Group
Group
Preferred Source
Google Discover
Aldehydes, Ketones & Carboxylic Acids Formulas For JEE 2027
Aldehydes, Ketones and Carboxylic Acids in JEE Chemistry covers the structure, preparation, properties, and reactions of these organic compounds. Key concepts include nucleophilic addition to aldehydes and ketones, oxidation and reduction, and the acidity of carboxylic acids. Understanding how functional groups and substituents affect reactivity helps students predict products and solve organic conversion questions.
Focus on the reagents, conditions, and structural requirements of reactions such as aldol condensation, Cannizzaro, Rosenmund reduction, Clemmensen reduction, and Wolff–Kishner reduction. Revise distinguishing tests, including Tollens’ and Fehling’s tests, alongside their limitations. Use a JEE Mains formula sheet to review key reactions, then practise their application through chapter-wise questions. Taking a JEE Mains mock test can help you identify gaps in reagent recall, product prediction, and reaction sequencing.
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 2027: Conclusion
Preparing Aldehydes, Ketones and Carboxylic Acids well requires connecting the structure of each compound with its reactivity. Focus on nucleophilic addition, oxidation, reduction, and named reactions, paying attention to reagents, conditions, and the products formed. Understanding these relationships will help you approach unfamiliar questions more confidently.
Revise acidity trends and distinguishing tests alongside reaction mechanisms and organic conversions. Solve previous-year questions, review incorrect answers, and keep a concise reaction summary for quick revision. Consistent practice can strengthen your recall and improve accuracy in product prediction and reaction-based questions.
.webp)
