen bond between ether molecule.

Chemical Properties:

1. Action with HI: Ethers react with conc. HI to give alcohol and haloalkane.

\begin{align*} \underset{Ether}{ROR}+HI &\rightarrow \underset{Alcohol}{R-OH} + \underset{Iodoalkane}{R-I}\\ \underset{Ethoxy\ ethane}{C_{2}H_{5}OC_{2}H_{5}}+ &HI \rightarrow \underset{Ethanol}{C_{2}H_{5}OH}+ \underset{Iodoethane}{C_{2}H_{5}I} \end{align*}

*In the case of unsymmetrical ethers, the alkyl halide with a smaller alkyl group is formed.

\underset{Methoxy\ ethane}{CH_{3}OC_{2}H_{5}} + HI \rightarrow \underset{Ethanol}{C_{2}H_{5}OH} + \underset{Iodomethane}{CH_{3}I}

If excess HI is used, the alcohol formed above reacts again with the acid to give alkyl halide only.

2. Action with conc. HCl and conc. H₂SO₄: Ether dissolves in cold and conc. mineral acids to form oxonium salts.

3. Action with air: Ether reacts with air in presence of light to form ether peroxide.

Ether peroxide is explosive and thermally unstable. So, it is dangerous to distil old samples of ether.

4. Action with chlorine:

Uses of ethoxy ethane:

• As industrial solvent.
• As an anaesthetic agent.
• As a refrigerent.
• As solvent for many reactions like Wurtz reaction, Fittig
  reaction, preparation of Grignard’s reagent, etc.

Some Important Questions

1. Prepare the given compounds using Williamson’s synthesis:
   i. CH₃-O-CH₃ ii. C₂H₅-O-C₂H₅ iii. C₆H₅-O-C₂H₅ iv. 1-ethoxy propane
2. How would you convert ethoxy ethane to methoxy methane and vice versa.
3. Convert ethoxy methane into ethoxy ethane.
4. How is diethyl ether prepared from ethyl bromide and ethanol?
5. How would you convert diethyl ether into acetyl chloride and propanoic acid?
6. Define oxonium salt. What happens when an oxonium salt is dissolved in water?
7. Give reasons:
   i. It is dangerous to boil old sample of ether stored for long time.
   ii. Ether is stored in bottle containing iron wire.
   iii. C-O-C bond angle in ether is higher than H-O-H bond angle in water.
8. Methyl phenyl ether reacts with HI to give phenol and methyl iodide but not iodobenzene and methyl alcohol. Explain.
9. Write the action of sodium phenolate with iodomethane.
10. Convert phenol to anisole.