Ethers: Describe the Williamson synthesis of ethers and its limitations with tertiary halides

Chapter: Alcohols, Phenols and Ethers

Topic: Williamson Ether Synthesis and Its Limitations

Williamson Synthesis of Ethers

The Williamson ether synthesis is an important method for preparing ethers by reacting a suitable sodium alkoxide or sodium phenoxide with an alkyl halide.

R–ONa + R′–X → R–O–R′ + NaX

The reaction generally proceeds through an SN2 mechanism.

Alkoxide / Phenoxide + Alkyl halide
↓
SN2 substitution
↓
Ether

An alcohol reacts with sodium metal to form sodium alkoxide.

2ROH + 2Na → 2RONa + H₂↑

The resulting sodium alkoxide acts as the nucleophile in the Williamson synthesis.

ROH → RONa → Nucleophile

R–ONa + R′–X → R–O–R′ + NaX

Where:

• R–ONa = sodium alkoxide
• R′–X = alkyl halide
• R–O–R′ = ether
• NaX = sodium halide

The alkyl halide should preferably be a primary alkyl halide for a good yield.

The alkoxide ion acts as a nucleophile and attacks the carbon atom attached to the leaving group.

RO⁻ + R′–X
↓
Back-side attack
↓
R–O–R′ + X⁻

The mechanism is a single-step SN2 nucleophilic substitution.

SN2 → One-step reaction → Ether formation

Sodium ethoxide reacts with bromoethane to form ethoxyethane (diethyl ether).

C₂H₅ONa + C₂H₅Br
↓
C₂H₅–O–C₂H₅ + NaBr

Here:

• C₂H₅ONa = sodium ethoxide
• C₂H₅Br = bromoethane
• C₂H₅–O–C₂H₅ = ethoxyethane
• NaBr = sodium bromide

Bromoethane + Sodium ethoxide → Diethyl ether

Williamson synthesis is particularly useful for preparing unsymmetrical ethers.

R–ONa + R′–X
↓
R–O–R′

For example, sodium ethoxide and methyl iodide give methoxyethane.

C₂H₅ONa + CH₃I
↓
CH₃OC₂H₅ + NaI

Methyl/primary alkyl halide + Alkoxide → Good Williamson synthesis

Tertiary alkyl halides are generally unsuitable for Williamson ether synthesis because the reaction proceeds through an SN2 mechanism.

In a tertiary alkyl halide, the carbon bearing the halogen is highly sterically hindered. Therefore, backside attack by the alkoxide ion becomes difficult.

3° R–X
↓
Steric hindrance
↓
SN2 attack difficult
↓
Elimination (E2) favoured
↓
Alkene

Tertiary halide + Alkoxide
→ E2 elimination predominates
→ Alkene forms instead of ether

Consider tert-butyl bromide with sodium ethoxide.

(CH₃)₃C–Br + C₂H₅ONa
↓
E2 elimination
↓
(CH₃)₂C=CH₂

Instead of ether formation, the strong alkoxide base removes a β-hydrogen and an alkene is formed.

Exam Point: Tertiary alkyl halides favour elimination because SN2 backside attack is sterically hindered.

Halide	Williamson Reaction	Reason
Methyl	Excellent	Very low steric hindrance
Primary	Very good	SN2 favoured
Secondary	Less suitable	Some elimination may occur
Tertiary	Not suitable	E2 predominates

1. The reaction works best with methyl and primary alkyl halides.
2. Secondary alkyl halides may give a mixture of substitution and elimination products.
3. Tertiary alkyl halides generally undergo E2 elimination instead of SN2 substitution.
4. Strongly hindered substrates are poor choices because SN2 attack is sterically difficult.
5. Aryl halides such as chlorobenzene generally do not undergo ordinary SN2 Williamson substitution.

For Williamson synthesis:

Choose a methyl or primary alkyl halide.

Q. What is Williamson ether synthesis?

Answer:

Williamson ether synthesis is the preparation of ethers by reacting sodium alkoxide or sodium phenoxide with a suitable alkyl halide. The reaction generally follows the SN2 mechanism.

RONa + R′X → ROR′ + NaX

Q. Explain Williamson ether synthesis with an example.

Answer:

Sodium alkoxide reacts with an alkyl halide through an SN2 nucleophilic substitution reaction to form an ether.

C₂H₅ONa + CH₃I → CH₃OC₂H₅ + NaI

Here sodium ethoxide acts as the nucleophile and methyl iodide acts as the substrate.

Q. Why are tertiary alkyl halides not suitable for Williamson ether synthesis?

Answer:

1. Williamson synthesis proceeds by an SN2 mechanism.
2. SN2 requires backside attack on the carbon attached to the leaving group.
3. In tertiary alkyl halides, this carbon is highly sterically hindered.
4. Therefore SN2 substitution is difficult and the strong alkoxide ion acts as a base, causing E2 elimination to form an alkene.

Tertiary halide → SN2 difficult → E2 elimination favoured

Q. Describe Williamson ether synthesis and discuss its limitations.

Williamson Reaction

R–ONa + R′–X → R–O–R′ + NaX

It is an SN2 reaction in which an alkoxide ion attacks an alkyl halide.

Limitations

1. Methyl and primary alkyl halides give the best results.
2. Secondary halides may undergo competing elimination.
3. Tertiary halides do not favour SN2 because of steric hindrance.
4. Tertiary halides generally give alkenes by E2 elimination.
5. Aryl halides are not suitable for ordinary SN2 Williamson synthesis.

Williamson synthesis is most effective when the alkyl halide is methyl or primary.

Q. Explain the Williamson synthesis of ethers, its mechanism, examples and limitations with tertiary alkyl halides.

1. Definition

Williamson ether synthesis is a method for preparing ethers by reacting a sodium alkoxide or sodium phenoxide with an alkyl halide.

R–ONa + R′–X → R–O–R′ + NaX

2. Mechanism

The alkoxide ion attacks the carbon atom of the alkyl halide from the back side and displaces the halide ion.

RO⁻ + R′–X
↓
Back-side SN2 attack
↓
R–O–R′ + X⁻

3. Example

C₂H₅ONa + C₂H₅Br → C₂H₅OC₂H₅ + NaBr

This produces ethoxyethane (diethyl ether).

4. Limitation with Tertiary Halides

Tertiary alkyl halides are highly sterically hindered. Therefore, backside attack required for SN2 is difficult.

3° R–X + RO⁻
↓
SN2 hindered
↓
E2 elimination
↓
Alkene

5. Final Conclusion

Methyl / Primary alkyl halide → SN2 → Ether formation

Tertiary alkyl halide → SN2 hindered → E2 elimination → Alkene formation

1. Williamson synthesis is used for the preparation of:

A) Aldehydes

B) Ethers

C) Ketones only

D) Amines

✅ Answer

B) Ethers

---

2. The nucleophile in Williamson synthesis is generally:

A) Alkoxide ion

B) H⁺

C) Cl⁻ only

D) Water

✅ Answer

A) Alkoxide ion

---

3. Williamson synthesis generally follows:

A) SN1

B) SN2

C) E1 only

D) Addition

✅ Answer

B) SN2

---

4. General reaction of Williamson synthesis is:

A) RONa + R′X → ROR′ + NaX

B) ROH → RCHO

C) RX → RH

D) RCOOH → RCHO

✅ Answer

A

---

5. Which substrate gives the best Williamson reaction?

A) Tertiary alkyl halide

B) Primary alkyl halide

C) Highly hindered tertiary alcohol

D) Quaternary carbon compound

✅ Answer

B) Primary alkyl halide

---

6. Methyl halides are:

A) Very suitable

B) Unsuitable

C) Always eliminated

D) Radical substrates

✅ Answer

A) Very suitable

---

7. Tertiary alkyl halides are unsuitable mainly because:

A) SN2 is sterically hindered

B) Oxygen is absent

C) They cannot contain halogen

D) They are gases

✅ Answer

A

---

8. With tertiary halides, alkoxide ions generally favour:

A) E2 elimination

B) SN2 strongly

C) Addition

D) Reduction

✅ Answer

A) E2 elimination

---

9. The product of E2 reaction of a tertiary halide is generally:

A) Alkene

B) Ether

C) Alcohol only

D) Aldehyde

✅ Answer

A) Alkene

---

10. SN2 reaction involves:

A) Backside attack

B) Carbocation only

C) Free radical

D) Electrophilic attack only

✅ Answer

A

---

11. Sodium ethoxide + bromoethane gives:

A) Ethoxyethane

B) Ethene only

C) Ethanal

D) Ethanoic acid

✅ Answer

A) Ethoxyethane

---

12. The leaving group in an alkyl bromide is:

A) Br⁻

B) Na⁺

C) O⁻

D) H⁺

✅ Answer

A) Br⁻

---

13. Williamson synthesis can prepare:

A) Symmetrical ethers

B) Unsymmetrical ethers

C) Both

D) Neither

✅ Answer

C) Both

---

14. For unsymmetrical ethers, the less hindered alkyl halide is usually preferred because:

A) SN2 is easier

B) SN1 is always required

C) E1 is required

D) No reaction occurs

✅ Answer

A

---

15. Secondary alkyl halides in Williamson synthesis may undergo:

A) Only substitution

B) Competing elimination

C) Only oxidation

D) No reaction

✅ Answer

B

---

16. Which is most suitable for Williamson synthesis?

A) CH₃I

B) (CH₃)₃CBr

C) Highly hindered tertiary bromide

D) Aryl halide

✅ Answer

A) CH₃I

---

17. Sodium phenoxide can react with a suitable alkyl halide to form:

A) Aryl ether

B) Alkane only

C) Ketone only

D) Amide

✅ Answer

A) Aryl ether

---

18. Williamson synthesis is essentially a:

A) Nucleophilic substitution

B) Electrophilic addition

C) Free-radical polymerisation

D) Oxidation

✅ Answer

A

---

19. Tertiary halide + strong alkoxide generally produces:

A) Ether as major product

B) Alkene by elimination

C) Aldehyde

D) Acid

✅ Answer

B

---

20. The key requirement for a successful Williamson synthesis is:

A) Suitable alkyl halide capable of SN2

B) Tertiary halide only

C) Carbocation formation

D) Aromatic halide only

✅ Answer

A

🎯 Quick Revision

Williamson synthesis: RONa + R′X → ROR′ + NaX
Mechanism: SN2
Nucleophile: Alkoxide / Phenoxide ion
Best substrate: Methyl or Primary alkyl halide
Secondary halide: Substitution + elimination may compete
Tertiary halide: SN2 strongly hindered
Tertiary halide + alkoxide: E2 elimination favoured
Main product with tertiary halide: Alkene