Named Reactions Recap
On this page
Direct answer
Roughly a dozen name reactions supply the organic questions every session, and mastery means holding three columns together for each — substrate, reagent with conditions, product. The carbonyl pair splits on one question: alpha hydrogens present gives the aldol condensation under dilute alkali; absent gives Cannizzaro disproportionation under concentrated alkali. Reduction of C=O to CH2 comes via Clemmensen (Zn-Hg with HCl, acidic) or Wolff-Kishner (hydrazine with KOH in ethylene glycol, basic) — chosen by what else the molecule tolerates. The nitrogen cluster runs on diazonium chemistry: Sandmeyer with copper(I) salts, Gattermann with copper powder, Balz-Schiemann with HBF4 for fluoroarenes. And the aromatic trio — Reimer-Tiemann, Kolbe, Friedel-Crafts — turns phenol and benzene into the aldehydes, acids and alkylated rings the exam sketches.
What you must remember
- Aldol versus Cannizzaro: alpha-H plus dilute NaOH gives the beta-hydroxy carbonyl (heat dehydrates it); no alpha-H (HCHO, benzaldehyde) plus concentrated alkali disproportionates to alcohol plus salt.
- C=O to CH2 pair: Clemmensen Zn(Hg)/HCl — acid-stable substrates only; Wolff-Kishner NH2NH2, KOH, ethylene glycol — base-stable substrates.
- Hofmann bromamide: RCONH2 + Br2 + KOH gives RNH2 with one carbon fewer — the amine ladder-shrinker.
- Gabriel phthalimide: phthalimide's potassium salt alkylated then hydrolysed gives pure primary amines; secondary, tertiary and aryl amines cannot be made this way.
- Diazonium hub: ArN2+ + Cu2Cl2/HCl → ArCl (Sandmeyer); + Cu powder/HCl → ArCl (Gattermann); + HBF4 then heat → ArF (Balz-Schiemann); + water warm → phenol; + CuCN/KCN → benzonitrile.
- Aromatic two-step: Reimer-Tiemann (CHCl3 + NaOH on phenol) → salicylaldehyde; Kolbe (phenoxide + CO2 then acid) → salicylic acid, the aspirin precursor.
- HVZ: carboxylic acid + red phosphorus and Br2 gives alpha-bromo acid; Etard (CrO2Cl2 on toluene) stops at benzaldehyde; Rosenmund (H2 over Pd-BaSO4, poisoned by quinoline-sulphur) reduces acid chloride to aldehyde.
- Halide toolkit: Wurtz (Na, dry ether, R-X + R-X → R–R), Fittig (aryl), Wurtz-Fittig (alkyl + aryl), Swarts (AgF swaps F for Cl/Br), Finkelstein (NaI in acetone swaps I for Cl/Br).
One-line recall of the twelve, then two mechanisms
Recall as triplets. Aldehyde-making: Rosenmund (acid chloride), Stephen (nitrile with SnCl2/HCl), Etard (toluene). Nitrogen: Hofmann (−1 carbon), Gabriel (pure primary), carbylamine (isocyanide stink test for primary amines). Carbonyl: aldol, Cannizzaro, with Clemmensen and Wolff-Kishner as the third axis. Aromatic: Friedel-Crafts alkylation and acylation (AlCl3; acylation avoids rearrangement), Reimer-Tiemann, Kolbe. Halides: Wurtz, Fittig, Wurtz-Fittig, Swarts, Finkelstein.
Two mechanisms repay deeper reading. Cannizzaro: concentrated hydroxide adds to one aldehyde; the resulting intermediate transfers a hydride to a second aldehyde — one molecule oxidised to acid (as its salt), the other reduced to alcohol; formaldehyde, always the easiest oxidiser, drags benzaldehyde to benzyl alcohol in a crossed reaction. Hofmann: N-bromination, base abstraction, then the rearrangement in which the R group migrates from carbonyl carbon to nitrogen as bromide leaves — the isocyanate hydrolyses instantly to the amine with one carbon fewer. The migration step explains why the product cleanly loses exactly one carbon, and "one carbon fewer" is the phrase to match in the options.
Hinge facts that decide the name
The hinge facts fail first: aldol or Cannizzaro is decided by alpha hydrogens, not by the aldehyde's size; formaldehyde undergoes only Cannizzaro (no alpha carbon at all). Clemmensen or Wolff-Kishner is decided by acid-sensitive or base-sensitive groups elsewhere in the molecule. Sandmeyer against Gattermann: copper(I) salt against copper powder — the products coincide for chlorine, but Sandmeyer alone works well for cyano. Gabriel's limitation is a favourite statement to falsify: no secondary, no tertiary, no aniline. Rosenmund's poisoned catalyst is "why not over-reduce to the alcohol" — the answer the reason-statement wants. And in Wurtz reactions, an odd result with two different alkyl halides gives a three-product mixture, so the exam only runs it with identical partners.
Frequently asked questions
Which reaction converts an amide to an amine with one carbon fewer?
Hofmann bromamide degradation — Br2 with KOH on RCONH2 yields RNH2, losing the carbonyl carbon as carbonate.
What decides aldol versus Cannizzaro for an aldehyde?
Alpha hydrogens: present means aldol under dilute alkali; absent means Cannizzaro disproportionation under concentrated alkali.
Why is the palladium poisoned in Rosenmund reduction?
BaSO4 with quinoline-sulphur partially deactivates the catalyst so the acid chloride stops at the aldehyde instead of reducing to the alcohol.
What does the HVZ reaction accomplish?
Alpha-bromination of a carboxylic acid using red phosphorus and bromine, stopping cleanly at the alpha-bromo acid.
How is fluorobenzene prepared from benzenediazonium salt?
Balz-Schiemann: diazonium tetrafluoroborate with HBF4, gently heated, gives fluorobenzene with nitrogen and BF3 leaving.