MHT-CET Chemistry · Alkanes

Making Alkanes: Wurtz, Grignard Reagents and Decarboxylation

Alkanes are made by joining two alkyl halides with sodium (Wurtz), by giving a Grignard reagent a proton from water, an alcohol or ammonia, by heating a sodium carboxylate with soda-lime, and by hydrogenating CO over nickel.

All Alkanes notes MHT-CET Chemistry strategy

Why this matters

17 PYQs, the chapter's heaviest page and most of its MODERATE rows. Eight are Wurtz — which product cannot form from a mixed pair, mole counts, the Fittig variant; seven are Grignard reagents meeting an active hydrogen; two are decarboxylation and CO + H₂. Three cards.

Concept 1 of 3: The Wurtz Reaction and Mixed Halides

Two alkyl groups lose their halogens to sodium and join. With ONE halide you get one product with double the carbons. With a MIXTURE of RX and R′X, every pairing happens — R–R, R–R′ and R′–R′ — so the question is which alkane is NOT among the three.

Definition

2RX + 2Na → R–R + 2NaX (dry ether). 2 mol halide give 1 mol alkane.
Mixed halides RX + R′X → R–R, R–R′, R′–R′. From C₂ + C₃ halides: butane, pentane, hexane — not propane. From C₁ + C₃: ethane, butane, hexane — not propane. From C₁ + C₂: ethane, propane, butane — not pentane.
A secondary halide couples at its own carbon: isopropyl bromide → 2,3-dimethylbutane.
Wurtz–Fittig: aryl + alkyl halide. Fittig: two aryl halides → biphenyl.

Wurtz reaction

\mathrm{2R{-}X + 2Na \xrightarrow{\text{dry ether}} R{-}R + 2NaX}

Worked example

A mixture of bromoethane and 1-bromopropane is treated with sodium in dry ether. Which alkane is NOT formed: propane, butane, pentane, hexane?

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 2nd May Shift 1 · Q80Moderate

Example 1 · Alkanes · Preparation of Alkanes, Wurtz, Grignard and Decarboxylation

Which of the following is NOT obtained when a mixture of bromoethane and 1-bromopropane is treated with sodium metal in dry ether?

Forgetting the self-coupled products

A mixed Wurtz does not only give the cross product. Both halides also couple with themselves, so check all three alkanes before picking the one that cannot form.

Drill 7 more on the wurtz reaction and mixed halides

Concept 2 of 3: Grignard Reagents Meeting an Active Hydrogen

The carbon of R–MgX carries a negative charge and grabs any proton on offer. Water, an alcohol and ammonia all have an H on O or N, so each one turns the Grignard reagent into the alkane R–H. One mole of Grignard needs one mole of that proton source.

Definition

Making it: RX + Mg (dry ether) → R–MgX.
R–MgX + H–Z → R–H + Mg(Z)X, where H–Z is H₂O, ROH or NH₃.
CH₃MgI + H₂O → CH₄ + Mg(OH)I, 1 : 1. CH₃OH + CH₃MgX → CH₄ + CH₃OMgX. C₂H₅MgBr + CH₃OH → C₂H₆.
Ethanol → (SOCl₂) C₂H₅Cl → (Mg) C₂H₅MgCl → (NH₃) ethane + Mg(NH₂)Cl.

Grignard protonation

\mathrm{R{-}MgX + H{-}Z \rightarrow R{-}H + Mg(Z)X}\quad (Z = OH,\ OR,\ NH_2)

Worked example

C₂H₅Br → (Mg / dry ether) A → (CH₃OH) B. Identify B.

Practice this conceptself-check · 2 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 11th May Shift 2 · Q86Moderate

Example 2 · Alkanes · Preparation of Alkanes, Wurtz, Grignard and Decarboxylation

Identify the product 'B' in the following reaction sequence.

\text{C}_2\text{H}_5\text{Br} \xrightarrow{\text{Mg/dry ether}} A \xrightarrow{\text{CH}_3\text{OH}} B

Expecting an amine or an alcohol

NH₃ and CH₃OH look like reagents that would add their group. With a Grignard reagent they only give up a proton: the product is the alkane every time.

Drill 6 more on grignard reagents meeting an active hydrogen

Concept 3 of 3: Decarboxylation and Hydrogenation of CO

Soda-lime removes CO₂ from a sodium carboxylate, so the alkane has one carbon fewer than the acid. Nickel lets hydrogen reduce CO all the way to methane.

Definition

Decarboxylation: RCOONa + NaOH (soda-lime, Δ) → R–H + Na₂CO₃. Sodium propanoate → ethane, not propane.
CO + 3H₂ (Ni) → CH₄ + H₂O.

Reaction	Product
Sodium propanoate + soda-lime, Δ	Ethane + Na₂CO₃ (paper keys propane)Q Decarboxylation loses a carbon, so C₂H₅COONa gives ethane. The 9 May 2023 key is propane; learn the rule, and expect the paper's answer.
CO + H₂ over Ni	Methane and waterQ

Practice this conceptself-check

The same idea in a real exam question:

MHT-CET · 2024 · 13th May Shift 2 · Q92Moderate

Example 3 · Alkanes · Preparation of Alkanes, Wurtz, Grignard and Decarboxylation

Identify the product formed by the action of

\text{H}_2\text{(g)}

with

\text{CO(g)}

in presence of Ni?

Drill 1 more on decarboxylation and hydrogenation of co

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

The Wurtz Reaction and Mixed Halides
Wurtz reaction
$\mathrm{2R{-}X + 2Na \xrightarrow{\text{dry ether}} R{-}R + 2NaX}$
Grignard Reagents Meeting an Active Hydrogen
Grignard protonation
$\mathrm{R{-}MgX + H{-}Z \rightarrow R{-}H + Mg(Z)X}\quad (Z = OH,\ OR,\ NH_2)$

Reference tables (1)

Decarboxylation and Hydrogenation of CO2 rows

Reaction	Product
Sodium propanoate + soda-lime, Δ	Ethane + Na₂CO₃ (paper keys propane)Q Decarboxylation loses a carbon, so C₂H₅COONa gives ethane. The 9 May 2023 key is propane; learn the rule, and expect the paper's answer.
CO + H₂ over Ni	Methane and waterQ

Watch out for (2)

Forgetting the self-coupled products
→ The Wurtz Reaction and Mixed Halides
Expecting an amine or an alcohol
→ Grignard Reagents Meeting an Active Hydrogen

Test yourself on Alkanes

15 past MHT-CET questions from this chapter, timed at 14 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the Alkanes chapter test Drill all 17 questions