Ethers – NEB Class 12 Notes – Organic Chemistry

Chapter 12: Ethers

Unit 12: Ethers in Class 12 Chemistry covers introduction, nomenclature, classification, preparation methods (Williamson’s synthesis), physical and chemical properties (including reactions with HI, Conc. HCl, Conc. H2SO4, air, and Cl2), and practical uses.

Ethers Syllabus

Chapter 12	Ethers (2 hours)
Section	Topic
12.1	Introduction
12.2	Nomenclature, classification, and isomerism of ethers
12.3	Preparation of aliphatic and aromatic ethers from Williamson’s synthesis
12.4	Physical properties of ether
12.5	Chemical properties of ethoxyethane: Action with HIConc. HClConc. H2SO4AirCl2
12.6	Uses of ethers

Introduction to Ethers

Aliphatic Ethers

Organic compounds containing alkoxy (-OR) functional group on their molecule are called ethers. They are considered as alkyl derivatives of water.

They are also considered as alkyl derivative of alcohol in which hydrogen atom of alcohol is replaced by alkyl group.

Aromatic ethers

An alkoxy or phenoxy derivative of aromatic ring are called aromatic ethers.

Nomenclature of ethers

General formula = prefix + word root + primary suffix
(-OR → alkoxy)

Classification of Ethers

1. Simple or symmetrical ethers
   Same alkyl groups on both side of etheral oxygen atom.
   R-O-R

2. Mixed or unsymmetrical ethers
   Different alkyl groups on both side of etheral oxygen atom.
   R-O-R’

Isomerism in ethers

1. Chain isomerism
   Ethers having same molecular formula but different number of carbon chain length are called chain isomers.

2. Position isomerism
   Ethers having same molecular formula but different position of functional group are called chain isomers.

3. Functional isomerism
   Ethers having same molecular formula but different functional group are called functional isomers.

4. Metamerism
   Ethers having same molecular formula but different number of carbon atom on either side of Ethereal oxygen are called metamers.

General methods of preparation of ethers

1. From haloalkane

(a) By Williamson’s ether synthesis process

When haloalkane reacts with sodium alkoxide then ether is formed. This reaction is called Williamson’s etherification reaction.

By this process both symmetrical and unsymmetrical ethers can be prepared.

Tertiary haloalkane can not form ether with sodium alkoxide as it forms alkene.

Preparation of aromatic ether (Anisole)

(Williamson’s ether synthesis process)
When sodium phenoxide reacts with alkyl halide (methyl bromide) then aromatic ether (anisole) is formed.

During this process, sodium alkoxide and haloarene do not react to give anisole because C-X bond in haloarene is stronger than haloalkane due to partial double bond character. (Resonance effect)

Physical properties of ethers:

• Lower members like dimethyl ether and ethyl methyl ether are colorless gases, while others are highly volatile and inflammable colorless liquids at ordinary temperatures, characterized by a sweet, ethereal smell.
• The boiling point of diethyl ether is 34.6°C, and its freezing point is -116°C.
• Ethers are partially soluble in water and soluble in almost all organic solvents.
• They have lower boiling points than alcohols of comparable molecular mass due to the absence of intermolecular hydrogen bonding in ethers.
• The boiling point of ethers increases gradually with an increase in molecular mass.

Chemical properties of ethers

[A] Reaction due to etheral oxygen(-O-)

1.Reaction with air (atmospheric oxygen)
(Auto-oxidation of ether / Formation of peroxide)

When ether reacts slowly with atmospheric oxygen in the presence of sunlight, peroxides are formed. These peroxides have higher boiling points than ethers and can be highly explosive when heated. Therefore, it is dangerous to heat old samples of ether.

The presence of peroxides in ether can be detected by adding FeSO₄ solution followed by KCNS. The appearance of a blood-red color indicates the presence of peroxides.

To prevent peroxide formation, a small amount of cuprous oxide (Cu₂O) can be added. Ether is often stored with iron wire because iron readily reacts with oxygen, preventing peroxide formation:

3Fe+2O2​→Fe3​O4​

2.Reaction with cold mineral acid
(Formation of oxonium salt)

When ether reacts with Conc. HCl and Conc. H₂SO₄ under ice cold condition then oxonium salt are formed. The oxonium salt give ether when dissolved in water. Here ether acts as Lewis base.

[B] Reaction due to breaking of C-O bond

1.Reaction with dilute H₂SO₄ (Hydrolysis of ether)

Heating ether with dilute sulfuric acid results in the formation of alcohols.

2. Reaction with concentrated H₂SO₄

Heating ether with concentrated sulfuric acid produces alcohols and alkyl hydrogen sulfates.

3. Reaction with HBr and HI

Reacting ether with concentrated HBr or HI results in the formation of alcohols and haloalkanes.

If the ether is unsymmetrical, the larger alkyl group forms the alcohol and the smaller alkyl group forms the haloalkane.

[C] Reaction due to alkyl group

(Halogenation of ether or α-halogenation)

When ethers react with halogens, particularly in the dark, α-halogenated products are formed.

In the presence of sunlight and excess Cl₂, all hydrogens of the ether can be replaced, forming perchloro ethers.

Uses of Ethers:

1. Solvent Properties: Ethers are effective solvents for dissolving fats, oils, waxes, resins, petrol, gums, plastics, and various organic compounds.
2. Extraction Solvent: Ethers are commonly used as solvents for the extraction of organic compounds in both laboratory and industrial settings.
3. Applications in Pharmaceuticals and Chemicals:
   • Anesthetics: Diethyl ether is historically known for its use as a general anesthetic.
   • Perfumes: Ethers like anisole are utilized in the formulation of perfumes.
   • Coolants: Dimethyl ether finds application as a coolant in refrigerants.
4. Non-polar Solvents: Ethers serve as non-polar solvents (inert solvents) in various chemical reactions, such as:
   • Wurtz’s Reaction: Used in the synthesis of alkanes from alkyl halides.
   • Grignard Reaction: Facilitates the formation of organomagnesium compounds, vital in organic synthesis.

Ethers Important Questions:

Short Questions:

1. Write an example of Williamson’s etherification reaction. What is its importance?
2. Convert methoxy ethane to ethoxy ethane.
3. How is methoxy ethane obtained from ethoxyethane?
4. Write down possible unsymmetrical ethers of C₄H₁₀O and write their IUPAC names.
5. How is an unsymmetrical ether prepared from Williamson’s process?
6. Prepare methoxyethane using Williamson’s ether synthesis.
7. Write all possible isomers of C₃H₈O. How can you prepare unsymmetrical ethers using Williamson’s process?
8. It is dangerous to boil an old sample of ether. Give a reason.
9. Why is ether always stored in a dark brown-colored bottle containing iron wire?
10. What happens when ethoxyethane is exposed to air in the presence of sunlight?
11. What are oxonium salts? What happens when they are dissolved in water?
12. What product is obtained when methoxyethane reacts with concentrated HBr?
13. Name the isomer of C₂H₆O which reacts with excess HI, giving iodomethane as the major product. Write the reaction involved.
14. How would you prepare methoxybenzene (anisole) from phenol?
15. Prepare anisole using Williamson’s ether synthesis.
16. What happens when sodium phenoxide reacts with bromomethane?
17. How would you obtain anisole from chloromethane?
18. What happens when methoxybenzene is heated with aqueous HI solution?
19. Write the structure of anisole and prepare it using Williamson’s ether synthesis process.

Long Questions:

Some important questions for exam (5 marks):

1. Write down a suitable chemical reaction for the preparation of ethoxyethane from ethanol. How is ethoxyethane converted into methoxyethane?
2. Describe the laboratory method of preparation of ethoxyethane with a neat and well-labeled diagram.
3. How would you prepare diethyl ether (ethoxyethane) in the lab? Why is boiling an old sample of ether dangerous?
4. How is diethyl ether prepared in the laboratory? Write Williamson’s synthesis for the preparation of an unsymmetrical ether.
5. Write short notes on the laboratory preparation of diethyl ether (ethoxyethane).