GATE Organic Chemistry — Name Reagents and What They Do
Reading a synthesis scheme quickly depends on recognising a reagent by its formula and immediately knowing what it does and, just as importantly, what it will not touch. GATE loves questions built entirely around selectivity — two reagents that look interchangeable on paper but give completely different products. This is a reference list of the reagents that come up repeatedly, organised by what they do, with three worked comparisons that GATE tests most often.
Reducing agents
| Reagent | Reduces | Selectivity note |
|---|---|---|
| LiAlH₄ (lithium aluminium hydride) | Esters, acids, amides, nitriles and epoxides — all the way to the alcohol/amine stage | A powerful, non-selective hydride source; reacts violently with water, must be used in dry ether |
| NaBH₄ (sodium borohydride) | Aldehydes and ketones to alcohols | Much milder — does not reduce esters, acids or amides; can be used directly in methanol/ethanol |
| DIBAL-H (diisobutylaluminium hydride) | Esters and nitriles, but only to the aldehyde stage at low temperature (−78 °C) | A controlled partial reduction; at room temperature it over-reduces, behaving like LiAlH₄ |
| Zn(Hg) / conc. HCl (Clemmensen) | Aldehydes/ketones fully to CH₂ | Strongly acidic conditions — see the named-reactions article for the base-stable alternative, Wolff-Kishner |
Worked example 1 — LiAlH₄ vs NaBH₄ on the same ester. Ethyl acetate,
CH₃COOC₂H₅, is treated separately with (a) LiAlH₄ and (b) NaBH₄.
(a) LiAlH₄ reduces an ester all the way: RCOOR' + 4[H] → RCH₂OH + R'OH. For ethyl acetate,
the acyl part (R = CH₃) becomes CH₃CH₂OH and the alkoxy part (R' = C₂H₅) is released as
C₂H₅OH — both are simply ethanol, so the product is ethanol in both
fragments.
(b) NaBH₄ is far too mild to attack an ester under ordinary conditions — the ester is
essentially unreacted.
This is the single most-tested selectivity fact between the two hydrides: NaBH₄ is safe to use
on a molecule that also carries an ester group you want to leave alone; LiAlH₄ is not.
Oxidising agents
| Reagent | Oxidises | Selectivity note |
|---|---|---|
| PCC (pyridinium chlorochromate) | 1° alcohol → aldehyde (stops there); 2° alcohol → ketone | Mild, anhydrous — the reagent of choice when you must stop at the aldehyde |
| Jones reagent (CrO₃/H₂SO₄) | 1° alcohol → carboxylic acid (goes all the way); 2° alcohol → ketone | Aqueous acidic conditions push a 1° alcohol past the aldehyde stage |
| KMnO₄ (hot, acidic or alkaline) | 1° alcohols/aldehydes → acids; alkyl benzylic side chains on a ring → −COOH regardless of chain length | A strong, non-selective oxidant; also cleaves alkenes oxidatively under hot concentrated conditions |
| Baeyer's reagent (cold dilute alkaline KMnO₄) | Alkene → vicinal diol (syn addition) | The standard test for unsaturation — the purple colour is decolorised |
| Etard reagent (CrO₂Cl₂, chromyl chloride) | A toluene-type aromatic methyl group → benzaldehyde | Stops cleanly at the aldehyde via a stable addition complex, unlike KMnO₄ which continues to the acid |
Worked example 2 — PCC vs Jones reagent on the same alcohol. 1-Butanol,
CH₃CH₂CH₂CH₂OH, is oxidised separately with (a) PCC and (b) Jones reagent.
(a) PCC stops at the aldehyde: product is butanal, CH₃CH₂CH₂CHO.
(b) Jones reagent continues through to the acid: product is
butanoic acid, CH₃CH₂CH₂COOH.
Same starting alcohol, same oxidation direction, genuinely different products — this is the
exact reagent-choice question GATE is fond of asking.
Identification reagents (qualitative tests)
| Reagent | Positive result | Note |
|---|---|---|
| Tollens' reagent, [Ag(NH₃)₂]⁺ | Silver mirror on the test-tube wall | Oxidises any aldehyde (aliphatic or aromatic); does not react with ketones |
| Fehling's solution (alkaline Cu²⁺/tartrate) | Brick-red Cu₂O precipitate | Oxidises aliphatic aldehydes only — aromatic aldehydes such as benzaldehyde give a negative test |
| Lucas reagent (conc. HCl + anhydrous ZnCl₂) | Turbidity, at a rate that identifies the alcohol class | 3° alcohol turns turbid immediately, 2° within a few minutes, 1° shows no reaction at room temperature |
Worked example 3 — telling three unknowns apart. Three unlabelled bottles contain acetaldehyde (an aliphatic aldehyde), benzaldehyde (an aromatic aldehyde) and acetone (a ketone). Use Tollens' and Fehling's tests to identify each.
| Compound | Tollens' test | Fehling's test |
|---|---|---|
| Acetaldehyde | Positive (mirror) | Positive (brick-red) |
| Benzaldehyde | Positive (mirror) | Negative |
| Acetone | Negative | Negative |
Tollens' positive plus Fehling's positive identifies the aliphatic aldehyde; Tollens' positive plus Fehling's negative identifies the aromatic aldehyde; both negative leaves only the ketone. Two cheap tests, three unambiguous identifications.
Functional-group interconversion reagents
| Reagent | Converts | Note |
|---|---|---|
| PBr₃ | R−OH → R−Br | Alcohol to alkyl bromide |
| SOCl₂ (thionyl chloride) | R−OH → R−Cl | Byproducts (SO₂, HCl) are gases, so the product is very easy to isolate cleanly |
| PCl₅ | R−OH → R−Cl | Also gives POCl₃ as a byproduct — messier workup than SOCl₂ |
| NBS (N-bromosuccinimide) | Allylic/benzylic C−H → C−Br | Maintains a low, steady Br₂ concentration so the alkene itself is not brominated by addition |
| O₃, then Zn/H₂O or H₂O₂ | Alkene → two carbonyl fragments (ozonolysis) | Reductive workup (Zn/H₂O) gives aldehydes/ketones; oxidative workup (H₂O₂) carries any aldehyde fragment on to a carboxylic acid |
| Swarts reagent (AgF, SbF₃, or similar) | R−Cl / R−Br → R−F | Halogen-exchange route to alkyl fluorides, which are not accessible by direct fluorination |
Common mistakes that cost marks
- Assuming NaBH₄ can substitute for LiAlH₄. It cannot touch an ester, acid or amide under normal conditions — this is the most-tested trap on this whole list.
- Forgetting DIBAL-H needs low temperature to stop at the aldehyde. At room temperature it behaves like a full hydride reducer and over-reduces to the alcohol.
- Expecting Fehling's to detect benzaldehyde. Aromatic aldehydes give a negative Fehling's test — a genuine and frequently-tested exception.
- Reading the Lucas test as a simple yes/no. The result is the rate of turbidity, not its presence or absence — a 1° alcohol does eventually react, just not within the timescale of the test at room temperature.
- Confusing PCC (stops at the aldehyde) with stronger oxidants like Jones reagent or hot KMnO₄ (go to the acid). The choice of reagent, not just the substrate, decides where oxidation stops.
Where this appears in GATE Chemistry
| Question style | What it is testing |
|---|---|
| "Predict the product" with a named reagent given | Recognising the reagent and its known selectivity |
| "Which reagent distinguishes X from Y?" | Choosing a test that gives different results for two similar compounds |
| "Which reagent would leave [a functional group] untouched?" | Reagent selectivity, not just reactivity |
Verify the stoichiometry of any transformation. The 4AB Calculator Suite's molar mass tool is a quick way to check a reagent's equivalents in a multi-step synthesis problem.
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