GATE Organic Chemistry — The Named Reactions That Repeat
A large share of GATE organic questions boil down to recognising a named reaction from its reagents and predicting the product — or the reverse, naming the reaction that would achieve a given transformation. This is a reference list of the bond-forming, reduction and substitution named reactions that come up again and again, organised so you can scan for the reagent you recognise, plus three fully worked product-prediction examples.
Bond-forming named reactions
| Name | Reagents / conditions | What it does |
|---|---|---|
| Grignard reaction | RMgX (from RX + Mg, dry ether), then aqueous acid workup | Adds R to a carbonyl carbon; product depends on the substrate (see table below) |
| Wittig reaction | Ph₃P=CHR (a phosphorus ylide) + aldehyde/ketone | Converts C=O directly to C=CHR, releasing Ph₃P=O; installs a double bond at a defined position |
| Aldol condensation | Enolisable aldehyde/ketone + dilute base (or acid), then heat | Self-addition gives a β-hydroxy carbonyl; heating dehydrates it to an α,β-unsaturated carbonyl |
| Claisen condensation | Two ester molecules + strong base (NaOEt) | Forms a β-ketoester (e.g. ethyl acetoacetate from ethyl acetate); needs an α-hydrogen on the ester |
| Reformatsky reaction | α-haloester + Zn, then a carbonyl compound | A milder, zinc-enolate version of the Grignard addition; forms a β-hydroxyester |
| Perkin reaction | Aromatic aldehyde + acid anhydride, base catalyst (e.g. sodium acetate) | Forms an α,β-unsaturated aromatic acid (e.g. cinnamic acid) |
| Knoevenagel condensation | Aldehyde/ketone + active-methylene compound (e.g. malonic ester), weak base (piperidine) | Milder condensation with active-methylene nucleophiles, giving an α,β-unsaturated product |
| Diels-Alder reaction | Conjugated diene + dienophile (alkene bearing an electron-withdrawing group) | [4+2] pericyclic cycloaddition forming a six-membered ring, stereospecific |
| Williamson ether synthesis | Alkoxide + primary alkyl halide (SN2) | Forms an ether; works cleanly only with a primary (or methyl) halide |
| Gabriel synthesis | Potassium phthalimide + RX (SN2), then hydrolysis or hydrazinolysis | Gives a pure primary amine with no over-alkylation, unlike direct amine alkylation |
| Sandmeyer reaction | Aryl diazonium salt + CuCl, CuBr or CuCN | Replaces −N₂⁺ with Cl, Br or CN on the ring — a route to aryl halides/nitriles not open to direct substitution |
Aromatic substitution and reduction named reactions
| Name | Reagents / conditions | What it does |
|---|---|---|
| Friedel-Crafts alkylation | Arene + RX / AlCl₃ | Installs an alkyl group; carbocation can rearrange, and the product ring (more activated) tends toward over-alkylation |
| Friedel-Crafts acylation | Arene + RCOCl (or anhydride) / AlCl₃ | Installs an acyl group with no rearrangement (the acylium ion is resonance-stabilised); the ketone product deactivates the ring, so it stops cleanly at mono-substitution |
| Clemmensen reduction | Carbonyl + Zn(Hg) / conc. HCl | Reduces C=O all the way to CH₂ under strongly acidic conditions |
| Wolff-Kishner reduction | Carbonyl + N₂H₄ (hydrazine), then KOH, heat | Also reduces C=O to CH₂, but under basic conditions — the complementary choice when the substrate cannot survive acid |
| Cannizzaro reaction | Aldehyde with no α-hydrogen + conc. NaOH | Disproportionation: one molecule is oxidised to the carboxylate, another reduced to the alcohol |
Worked example 1 — Grignard, and how the product changes with the substrate. A Grignard reagent RMgX behaves differently depending on what it adds to:
| Substrate | Product after H₃O⁺ workup |
|---|---|
| Formaldehyde, HCHO | Primary alcohol, RCH₂OH |
| Any other aldehyde, R'CHO | Secondary alcohol, R'CH(OH)R |
| Ketone | Tertiary alcohol |
| CO₂ | Carboxylic acid, RCOOH |
| Ester (2 equivalents of RMgX) | Tertiary alcohol |
| Epoxide (e.g. ethylene oxide) | Primary alcohol, two carbons longer |
So C₆H₅MgBr reacting with acetaldehyde (CH₃CHO, an aldehyde other than formaldehyde) gives the secondary alcohol C₆H₅CH(OH)CH₃ (1-phenylethanol) after workup — not a primary alcohol, because only formaldehyde gives that outcome.
Worked example 2 — Wittig reaction. Methylenetriphenylphosphorane
(Ph₃P=CH₂, generated from methyltriphenylphosphonium bromide and a strong base such as
n-BuLi) reacts with cyclohexanone.
The ylide's carbanion attacks the carbonyl carbon, and the resulting four-membered
oxaphosphetane collapses to expel triphenylphosphine oxide, Ph₃P=O, leaving the new C=C bond
exactly where the C=O bond was.
Product: methylenecyclohexane (the ring with an exocyclic =CH₂), plus Ph₃P=O
as the by-product. Unlike acid-catalysed dehydration of an alcohol, the Wittig places the
double bond with no ambiguity about position — a major reason it is preferred for planned
alkene synthesis.
Worked example 3 — Cannizzaro reaction. Benzaldehyde has no α-hydrogen (the
carbon next to the carbonyl is the aromatic ring), so it cannot undergo an aldol reaction.
Treated with concentrated NaOH, it instead disproportionates:
2 C₆H₅CHO + NaOH → C₆H₅CH₂OH + C₆H₅COONa
One molecule of benzaldehyde is reduced to benzyl alcohol; the other is oxidised to sodium
benzoate. The reaction only runs this way because there is no α-hydrogen available for the
base to remove — with one present, aldol chemistry would dominate instead.
Common mistakes that cost marks
- Mixing up Clemmensen and Wolff-Kishner conditions. Clemmensen is strongly acidic (Zn(Hg)/HCl) and will not tolerate acid-sensitive groups; Wolff-Kishner is basic (NH₂NH₂/KOH) and will not tolerate base-sensitive groups. Picking the wrong one for a substrate that cannot survive those conditions is a classic trap.
- Forgetting Friedel-Crafts fails on a strongly deactivated ring. Neither alkylation nor acylation proceeds on nitrobenzene-type substrates — the ring is too electron-poor to attack the electrophile.
- Applying Cannizzaro to an aldehyde that has an α-hydrogen. If an α-hydrogen is present, base-catalysed self-condensation (aldol) happens instead of disproportionation.
- Confusing the aldol addition product with the aldol condensation product. The addition product is the β-hydroxy carbonyl formed first; "condensation" specifically refers to the dehydrated, α,β-unsaturated product formed on heating.
- Trying to make an aromatic amine by the Gabriel synthesis. The method needs an SN2-reactive alkyl halide; aryl halides do not undergo SN2, so Gabriel synthesis only ever produces a primary alkyl amine.
Where this appears in GATE Chemistry
| Question style | What it is testing |
|---|---|
| "Identify the major product" given reagents | Recognising the named reaction from its reagent combination |
| "Which reagent would you use to convert X to Y?" | The reverse — choosing the right named reaction for a target transformation |
| "Which of these conditions would fail?" | Knowing the scope limits (no α-H, deactivated ring, acid/base sensitivity) |
Once you have identified the product, verify related numericals. The ABC Chemistry Calculator Suite's molar mass and formula tools are useful for checking the stoichiometry of any synthesis scheme you work through.
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