On this page
Preparation questions on alkanes usually ask one of two things: which alkane does this method give, or which method would you use to make this alkane? Both are carbon-counting problems. This post sets out the four NCERT methods, gives one counting rule for each, works examples including a mass calculation, and lists the slips that cost marks.
What does the syllabus include?
Section 9.2.2 of the NCERT Class 11 chapter Hydrocarbons covers preparation from unsaturated hydrocarbons, from alkyl halides and from carboxylic acids. The JEE (Advanced) 2026 brochure lists alkane "Preparation from alkyl halides and aliphatic carboxylic acids". The JEE (Main) 2026 syllabus lists "general methods of preparation" for hydrocarbons. Check the syllabus for your exam year.
What is the carbon-count rule for each method?
Reduction with zinc and dilute acid: R–X → R–H. Same number of carbons. NCERT's examples turn chloroethane into ethane and 1-chloropropane into propane.
Wurtz reaction: 2R–X + 2Na → R–R + 2NaX, in dry ether. Two chains join, so the carbon count doubles. NCERT notes it is used for higher alkanes "containing even number of carbon atoms".
Decarboxylation: RCOONa heated with soda lime (NaOH and CaO) gives R–H and Na₂CO₃. One carbon fewer than the acid.
Kolbe's electrolytic method: electrolysis of RCOONa (or the potassium salt) in water gives R–R at the anode. Each acid with n carbons loses one carbon as CO₂ and the two pieces join, so the product has 2(n − 1) carbons.
Hermann Kolbe reported his work on the electrolysis of organic compounds in Liebigs Annalen in 1849, and the reaction still carries his name.
How does the Kolbe reaction work?
NCERT gives the path for sodium acetate. In water the salt gives acetate ions. At the anode, each acetate ion loses an electron to form an acetate free radical. This splits into a methyl radical and CO₂, and two methyl radicals join to give ethane. At the cathode, water is reduced to OH⁻ and hydrogen gas. The overall equation is:
2CH₃COO⁻Na⁺ + 2H₂O → CH₃–CH₃ + 2CO₂ + H₂ + 2NaOH
NCERT then asks why methane cannot be made this way. Two methyl radicals join, so the smallest possible product is ethane.
Worked example 1: four routes to butane, and the hard targets
Butane has four carbons. Work backwards with each rule:
Zinc and acid: from 1-chlorobutane (4 → 4).
Wurtz: from bromoethane, 2 × 2 = 4. NCERT's equation 9.8.
Decarboxylation: from sodium pentanoate, 5 − 1 = 4.
Kolbe: from sodium propanoate, 2 × (3 − 1) = 4.
Now try the awkward targets.
Methane: Wurtz and Kolbe both join two pieces, so neither can give a one-carbon product. Decarboxylation of sodium ethanoate works, as NCERT shows.
Propane: Wurtz with a single halide gives only even chains. NCERT's Problem 9.6 makes propane by decarboxylating sodium butanoate.
Mixed Wurtz: NCERT asks what happens with two different halides. With CH₃Br and C₂H₅Br, methyl can join methyl, ethyl can join ethyl, or methyl can join ethyl. You get ethane, butane and propane together, a mixture that is hard to separate.
Worked example 2: how much alkane from 8.2 g of sodium ethanoate?
The molar mass of CH₃COONa is 2(12) + 3(1) + 2(16) + 23 = 82 g mol⁻¹, so 8.2 g is 0.100 mol. Assume complete reaction (an idealised classroom case).
Decarboxylation: one mole of salt gives one mole of methane. 0.100 mol CH₄ × 16 g mol⁻¹ = 1.6 g.
Kolbe: two moles of salt give one mole of ethane. 0.050 mol C₂H₆ × 30 g mol⁻¹ = 1.5 g. The same equation also gives 0.050 mol of hydrogen at the cathode.
The same salt gives different products and different amounts. The 2 : 1 ratio in Kolbe's equation is the step students most often miss. Our post on mole concept errors covers the general habit of reading ratios off the balanced equation.
Common mistakes and how to avoid them
Kolbe without the lost carbon. Each acid loses CO₂ before the chains join. Sodium propanoate gives butane, not hexane.
Plain NaOH for decarboxylation. NCERT uses soda lime, a mixture of sodium hydroxide and calcium oxide, with heating.
Wet ether in Wurtz. NCERT says dry ether, free from moisture.
Forgetting the cathode in Kolbe. Hydrogen and NaOH form there, not at the anode.
Using Wurtz for an odd alkane. With one halide the product is always even.
Where does this fit with the rest of the chapter?
Preparation is one branch of the wider reaction map. Our guide to hydrocarbon reactions mapped out shows how alkanes connect to alkenes, alkynes and benzene, so you can chain these methods into conversion questions.
A practice routine
Make four columns headed with the carbon-count rules. Write five target alkanes from C1 to C6 and fill in every method that can make each.
Write balanced equations for NCERT equations 9.5 to 9.9 from memory, then check them.
Do two mass calculations like the one above, using sodium propanoate (96 g mol⁻¹) for both decarboxylation and Kolbe.
A week later, mix these with halogenation questions so you have to choose between preparing an alkane and reacting one.
When you are unsure which route a question wants, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer. Use it to check your choice after you have counted the carbons yourself.
Start now: list every method in this post that can give ethane, and write the equation for each.
