Read Which Alkane-Synthesis Method Actually Forms the C-C Bond
Every named alkane-synthesis method (Wurtz, Kolbe, Corey-House) forms a new C-C bond a different way — knowing exactly which two carbon fragments combine, and what by-products form, resolves nearly every question in this cluster.
Alkane Preparation: Wurtz, Kolbe and Corey-House is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Wurtz reaction: symmetrical coupling, even-carbon bias
Wurtz reaction (R-X + 2Na + X-R' → R-R' + 2NaX, in dry ether) proceeds via a free-radical-adjacent mechanism and works best for preparing SYMMETRICAL alkanes with an even number of carbons — reacting ethyl chloride with n-propyl chloride gives a statistical MIXTURE of ethane-ethane, propane-propane, and ethane-propane coupling products, so 'which product is NOT obtained' questions test recognising which specific coupling combinations are geometrically possible from the given starting halides.
Kolbe electrolysis: radical coupling from carboxylate salts
Kolbe electrolysis (2 RCOO- → R-R + 2CO2 + 2e-, at the anode) couples two alkyl radicals released after decarboxylation — electrolysing a MIXTURE of two different potassium carboxylates gives a statistical mixture of all three possible coupling products (symmetrical-A, symmetrical-B, and cross-coupled A-B), exactly analogous to the Wurtz reaction's own statistical mixture.
Corey-House: the reliable route to unsymmetrical alkanes
R2CuLi + R'-X → R-R') is the preferred method specifically BECAUSE it avoids the statistical-mixture problem of Wurtz/Kolbe — it reliably forms ONE specific unsymmetrical product from two different alkyl halides, going through a well-defined organocuprate intermediate rather than free radicals.
Alkanes from other functional groups: reduction and protonation routes
Ketones convert to alkanes via Clemmensen reduction (Zn-Hg/HCl) or Wolff-Kishner reduction (NH2NH2/KOH) — but NOT via LiAlH4, which reduces a ketone only as far as the alcohol, since it can't remove the oxygen entirely. Separately, hydrolysing a Grignard reagent (R-MgX + H2O → R-H) is a genuine alkane-forming reaction, but it does NOT form a new C-C bond — the alkyl group simply gains a proton where the metal was.
OlympIQ exam lens
- ✓Wurtz and Kolbe both proceed through radical-type intermediates and give statistical mixtures when two different alkyl groups are used.
- ✓Corey-House synthesis reliably gives ONE unsymmetrical product via a lithium dialkylcuprate intermediate — the preferred method when a specific unsymmetrical alkane is needed.
- ✓Clemmensen and Wolff-Kishner reduce a ketone all the way to an alkane; LiAlH4 only reduces it to an alcohol.
- ✓Grignard hydrolysis forms an alkane but does NOT form a new C-C bond — only Wurtz, Kolbe, and Corey-House do that among these methods.
Don't assume every 'alkane-forming' reaction forms a new C-C bond — Grignard hydrolysis and other protonation routes just cap an existing alkyl group with hydrogen, while Wurtz, Kolbe and Corey-House genuinely join two separate carbon fragments together.
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