Unit 12

Organic Chemistry

Class 12 Chemistry

Ethers

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NEB/CDC syllabus scope: Ethers is a 2-teaching-hour unit covering introduction; nomenclature, classification and isomerism; preparation of aliphatic and aromatic ethers by Williamson’s synthesis; physical properties; chemical properties of ethoxyethane with HI, concentrated HCl, concentrated H₂SO₄, air and chlorine; and uses of ethers.

1. Introduction to Ethers

Definition Ethers are organic compounds in which an oxygen atom is bonded to two carbon-containing groups. These groups may be alkyl, aryl, or one of each.

General structure R–O–R′

R and R′ may be the same or different hydrocarbon groups.

Examples:

CH₃–O–CH₃ = methoxymethane (dimethyl ether) CH₃CH₂–O–CH₂CH₃ = ethoxyethane (diethyl ether) C₆H₅–O–CH₃ = methoxybenzene (anisole)

Diagram 1: General structure of an ether

2. Classification of Ethers

2.1 Symmetrical or Simple Ethers

Both groups attached to oxygen are identical.

R–O–R

Examples: CH₃OCH₃ and C₂H₅OC₂H₅.

2.2 Unsymmetrical or Mixed Ethers

The two groups attached to oxygen are different.

R–O–R′ where R ≠ R′

Example: CH₃OC₂H₅.

2.3 Aliphatic and Aromatic Ethers

Class	Structural feature	Example
Aliphatic ether	Oxygen bonded only to alkyl groups	C₂H₅OC₂H₅
Aromatic ether	At least one side is an aryl group	C₆H₅OCH₃ (anisole)
Symmetrical ether	Same groups on both sides	CH₃OCH₃
Unsymmetrical ether	Different groups on the two sides	CH₃OC₂H₅

Diagram 2: Classification of ethers

3. Nomenclature of Ethers

3.1 Common Nomenclature

Name the two groups attached to oxygen, followed by the word ether. If the groups are different, they are commonly written alphabetically.

Formula	IUPAC name	Common name
CH₃OCH₃	Methoxymethane	Dimethyl ether
CH₃OC₂H₅	Methoxyethane	Ethyl methyl ether
C₂H₅OC₂H₅	Ethoxyethane	Diethyl ether
C₆H₅OCH₃	Methoxybenzene	Anisole
C₆H₅OC₂H₅	Ethoxybenzene	Phenetole

3.2 IUPAC Alkoxyalkane Method

Choose the larger carbon chain as the parent alkane. The smaller group plus oxygen is named as an alkoxy substituent.

Example CH₃–O–CH₂CH₃

Parent chain = ethane; CH₃O– = methoxy.

IUPAC name: methoxyethane.

4. Isomerism in Ethers

4.1 Functional Isomerism

Ethers and monohydric alcohols can have the same molecular formula but different functional groups.

C₂H₆O: CH₃CH₂OH (ethanol) and CH₃OCH₃ (methoxymethane)

4.2 Metamerism

Ethers may have the same molecular formula but different distributions of carbon atoms on the two sides of oxygen.

Example: C₄H₁₀O ethers CH₃–O–CH₂CH₂CH₃ = 1-methoxypropane C₂H₅–O–C₂H₅ = ethoxyethane

They differ in how carbon atoms are distributed around oxygen, so they are metamers.

4.3 Chain Isomerism

Ethers may also differ in branching of the carbon skeleton.

Diagram 3: Main isomerism patterns of ethers

5. Williamson’s Ether Synthesis

Williamson synthesis An ether is prepared by reacting a sodium or potassium alkoxide/phenoxide with a suitable alkyl halide through nucleophilic substitution.

RONa + R′X → ROR′ + NaX

The alkoxide ion, RO⁻, acts as the nucleophile. The reaction generally works best when the alkyl halide is methyl or primary because the key substitution step is SN2-like and is slowed by steric crowding.

5.1 Preparation of Aliphatic Ethers

Symmetrical Ether

C₂H₅ONa + C₂H₅Br → C₂H₅OC₂H₅ + NaBr

Unsymmetrical Ether

CH₃ONa + C₂H₅Br → CH₃OC₂H₅ + NaBr

Diagram 4: Williamson synthesis of an aliphatic ether

5.2 Preparation of Aromatic Ether — Anisole

Anisole is prepared by reacting sodium phenoxide with a methyl halide:

C₆H₅ONa + CH₃I → C₆H₅OCH₃ + NaI

Very Important To prepare anisole by Williamson synthesis, use sodium phenoxide + methyl halide, not sodium methoxide + chlorobenzene. The aryl C–Cl bond in chlorobenzene resists ordinary SN2 displacement.

Diagram 5: Preparation of anisole by Williamson synthesis

6. Physical Properties of Ethers

The oxygen atom makes ethers polar and enables them to accept hydrogen bonds from water, but ethers do not contain an O–H bond and therefore cannot form strong intermolecular hydrogen-bond networks with themselves.

Property	General behaviour	Reason
State	Lower ethers are volatile liquids or gases.	Relatively weak intermolecular attractions compared with alcohols.
Boiling point	Lower than isomeric alcohols.	No intermolecular O–H hydrogen bonding between ether molecules.
Water solubility	Lower ethers are slightly soluble.	Ether oxygen can accept hydrogen bonds from water.
Organic-solvent solubility	Good solvent for many organic compounds.	Contains both polar C–O bonds and hydrocarbon groups.
Volatility	Ethoxyethane is highly volatile.	Low boiling point.
Flammability	Many ethers are highly flammable.	Volatile organic vapours ignite readily.

Diagram 6: Hydrogen bonding and boiling-point comparison

Exam Important Methoxymethane and ethanol can have the same molecular formula, but ethanol has a much higher boiling point because ethanol molecules form intermolecular O–H hydrogen bonds.

7. Chemical Properties of Ethoxyethane

Ethoxyethane (diethyl ether), C₂H₅OC₂H₅, is relatively unreactive toward many ordinary reagents, but the syllabus emphasizes its reactions with strong acids, air and chlorine.

7.1 Action with Hydroiodic Acid (HI)

Concentrated HI protonates the ether oxygen and then cleaves a C–O bond.

With One Equivalent of HI

C₂H₅OC₂H₅ + HI → C₂H₅I + C₂H₅OH

With Excess Hot HI

The ethanol initially produced also reacts with HI:

C₂H₅OH + HI → C₂H₅I + H₂O

Overall:

C₂H₅OC₂H₅ + 2HI → 2C₂H₅I + H₂O

Diagram 7: Stepwise cleavage of ethoxyethane by HI

7.2 Action with Concentrated HCl and H₂SO₄

Ethers possess lone pairs on oxygen and behave as weak Lewis/Brønsted bases toward strong mineral acids. In cold concentrated acid, oxygen is protonated and an oxonium salt is formed.

With Concentrated HCl

(C₂H₅)₂O + HCl ⇌ [(C₂H₅)₂OH]⁺Cl⁻

With Concentrated H₂SO₄

(C₂H₅)₂O + H₂SO₄ ⇌ [(C₂H₅)₂OH]⁺HSO₄⁻

Concept These equations show protonation at oxygen. Stronger cleavage of an ether C–O bond is especially associated with concentrated HI or HBr on heating.

Diagram 8: Protonation of ether oxygen by strong acids

7.3 Action with Air — Peroxide Formation

On prolonged storage in contact with air, especially in light, ethoxyethane can slowly form ether hydroperoxides and peroxides.

ethoxyethane + O₂ → ether peroxides / hydroperoxides (slow, air/light)

Why old ether can be dangerous Ether peroxides can become concentrated during evaporation or distillation and may be shock- or heat-sensitive. This is why aged ether is a laboratory safety concern and should only be handled under appropriate chemical-safety procedures.

Diagram 9: Slow peroxide formation in stored ether

7.4 Action with Chlorine

Chlorination occurs at carbon atoms adjacent to oxygen. The product depends on reaction conditions.

Chlorine in the Dark

Substitution at the α-carbons gives α,α′-dichlorodiethyl ether:

CH₃CH₂–O–CH₂CH₃ + 2Cl₂ → CH₃CHCl–O–CHClCH₃ + 2HCl

Excess Chlorine in Strong Light

Further extensive substitution of hydrogen atoms by chlorine can occur.

Exam Important If the question asks specifically about ethoxyethane with chlorine, mention that conditions control the extent of chlorination: limited α-chlorination in the dark versus extensive chlorination in strong light/excess chlorine.

8. Reaction Summary of Ethoxyethane

Reagent / condition	Main behaviour	Product / observation
HI	C–O bond cleavage	C₂H₅I + C₂H₅OH
Excess hot HI	Complete cleavage	2C₂H₅I + H₂O
Cold concentrated HCl	Protonation of ether oxygen	Oxonium chloride salt
Cold concentrated H₂SO₄	Protonation of ether oxygen	Oxonium hydrogen sulphate salt
Air / O₂ / light / storage	Slow oxidation	Ether peroxides / hydroperoxides
Cl₂ in dark	α-Chlorination	α,α′-Dichlorodiethyl ether + HCl
Excess Cl₂ / strong light	Extensive substitution	Highly chlorinated products

9. Uses of Ethers

Organic solvent: useful for dissolving oils, fats, resins and many organic compounds.
Reaction medium: dry ether is commonly used in reactions such as preparation and handling of Grignard reagents.
Extraction solvent: suitable for separating many organic compounds from aqueous mixtures because of limited water miscibility.
Historical anaesthetic: diethyl ether was formerly widely used as an inhalational general anaesthetic, though safer modern agents replaced it in most routine practice.
Industrial intermediate: different ethers are used as solvents, fuel-related components and synthesis intermediates.

Safety Note Ethoxyethane is extremely flammable and volatile. Its vapour can ignite easily, and stored samples may form hazardous peroxides. These properties are central to safe laboratory handling.

10. Important Comparisons

Ether vs Alcohol

Feature	Ether	Alcohol
Functional group	R–O–R′	R–OH
O–H bond	Absent	Present
Self hydrogen bonding	No strong O–H H-bond network	Strong intermolecular hydrogen bonding
Boiling point	Lower than isomeric alcohol	Higher
Reaction with Na metal	No acidic O–H hydrogen	Forms alkoxide + H₂
Typical formula relation	Often functional isomer of monohydric alcohol	Often functional isomer of ether

Symmetrical vs Unsymmetrical Ether

Feature	Symmetrical	Unsymmetrical
Groups attached to O	Same	Different
General form	R–O–R	R–O–R′
Example	C₂H₅OC₂H₅	CH₃OC₂H₅

11. Common Exam Mistakes

Writing the ether functional group as R–OH. Ether is R–O–R′.
Confusing a symmetrical ether with an unsymmetrical ether.
Forgetting that ethers and alcohols can be functional isomers.
Confusing metamerism with ordinary position isomerism.
Choosing a tertiary alkyl halide for a simple Williamson SN2 preparation; elimination can compete strongly.
Trying to prepare anisole from sodium methoxide + chlorobenzene. Use sodium phenoxide + methyl halide.
Writing that ethers have no interaction with water. Ether oxygen can accept hydrogen bonds from water.
Writing that ether molecules form strong intermolecular O–H hydrogen bonds with one another. They have no O–H bond.
Forgetting the second step when ethoxyethane reacts with excess hot HI.
Writing that concentrated HCl and H₂SO₄ immediately behave exactly like hot HI. Cold concentrated mineral acids mainly protonate ether oxygen to form oxonium salts.
Ignoring peroxide formation during storage in air.
Forgetting that chlorine reaction depends on light and chlorine concentration.

12. Worked Examples

Worked Example 1: Name CH₃OC₂H₅

The larger carbon group is ethyl/ethane and the smaller CH₃O– group is methoxy.

IUPAC name: methoxyethane.

Common name: ethyl methyl ether.

Worked Example 2: Prepare ethoxyethane by Williamson synthesis C₂H₅ONa + C₂H₅Br → C₂H₅OC₂H₅ + NaBr

Sodium ethoxide is the nucleophile and bromoethane supplies the alkyl carbon.

Worked Example 3: Prepare anisole correctly C₆H₅ONa + CH₃I → C₆H₅OCH₃ + NaI

The SN2 attack occurs at the methyl carbon. Chlorobenzene is not used as the SN2 substrate.

Worked Example 4: Ethoxyethane + excess HI C₂H₅OC₂H₅ + 2HI → 2C₂H₅I + H₂O

Answer: excess hot HI converts both ethyl groups into iodoethane.

Worked Example 5: Why does ethoxyethane boil below butan-1-ol?

Butan-1-ol molecules form intermolecular hydrogen bonds through O–H groups. Ethoxyethane has no O–H bond and cannot form a comparable self-associated hydrogen-bond network, so less energy is needed to separate ether molecules.

13. Important Exam Questions

Short-Answer Questions

Define ether and write its general structure.
Differentiate symmetrical and unsymmetrical ethers.
Differentiate aliphatic and aromatic ethers with examples.
Give the IUPAC names of CH₃OCH₃, CH₃OC₂H₅ and C₂H₅OC₂H₅.
What is metamerism? Give an ether example.
How are alcohol and ether related by functional isomerism?
Define Williamson’s ether synthesis.
Write the preparation of ethoxyethane by Williamson synthesis.
How is anisole prepared by Williamson synthesis?
Why is sodium methoxide + chlorobenzene not the preferred Williamson route to anisole?
Why do ethers have lower boiling points than isomeric alcohols?
Why are lower ethers slightly soluble in water?
What happens when ethoxyethane is treated with one equivalent of HI?
What happens when ethoxyethane is heated with excess HI?
How does ethoxyethane react with concentrated HCl?
How does ethoxyethane react with concentrated H₂SO₄?
Why can old ether become dangerous?
What happens when ethoxyethane reacts with chlorine in the dark?
State any three uses of ethers.

Long-Answer Questions

Explain nomenclature, classification and isomerism of ethers with suitable examples.
Describe Williamson synthesis for symmetrical and unsymmetrical aliphatic ethers.
Explain preparation of anisole by Williamson synthesis and why an aryl halide is not used as the SN2 substrate.
Discuss the physical properties of ethers and compare their boiling points with isomeric alcohols.
Explain the reaction of ethoxyethane with HI under limited and excess conditions.
Explain formation of oxonium salts of ethoxyethane with concentrated HCl and H₂SO₄.
Explain peroxide formation when ether is stored in air and why it is a safety concern.
Describe the action of chlorine on ethoxyethane under different conditions.

Conversion / Reaction Questions

Sodium ethoxide + bromoethane → ethoxyethane.
Sodium methoxide + bromoethane → methoxyethane.
Sodium phenoxide + methyl iodide → anisole.
Ethoxyethane + HI → iodoethane + ethanol.
Ethoxyethane + excess HI → iodoethane.
Ethoxyethane + concentrated HCl → oxonium salt.
Ethoxyethane + concentrated H₂SO₄ → oxonium salt.

Diagram Questions

Draw the general structure of an ether.
Draw a classification chart for ethers.
Illustrate functional isomerism and metamerism in ethers.
Draw Williamson synthesis of an aliphatic ether.
Draw Williamson preparation of anisole.
Illustrate why ethers boil below isomeric alcohols.
Draw stepwise cleavage of ethoxyethane by HI.
Draw oxonium salt formation with a strong acid.
Draw a peroxide-formation safety diagram for stored ether.

Exam Strategy Ethers is a short unit. Prioritize Williamson synthesis, isomerism, boiling-point reasoning, HI cleavage, oxonium-salt formation, peroxide formation and chlorine reaction conditions.

14. One-Minute Revision

Ethers have the functional structure R–O–R′.
R = R′ gives a symmetrical ether; R ≠ R′ gives an unsymmetrical ether.
Aliphatic ethers contain alkyl groups; aromatic ethers contain an aryl group.
IUPAC naming commonly uses the alkoxyalkane system.
Ethers show functional isomerism with alcohols.
Ethers also show metamerism due to different carbon distributions around oxygen.
Williamson synthesis: RONa + R′X → ROR′ + NaX.
Methyl and primary alkyl halides are especially suitable for Williamson SN2 reactions.
Anisole is prepared from sodium phenoxide + methyl halide.
Ethers boil below isomeric alcohols because they lack intermolecular O–H hydrogen bonding.
Ether oxygen can accept hydrogen bonds from water, so lower ethers have slight water solubility.
Ethoxyethane + HI gives iodoethane + ethanol.
With excess hot HI, ethoxyethane gives 2 molecules of iodoethane + water.
Concentrated HCl and H₂SO₄ protonate ether oxygen and form oxonium salts.
Stored ether can form hazardous peroxides in contact with air/light.
Chlorine in the dark causes α-chlorination of ethoxyethane.
Excess chlorine in strong light causes more extensive chlorination.
Ethers are important organic solvents and reaction media.

15. Diagram Practice

Students should practice these diagrams for the NEB examination:

General R–O–R′ structure of ether.
Classification of ethers.
Functional isomerism and metamerism.
Williamson synthesis of an aliphatic ether.
Williamson synthesis of anisole.
Hydrogen-bond comparison between alcohol and ether.
HI cleavage of ethoxyethane.
Oxonium salt formation with concentrated mineral acid.
Peroxide formation during storage in air.

Source handling: The original Nepal eNotes PDF remains embedded above. The typed section follows the verified NEB/CDC syllabus and is designed as a searchable, responsive study companion. Where the PDF viewer does not expose handwritten page text, the typed section is a syllabus-aligned reconstruction and is not claimed to be a word-for-word transcription.