Subtopics - Aldehydes and Ketones (NEET)
Carbonyl chemistry: preparation, nucleophilic addition, named reactions, and analytical tests
1) Preparation of Aldehydes and Ketones
Aldehydes: oxidation of 1 degree alcohols (PCC for selective aldehyde), Gattermann-Koch (CO + HCl + AlCl3 on ArH), Stephen reduction (RCN + SnCl2/HCl), Rosenmund reduction (RCOCl + H2/Pd-BaSO4), Reimer-Tiemann (phenol + CHCl3/KOH), dry distillation of Ca salts. Ketones: oxidation of 2 degree alcohols, Friedel-Crafts acylation (RCOCl + ArH/AlCl3), organocadmium with acid chloride, acetoacetic ester synthesis.
2) Nucleophilic Addition Reactions
General mechanism: nucleophile attacks electrophilic carbonyl carbon, then protonation. HCN addition: base-catalysed, gives cyanohydrin (racemic if chiral centre formed). Grignard: gives alcohol (1/2/3 degree depending on carbonyl). Ammonia derivatives: hydroxylamine gives oxime, hydrazine gives hydrazone, phenylhydrazine gives phenylhydrazone, semicarbazide gives semicarbazone, 2,4-DNP gives orange precipitate. Alcohol addition: hemiacetal then acetal (acid-catalysed). Tischenko reaction: Al(OEt)3 converts aldehyde to ester.
3) Condensation and Named Reactions
Aldol condensation: requires alpha-hydrogen, base-catalysed, gives beta-hydroxy aldehyde then alpha-beta-unsaturated aldehyde. Crossed aldol with different carbonyl compounds. Cannizzaro reaction: aldehydes without alpha-hydrogen undergo disproportionation (one molecule oxidised to acid, other reduced to alcohol). Crossed Cannizzaro (HCHO preferentially reduced). Perkin condensation: ArCHO + acid anhydride gives cinnamic acid derivative. Benzoin condensation: 2 ArCHO + KCN gives alpha-hydroxy ketone. Reformatsky: alpha-bromoester + Zn + C=O gives beta-hydroxy ester.
4) Oxidation, Reduction, and Rearrangements
Oxidation: aldehydes by KMnO4 to acids, Tollen test (silver mirror), Fehling test (red Cu2O). Haloform reaction for methyl ketones. Reduction: to alcohols (LiAlH4, NaBH4, H2/Pt), to hydrocarbons (Clemmensen Zn-Hg/HCl, Wolff-Kishner NH2NH2/KOH). MPV reduction with Al(OiPr)3. Baeyer-Villiger: ketone + peracid gives ester (oxygen insertion). Beckmann: ketoxime + acid gives N-substituted amide (anti migration). Pinacol-pinacolone: 1,2-diol + H+ gives ketone via methyl migration.
5) Analysis and Distinguishing Tests
2,4-DNP test positive for all aldehydes and ketones (orange/yellow precipitate). Tollen test positive for aldehydes only (silver mirror). Fehling test positive for aliphatic aldehydes (red Cu2O). Schiff test positive for aldehydes (magenta colour). Iodoform test positive for methyl ketones and ethanol. Comparison table for systematic identification.
Aldehydes and Ketones Download Notes & Weightage Plan
For each topic in the Aldehydes and Ketones chapter below, you get (2) the exact resources to download and how to use them, and (3) a simple importance & time plan so NEET students know what to do first and what to revise last.
Preparation of Aldehydes and Ketones
PCC selective oxidation, Rosenmund reduction, Gattermann-Koch, Stephen, Friedel-Crafts acylation, acetoacetic ester synthesis.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Rosenmund reduction and Gattermann-Koch are favourite named reaction questions. PCC selectivity is a common comparison question.
- High-risk Area: Students forget that Rosenmund reduction uses poisoned catalyst (Pd-BaSO4/quinoline). Without poison, further reduction occurs.
- Best Practice Style: Write equations with all reagents and conditions from memory.
Nucleophilic Addition Reactions
HCN cyanohydrin, Grignard addition, ammonia derivatives table, acetal formation, Tischenko reaction.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Ammonia derivative names are directly tested. Grignard product class (1/2/3 degree) is a standard question.
- High-risk Area: Students confuse semicarbazone with hydrazone. Remember: semicarbazide = H2NNHCONH2 (has extra CONH2).
- Best Practice Style: Memorise the table by writing it out three times without looking.
Condensation and Named Reactions
Aldol, Cannizzaro, crossed variants, Perkin, benzoin, Reformatsky.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Aldol vs Cannizzaro distinction based on alpha-hydrogen is the single most tested concept in this section.
- High-risk Area: Students apply aldol conditions to formaldehyde (no alpha-H) or Cannizzaro conditions to acetaldehyde (has alpha-H).
- Best Practice Style: Practice categorising aldehydes: which undergo aldol, which undergo Cannizzaro?
Oxidation, Reduction, and Rearrangements
Tollen, Fehling, haloform. Clemmensen, Wolff-Kishner, MPV. Baeyer-Villiger, Beckmann, pinacol-pinacolone.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Clemmensen vs Wolff-Kishner conditions are directly asked. Baeyer-Villiger migratory aptitude is a conceptual favourite.
- High-risk Area: Using Clemmensen (strongly acidic) on acid-sensitive substrates, or Wolff-Kishner (strongly basic) on base-sensitive substrates.
- Best Practice Style: For each substrate, decide: use Clemmensen (acid-stable) or Wolff-Kishner (base-stable)?
Analysis and Distinguishing Tests
2,4-DNP for all carbonyls, Tollen for aldehydes, Fehling for aliphatic aldehydes, Schiff for aldehydes, iodoform for methyl ketones.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Fehling test fails for aromatic aldehydes (benzaldehyde). This exception is frequently tested.
- High-risk Area: Students think all aldehydes give positive Fehling test. Benzaldehyde does not reduce Fehling solution.
- Best Practice Style: Memorise the exceptions: benzaldehyde fails Fehling but passes Tollen.
Aldehydes and Ketones Chapter NEET Traps & Common Mistakes (Topic-Wise)
Each subtopic below is of the Aldehydes and Ketones chapter and shows what NEET students usually do wrong in NEET examination, a short example of the mistake, and how NEET frames the question to trick you with close options are given below.
Mistake Snapshot (What Students Do Wrong)
- Applying aldol to no-alpha-H aldehydes: Aldol condensation requires at least one component with alpha-hydrogen. Formaldehyde, benzaldehyde, and trimethylacetaldehyde have no alpha-H and undergo Cannizzaro reaction instead.
- Applying Cannizzaro to acetaldehyde: Acetaldehyde has alpha-hydrogen and undergoes aldol condensation with NaOH, not Cannizzaro reaction.
Benzaldehyde (no alpha-H) + conc. NaOH gives benzyl alcohol + sodium benzoate (Cannizzaro). Acetaldehyde (has alpha-H) + dil. NaOH gives 3-hydroxybutanal (aldol).
How NEET Frames The Trap
NEET gives NaOH with an aldehyde and asks the product. Students must first check for alpha-hydrogen to decide between aldol and Cannizzaro.
Q. Formaldehyde on treatment with concentrated NaOH gives:
A. Aldol product B. Methanol + sodium formate C. Cannizzaro product and aldol product both D. No reaction
Trick: Methanol + sodium formate via Cannizzaro reaction because formaldehyde has no alpha-hydrogen. One HCHO is oxidised to HCOONa and the other is reduced to CH3OH.
Mistake Snapshot (What Students Do Wrong)
- Thinking all aldehydes reduce Fehling: Benzaldehyde does NOT reduce Fehling solution. Only aliphatic aldehydes and formic acid give a positive Fehling test (red Cu2O precipitate). Benzaldehyde does reduce Tollen reagent (silver mirror).
- Confusing Tollen and Fehling selectivity: Tollen test is positive for all aldehydes (including benzaldehyde). Fehling is positive only for aliphatic aldehydes. This distinction is a classic NEET question.
Benzaldehyde + Tollen reagent gives silver mirror (positive). Benzaldehyde + Fehling solution gives no reaction. Acetaldehyde gives positive result with both.
How NEET Frames The Trap
A question asks which aldehyde does NOT reduce Fehling solution. Students who assume all aldehydes pass Fehling choose incorrectly.
Q. Which of the following does NOT give a positive Fehling test?
A. Formaldehyde B. Acetaldehyde C. Benzaldehyde D. Propanal
Trick: Benzaldehyde does not reduce Fehling solution because aromatic aldehydes cannot be oxidised by the mild alkaline Cu2+ reagent. However, it does give a positive Tollen test.
Mistake Snapshot (What Students Do Wrong)
- Using the wrong reduction for acid/base sensitive substrates: Clemmensen uses strongly acidic conditions (Zn-Hg/conc. HCl). Wolff-Kishner uses strongly basic conditions (NH2NH2/KOH/high temperature). Acid-sensitive groups survive Wolff-Kishner; base-sensitive groups survive Clemmensen.
- Thinking both give different products: Both convert C=O to CH2 (same product). The choice depends on substrate compatibility with acid or base, not on the desired product.
For a substrate with an acid-labile protecting group, use Wolff-Kishner (basic). For a substrate with a base-labile ester group, use Clemmensen (acidic).
How NEET Frames The Trap
NEET gives a substrate with an acid- or base-sensitive group and asks which reduction method to use. Students who do not consider side reactions choose incorrectly.
Q. To reduce a carbonyl group to CH2 in a compound containing an ester group, the preferred method is:
A. Wolff-Kishner reduction B. Clemmensen reduction C. Catalytic hydrogenation D. LiAlH4 reduction
Trick: Clemmensen reduction because esters are base-sensitive and would hydrolyse under Wolff-Kishner conditions. Clemmensen uses acidic conditions that leave esters intact.
Mistake Snapshot (What Students Do Wrong)
- Wrong group migrates: In Baeyer-Villiger oxidation, the group with higher migratory aptitude migrates to oxygen. Order: aryl > tertiary > secondary > primary > methyl. Students often assume the smaller group migrates.
- Forgetting ring expansion in cyclic ketones: Cyclic ketones give lactones (ring expanded by one atom) via Baeyer-Villiger. Cyclopentanone gives a six-membered lactone.
PhCOCH3 + mCPBA gives PhOCOCH3 (phenyl migrates, not methyl) because aryl has higher migratory aptitude than methyl.
How NEET Frames The Trap
A question gives an unsymmetrical ketone and asks for the Baeyer-Villiger product. Students who pick the wrong migrating group get the wrong ester.
Q. Baeyer-Villiger oxidation of methyl phenyl ketone (acetophenone) with mCPBA gives:
A. Methyl benzoate B. Phenyl acetate C. Benzoic acid + methanol D. No reaction
Trick: Phenyl acetate (CH3COOPh) because the phenyl group has higher migratory aptitude than methyl and migrates to oxygen, placing it next to the ester oxygen.
Mistake Snapshot (What Students Do Wrong)
- Wrong molecule gets oxidised/reduced: In crossed Cannizzaro between HCHO and another no-alpha-H aldehyde, HCHO is always the one that gets oxidised to formate. The other aldehyde gets reduced to its alcohol.
- Thinking both undergo self-Cannizzaro: In a mixture, crossed Cannizzaro is faster than self-Cannizzaro. HCHO is a better hydride donor, so it preferentially gets oxidised.
PhCHO + HCHO + NaOH gives PhCH2OH (benzyl alcohol) + HCOONa (sodium formate). HCHO is oxidised; PhCHO is reduced.
How NEET Frames The Trap
NEET gives a mixture of HCHO and another aldehyde with NaOH. Students must identify which is oxidised and which is reduced.
Q. When a mixture of benzaldehyde and formaldehyde is treated with concentrated NaOH, the products are:
A. Benzyl alcohol + sodium formate B. Sodium benzoate + methanol C. Benzyl alcohol + sodium benzoate D. Methanol + sodium formate
Trick: Benzyl alcohol + sodium formate because in crossed Cannizzaro, formaldehyde acts as the hydride donor (oxidised to formate) and benzaldehyde is reduced to benzyl alcohol.