Mechanism (Free Radical / SET Mechanism)

Intramolecular Wurtz Reaction (Cyclic Formation)

JEE / NEET Important Points

1. Works best with primary alkyl halides
2. Secondary & tertiary → elimination side reactions
3. Dry ether is essential
4. Same alkyl halide → symmetrical alkane
5. Different alkyl halides → mixture of products
6. Free radical intermediate
7. SET (Single Electron Transfer) mechanism
8. Even-numbered carbon chain formed
9. Not suitable for unsymmetrical alkane synthesis
10. Intramolecular reaction gives cyclic alkane

Important Concept

✔ Only alkali metals (Na, K, Li) give Wurtz-type coupling
✔ Sodium is preferred due to controlled reactivity
✔ Mg and Zn mainly form organometallic intermediates instead of direct alkane

Wurtz Reaction – Metals Other Than Sodium

Although sodium (Na) is the classical metal used in Wurtz reaction, some other metals can also promote coupling of alkyl halides

Potassium (K)

$$2R–X + 2K \rightarrow R–R + 2KX$$

✔ More reactive than sodium
✔ Reaction is more vigorous
❌ Harder to control
📌 Rarely used in practice (safety issue)

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Lithium (Li)

$$2R–X + 2Li \rightarrow R–R + 2LiX$$

✔ Can participate in similar coupling
✔ Often forms organolithium intermediates
📌 Used more in organometallic synthesis than classical Wurtz

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Silver (Ag)

$$R–X + Ag \rightarrow R^\bullet$$

✔ Used mainly for radical formation studies
❌ Not common for alkane synthesis
📌 Often used in rearrangement or carbocation studies

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Zinc (Zn)

Zinc does not give classical Wurtz coupling easily but forms:$$R–X + Zn \rightarrow R–ZnX$$

✔ Forms organozinc compounds
✔ Used in coupling reactions (e.g., Reformatsky type)

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Magnesium (Mg)

R–X+Mg→R–MgX

✔ Forms Grignard reagent
❌ Does NOT directly give Wurtz alkane
📌 Very important alternative pathway

Cyclic Wurtz Reaction (Intramolecular Wurtz)

Cyclic Wurtz reaction is an intramolecular version of the Wurtz reaction in which a dihaloalkane reacts with sodium metal in dry ether to form a cycloalkane.

JEE / NEET Important Points

1. Best for small rings (3–5 members).
2. Large rings are difficult due to entropy factor.
3. Competes with intermolecular Wurtz (polymerization possible).
4. Primary dihalides give better yield.
5. Dry ether is essential.
6. Mechanism involves free radicals.

Product of Tertiary Alkyl Halide in Wurtz Reaction

When a tertiary alkyl halide (3° RX) is treated with sodium (Na) in dry ether, it does not give Wurtz coupling product efficiently.

Major product = Alkene (Elimination product)
Coupling product is minor or negligible.

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Example

Why Elimination Occurs?

✔ Tertiary halides easily form stable tertiary radicals / carbanions
✔ Strong base character of sodium promotes β-elimination
✔ Steric hindrance prevents effective coupling

Thus, E2-type elimination dominates over coupling.

JEE / NEET Important Points

1. Primary RX → best for Wurtz coupling
2. Secondary RX → mixture
3. Tertiary RX → alkene (major)
4. Elimination increases with substitution.
5. Reaction follows radical pathway but elimination competes strongly.

Reactivity Order (Bond Strength Basis)

$$R–I > R–Br > R–Cl \gg R–F$$

JEE / NEET Important Points

1. Alkyl bromides are preferred.
2. Primary halides give best results.
3. Aryl halides do NOT undergo normal Wurtz (need Wurtz–Fittig).
4. Fluorides are practically inactive.
5. Dry ether is essential for the reaction.

Reasons for Using Dry Ether

1️⃣ Provides Anhydrous Medium

✔ Sodium reacts violently with water:$$2Na + 2H_2O \rightarrow 2NaOH + H_2\uparrow$$2Na+2H2​O→2NaOH+H2​↑

If moisture is present, sodium will react with water instead of alkyl halide.
Therefore, ether must be dry.