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Aldehydes, Ketones & Carboxylic Acids Formulas For JEE 2026
Aldehydes, Ketones and Carboxylic Acids is an important chapter in JEE Chemistry, especially in the organic section. It mainly focuses on the reactions of carbonyl compounds, including nucleophilic addition, oxidation, and reduction reactions. Students also study key reactions like aldol condensation, Cannizzaro reaction, and reduction methods such as Clemmensen and Wolff–Kishner, which are commonly asked in exams.
The chapter further covers distinction tests like Tollens’ and Fehling’s tests, along with concepts related to the acidity of carboxylic acids and important named reactions such as Rosenmund, HVZ, and Wittig reactions. A clear understanding of these topics makes it easier to solve both conceptual and numerical questions. For quick revision, students can also use a well-organized JEE Mains Chemistry Formula PDF to review important reactions and key concepts efficiently.
Preparation of Aldehydes and Ketones
Oxidation of Alcohols
- $$1°$$ alcohol $$\xrightarrow{\text{PCC}}$$ Aldehyde
CH$$_3$$CH$$_2$$OH $$\xrightarrow{\text{PCC}}$$ CH$$_3$$CHO - $$2°$$ alcohol $$\xrightarrow{\text{CrO}_3/\text{H}^+ \text{ or PCC}}$$ Ketone
(CH$$_3$$)$$_2$$CHOH $$\xrightarrow{\text{Jones}}$$ (CH$$_3$$)$$_2$$CO
Rosenmund Reduction
$$$\text{R--COCl} + \text{H}_2 \xrightarrow{\text{Pd/BaSO}_4, \;\text{S}} \text{R--CHO} + \text{HCl}$$$
An acid chloride is selectively reduced to an aldehyde using H$$_2$$ with poisoned palladium catalyst.
Important
Rosenmund reduction cannot prepare formaldehyde (HCHO) because the starting material HCOCl (formyl chloride) is unstable.
Gattermann–Koch Reaction
$$$\text{C}_6\text{H}_6 + \text{CO} + \text{HCl} \xrightarrow{\text{AlCl}_3/\text{CuCl}} \text{C}_6\text{H}_5\text{CHO} + \text{HCl}$$$
Directly introduces a –CHO group onto benzene to give benzaldehyde.
Friedel–Crafts Acylation
$$$\text{C}_6\text{H}_6 + \text{R--COCl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{--CO--R} + \text{HCl}$$$
Gives an aromatic ketone. Example: C$$_6$$H$$_6$$ + CH$$_3$$COCl → C$$_6$$H$$_5$$COCH$$_3$$ (acetophenone).
Nucleophilic Addition Reactions
General Mechanism of Nucleophilic Addition
$$$\text{Nu}^- + \overset{\delta^+}{\text{C}}=\overset{\delta^-}{\text{O}} \longrightarrow \text{Nu--C--O}^- \xrightarrow{\text{H}^+} \text{Nu--C--OH}$$$
Reactivity: Aldehydes > Ketones (less steric hindrance and more electrophilic carbon in aldehydes).
Cyanohydrin Formation
$$$\text{R--CHO} + \text{HCN} \longrightarrow \text{R--CH(OH)(CN)}$$$
The –CN group can later be hydrolysed to –COOH, making this useful for chain extension.
Bisulphite Addition
$$$\text{R--CHO} + \text{NaHSO}_3 \longrightarrow \text{R--CH(OH)(SO}_3\text{Na)}$$$
Used for purification of aldehydes and methyl ketones.
Grignard Addition
- HCHO + RMgX $$\xrightarrow{\text{H}_3\text{O}^+}$$ R–CH$$_2$$OH ($$1°$$ alcohol)
- R'CHO + RMgX $$\xrightarrow{\text{H}_3\text{O}^+}$$ R'CH(OH)R ($$2°$$ alcohol)
- R'COR'' + RMgX $$\xrightarrow{\text{H}_3\text{O}^+}$$ R'R''C(OH)R ($$3°$$ alcohol)
Reactions Involving $$\alpha$$ Hydrogen
Aldol Condensation
$$$2 \;\text{CH}_3\text{CHO} \xrightarrow{\text{dil. NaOH}} \text{CH}_3\text{CH(OH)CH}_2\text{CHO}$$$
(3-hydroxybutanal — the "aldol")
On heating, the aldol loses water to give an $$\alpha$$,$$\beta$$-unsaturated aldehyde:
$$$\text{CH}_3\text{CH(OH)CH}_2\text{CHO} \xrightarrow{\Delta} \text{CH}_3\text{CH=CHCHO} + \text{H}_2\text{O}$$$
(crotonaldehyde)
Cross Aldol Condensation
$$$\text{C}_6\text{H}_5\text{CHO} + \text{CH}_3\text{CHO} \xrightarrow{\text{NaOH}} \text{C}_6\text{H}_5\text{CH=CHCHO} + \text{H}_2\text{O}$$$
(cinnamaldehyde) — Benzaldehyde has no $$\alpha$$-H, so it can only act as the electrophile.
Cannizzaro Reaction
Aldehydes with no $$\alpha$$-hydrogen undergo self-disproportionation in concentrated NaOH:
$$$2\;\text{HCHO} \xrightarrow{\text{conc. NaOH}} \text{HCOONa} + \text{CH}_3\text{OH}$$$
$$$2\;\text{C}_6\text{H}_5\text{CHO} \xrightarrow{\text{conc. NaOH}} \text{C}_6\text{H}_5\text{COONa} + \text{C}_6\text{H}_5\text{CH}_2\text{OH}$$$
Tip: Cannizzaro reaction is given only by aldehydes with no $$\alpha$$-hydrogen: HCHO, C$$_6$$H$$_5$$CHO, (CH$$_3$$)$$_3$$CCHO, etc. If $$\alpha$$-H is present, aldol condensation occurs instead.
Oxidation and Distinction Tests
Tollens' Test (Silver Mirror Test)
$$$\text{R--CHO} + 2[\text{Ag(NH}_3)_2]^+ + 2\text{OH}^- \longrightarrow \text{R--COO}^- + 2\text{Ag}\downarrow + 3\text{NH}_3 + \text{H}_2\text{O}$$$
Positive test: A shiny silver mirror deposits on the test tube wall.
Result: Aldehydes give a positive test; ketones do not.
Fehling's Test
$$$\text{R--CHO} + 2\text{Cu}^{2+}\text{(tartrate)} + 5\text{OH}^- \longrightarrow \text{R--COO}^- + \text{Cu}_2\text{O}\downarrow\text{(red)} + 3\text{H}_2\text{O}$$$
Positive test: Red precipitate of Cu$$_2$$O.
Result: Aliphatic aldehydes give positive test; aromatic aldehydes (like benzaldehyde) do not.
How to Identify Aldehydes vs. Ketones vs. Carboxylic Acids
| Test | Aldehyde | Ketone | Carboxylic Acid |
|---|---|---|---|
| Tollens' test | Silver mirror | No reaction | No reaction |
| Fehling's test | Red ppt. | No reaction | No reaction |
| NaHCO$$_3$$ test | No effervescence | No effervescence | CO$$_2$$ gas (brisk) |
| 2,4-DNP test | Yellow/orange ppt. | Yellow/orange ppt. | No reaction |
Reduction of Aldehydes and Ketones
Reduction to Alcohols
- NaBH$$_4$$ (mild, selective) — reduces aldehydes and ketones but not esters or acids.
- LiAlH$$_4$$ (strong) — reduces almost all C=O groups.
R–CHO $$\xrightarrow{\text{NaBH}_4}$$ R–CH$$_2$$OH ; R–CO–R' $$\xrightarrow{\text{NaBH}_4}$$ R–CH(OH)–R'
Reduction to Methylene (–CH$$_2$$–)
- Clemmensen reduction: Zn(Hg) / conc. HCl (acidic conditions) $$$\text{R--CO--R'} \xrightarrow{\text{Zn(Hg), HCl}} \text{R--CH}_2\text{--R'}$$$
- Wolff–Kishner reduction: NH$$_2$$NH$$_2$$ / KOH, ethylene glycol, $$\Delta$$ (basic conditions) $$$\text{R--CO--R'} \xrightarrow{\text{NH}_2\text{NH}_2,\;\text{KOH},\;\Delta} \text{R--CH}_2\text{--R'}$$$
Both give the same product but under different conditions (acid vs. base).
Tip: Clemmensen = acidic (Zn/HCl), Wolff–Kishner = basic (NH$$_2$$NH$$_2$$/KOH). Choose based on whether the substrate is sensitive to acid or base. Remember: "Clemmensen = aCidic".
Carboxylic Acids
Acidity
Factors Affecting Acid Strength
- Resonance stabilisation of RCOO$$^-$$ makes carboxylic acids much more acidic than alcohols. Typical pK$$_a$$ ≈ 4–5 (vs. ~16 for alcohols).
- Electron-withdrawing groups (EWG) near –COOH increase acidity (stabilise carboxylate ion by inductive effect).
Order: Cl$$_3$$CCOOH > Cl$$_2$$CHCOOH > ClCH$$_2$$COOH > CH$$_3$$COOH - Electron-donating groups (EDG) decrease acidity.
- Proximity effect: The closer the EWG to –COOH, the stronger the acid.
Preparation and Reactions
Methods of Preparation
- From nitriles: $$\text{R--CN} + 2\text{H}_2\text{O} \xrightarrow{\text{H}^+\text{ or OH}^-} \text{R--COOH} + \text{NH}_3$$
- From Grignard reagent: $$\text{RMgX} + \text{CO}_2 \xrightarrow{\text{H}_3\text{O}^+} \text{R--COOH}$$ (adds one carbon)
- From primary alcohols or aldehydes: $$\text{R--CH}_2\text{OH} \xrightarrow{\text{KMnO}_4/\text{H}^+} \text{R--COOH}$$
Fischer Esterification
$$$\text{R--COOH} + \text{R'--OH} \underset{\text{H}^+,\;\Delta}{\rightleftharpoons} \text{R--COOR'} + \text{H}_2\text{O}$$$
Acid Chloride Formation
$$$\text{R--COOH} + \text{SOCl}_2 \longrightarrow \text{R--COCl} + \text{SO}_2 + \text{HCl}$$$
Hell–Volhard–Zelinsky (HVZ) Reaction
$$$\text{R--CH}_2\text{--COOH} \xrightarrow{\text{Br}_2/\text{P}} \text{R--CHBr--COOH} + \text{HBr}$$$
Introduces a bromine at the $$\alpha$$-carbon of a carboxylic acid.
Decarboxylation
$$$\text{R--COONa} \xrightarrow{\text{NaOH/CaO},\;\Delta} \text{R--H} + \text{Na}_2\text{CO}_3$$$
The product has one fewer carbon than the starting acid.
Key Named Reactions Summary
Named Reactions Quick Reference
| Named Reaction | What It Does |
|---|---|
| Rosenmund reduction | RCOCl → RCHO |
| Gattermann–Koch | C$$_6$$H$$_6$$ → C$$_6$$H$$_5$$CHO |
| Friedel–Crafts acylation | C$$_6$$H$$_6$$ + RCOCl → ArCOR |
| Aldol condensation | 2 RCHO → $$\beta$$-hydroxy aldehyde |
| Cannizzaro | 2 RCHO (no $$\alpha$$-H) → RCOO$$^-$$ + RCH$$_2$$OH |
| Clemmensen reduction | RCOR' → RCH$$_2$$R' (Zn/HCl) |
| Wolff–Kishner reduction | RCOR' → RCH$$_2$$R' (NH$$_2$$NH$$_2$$/KOH) |
| Wittig reaction | R$$_2$$C=O → R$$_2$$C=CR'$$_2$$ |
| HVZ reaction | RCH$$_2$$COOH → RCHBrCOOH |
Aldehydes, Ketones & Carboxylic Acids Formulas For JEE 2026: Conclusion
Aldehydes, ketones, and carboxylic acids form a crucial part of organic chemistry in JEE preparation, as they involve a wide range of reactions and mechanisms. Understanding nucleophilic addition, oxidation, reduction, and important named reactions helps students build a strong conceptual foundation.
In addition, concepts like acidity of carboxylic acids and distinction tests play an important role in solving both theoretical and application-based questions. Regular revision of these reactions and concepts helps improve accuracy and confidence while attempting organic chemistry questions in competitive exams.
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