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Class 12 Chemistry Chapter 8 Aldehydes, Ketones and Carboxylic Acids — Formulas & Key Points

Chapter 8 of NCERT Class 12 Chemistry focuses on three interconnected families — aldehydes, ketones and carboxylic acids — all built around the carbonyl functional group. Mastery of their nomenclature, interconversion reactions and mechanisms is critical for both board exams and competitive tests like JEE and NEET. This formula sheet compiles every reaction pathway, reagent condition, and mechanistic step in quick-reference tables, so you can revise the entire chapter in one sitting before your chemistry paper.

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Key takeaways

✓Aldehydes (R-CHO) and ketones (R-CO-R') both contain the carbonyl group C=O but differ in oxidation behavior — aldehydes oxidize easily, ketones resist.
✓IUPAC names use suffix '-al' for aldehydes and '-one' for ketones; carboxylic acids take '-oic acid' with carbon-1 always at the carboxyl group.
✓Nucleophilic addition to C=O proceeds via partial δ+ on carbon: HCN adds to form cyanohydrins, NH₃ derivatives form imines and oximes.
✓Tollen's reagent (silver mirror) and Fehling's test distinguish aldehydes from ketones — only aldehydes reduce these reagents.
✓Carboxylic acids are prepared by oxidation of primary alcohols or aldehydes with KMnO₄ or K₂Cr₂O₇, and by hydrolysis of nitriles.
✓Esterification of RCOOH with R'OH in presence of conc. H₂SO₄ yields esters RCOOR'; saponification reverses this with aqueous NaOH.
✓Hell-Volhard-Zelinsky reaction introduces α-halogen in carboxylic acids using Cl₂/Br₂ with red phosphorus.

Functional Group Structures and General Formulas

Understanding the structural differences between aldehydes, ketones and carboxylic acids is the foundation of this chapter. Aldehydes have the carbonyl carbon bonded to at least one hydrogen atom, making them terminal groups in carbon chains. Ketones have the carbonyl flanked by two carbon atoms, placing it within the chain. Carboxylic acids feature a hydroxyl group directly attached to the carbonyl carbon, creating the carboxyl functional group -COOH. These structural distinctions dictate reactivity: the C-H bond in aldehydes is susceptible to oxidation, while the ketone carbonyl is more stable. The table below captures these core structures and their homologous series formulas.

Aldehyde general formula: CₙH₂ₙ₊₁CHO or CₙH₂ₙO (for aliphatic)
Ketone general formula: CₙH₂ₙO (same molecular formula as aldehydes, structural isomers)
Carboxylic acid general formula: CₙH₂ₙ₊₁COOH or CₙH₂ₙO₂
Aromatic aldehydes: benzaldehyde C₆H₅CHO; aromatic acids: benzoic acid C₆H₅COOH

IUPAC Nomenclature Rules and Common Names

NCERT Class 12 Chemistry emphasises systematic IUPAC naming. For aldehydes, identify the longest carbon chain containing the -CHO group, number from the aldehyde carbon as C-1, replace the terminal 'e' of the alkane name with '-al'. For ketones, select the longest chain containing the carbonyl, number to give the carbonyl the lowest locant, and use suffix '-one'. Carboxylic acids follow similar logic: the -COOH carbon is always C-1, suffix is '-oic acid'. Substituents (methyl, chloro, nitro) are prefixed with their position numbers in alphabetical order. Aromatic compounds often retain trivial names: benzaldehyde, acetophenone, benzoic acid are widely accepted alongside IUPAC alternatives.

Methanal (formaldehyde), ethanal (acetaldehyde), propanal, butanal — aldehyde series
Propanone (acetone), butanone (methyl ethyl ketone), pentan-2-one, pentan-3-one — ketone series
Methanoic acid (formic acid), ethanoic acid (acetic acid), propanoic acid, butanoic acid — carboxylic acid series
For dicarboxylic acids: ethanedioic acid (oxalic), butanedioic acid (succinic), benzenedicarboxylic acids (phthalic, isophthalic, terephthalic)

Preparation of Aldehydes and Ketones — All Methods

The CBSE Class 12 syllabus covers five principal laboratory and industrial methods. (1) Oxidation of alcohols: primary alcohols (RCH₂OH) with PCC (pyridinium chlorochromate) in CH₂Cl₂ yield aldehydes without over-oxidation to acids; secondary alcohols (R₂CHOH) with K₂Cr₂O₇/H⁺ or CrO₃ give ketones. (2) Dehydrogenation of alcohols over copper at 573 K is catalytic and yields aldehydes or ketones. (3) Ozonolysis of alkenes followed by reductive workup with Zn/H₂O cleaves the double bond to form carbonyl compounds. (4) Calcium salt dry distillation: heating calcium salts of carboxylic acids (e.g. calcium acetate gives acetone). (5) Friedel-Crafts acylation of benzene with acid chlorides and AlCl₃ produces aromatic ketones. Each method is reagent-specific and condition-sensitive.

Primary alcohol + PCC → Aldehyde (stops at aldehyde, no further oxidation)
Secondary alcohol + K₂Cr₂O₇/H₂SO₄ → Ketone
Alkene + O₃ then Zn/H₂O → two carbonyl fragments (aldehydes or ketones depending on substitution)
RCOO⁻Ca²⁺ (calcium salt) dry distillation → R-CO-R (ketone) + CaCO₃

Preparation of Carboxylic Acids — All Routes

Carboxylic acids are synthesised by oxidation or hydrolysis. (1) Oxidation of primary alcohols or aldehydes with strong oxidising agents such as alkaline KMnO₄ or acidified K₂Cr₂O₇ converts them to carboxylic acids. (2) Hydrolysis of nitriles (R-CN) with dilute H₂SO₄ or NaOH at elevated temperature yields RCOOH plus ammonia or ammonium salt. (3) Hydrolysis of esters (RCOOR') with aqueous acid or base regenerates the parent carboxylic acid. (4) Grignard reagent reaction: RMgX reacts with CO₂ to form RCOOMgX, which on acidification gives RCOOH. (5) Oxidation of alkyl benzenes: toluene with hot alkaline KMnO₄ produces benzoic acid regardless of side-chain length. Each method is chosen based on starting material availability and desired scale.

Primary alcohol or aldehyde + KMnO₄ (alkaline) → RCOOH
Nitrile R-CN + H₂O / H⁺ (or OH⁻) → RCOOH + NH₃ (or NH₄⁺)
Ester RCOOR' + H₂O / H⁺ or NaOH → RCOOH + R'OH
Grignard: RMgX + CO₂ → RCOOMgX, then H₃O⁺ → RCOOH + Mg(OH)X

Nucleophilic Addition Reactions of Aldehydes and Ketones

The carbonyl carbon in aldehydes and ketones is electrophilic (δ⁺) due to oxygen electronegativity, making it prone to nucleophilic attack. (1) Addition of HCN forms cyanohydrins: R₂C=O + HCN → R₂C(OH)CN, important for chain extension. (2) Addition of sodium bisulfite NaHSO₃ yields crystalline adducts used for purification. (3) Addition of Grignard reagents RMgX followed by hydrolysis gives alcohols: aldehydes produce secondary alcohols, ketones produce tertiary alcohols. (4) Addition of alcohols in acid catalyst forms hemiacetals and then acetals (from aldehydes) or hemiketals and ketals (from ketones). (5) Addition of ammonia derivatives — NH₂OH (hydroxylamine) gives oximes, NH₂NH₂ (hydrazine) gives hydrazones, and NH₂-NH-C₆H₅ (phenylhydrazine) gives phenylhydrazones. Reaction rate: aldehydes react faster than ketones due to steric and electronic factors.

RCHO + HCN → RCH(OH)CN (cyanohydrin, catalysed by base)
RCHO + NaHSO₃ → RCH(OH)SO₃Na (bisulfite adduct)
RCHO + RMgX, then H₃O⁺ → secondary alcohol; R₂CO + RMgX → tertiary alcohol
RCHO + 2 R'OH / H⁺ → RCH(OR')₂ (acetal); R₂CO + 2 R'OH / H⁺ → R₂C(OR')₂ (ketal)

Oxidation and Reduction Reactions — Aldehydes vs Ketones

Oxidation behaviour cleanly distinguishes aldehydes from ketones. Aldehydes are easily oxidised to carboxylic acids by mild oxidising agents such as Tollen's reagent (ammoniacal AgNO₃), Fehling's solution (Cu²⁺ in alkaline tartrate), or even atmospheric oxygen. Ketones resist oxidation under normal conditions; harsh oxidants like conc. HNO₃ or KMnO₄ cleave the carbon chain. Reduction converts both to alcohols: catalytic hydrogenation (H₂/Ni or Pt) or metal hydrides like NaBH₄ and LiAlH₄ reduce aldehydes to primary alcohols and ketones to secondary alcohols. Clemmensen reduction (Zn-Hg/conc. HCl) and Wolff-Kishner reduction (NH₂NH₂/KOH, heat) convert carbonyl groups directly to -CH₂- (complete reduction to alkane). These methods are vital for synthesis and functional-group interconversion.

Aldehyde + Tollen's reagent → silver mirror (RCHO → RCOOH, Ag⁺ → Ag)
Aldehyde + Fehling's → red precipitate Cu₂O (RCHO → RCOOH, Cu²⁺ → Cu⁺)
Aldehyde + H₂/Ni → primary alcohol; Ketone + H₂/Ni → secondary alcohol
RCHO or R₂CO + Zn-Hg/HCl (Clemmensen) → RCH₃ or R₂CH₂ (complete reduction)

Key Reactions and Tests to Distinguish Aldehydes from Ketones

CBSE board exams frequently ask for chemical tests. (1) Tollen's test: warm aldehyde with ammoniacal silver nitrate; a shiny silver mirror forms on the test tube wall. Ketones show no reaction. (2) Fehling's test: warm aldehyde with Fehling's A (CuSO₄) and B (alkaline Rochelle salt); a brick-red precipitate of Cu₂O appears. Aromatic aldehydes like benzaldehyde do not reduce Fehling's due to electron withdrawal from the ring. (3) Iodoform test: methyl ketones (CH₃COR) and ethanal give a yellow precipitate of CHI₃ with I₂/NaOH. Other aldehydes and ketones are negative. (4) Sodium bisulfite test: both aldehydes and methyl ketones form white crystalline adducts; useful for purification, not differentiation. Remembering reagent colour changes and precipitate colours is crucial for practical viva and theory answers.

Tollen's reagent: [Ag(NH₃)₂]⁺ + RCHO → RCOO⁻ + Ag (mirror) + NH₃
Fehling's: Cu²⁺ (blue) + RCHO → Cu₂O (red ppt) + RCOOH
Iodoform: CH₃COR + I₂/NaOH → CHI₃ (yellow ppt) + RCOONa
Benzaldehyde does not reduce Fehling's but reduces Tollen's reagent

Reactions of Carboxylic Acids — Acidity, Salt Formation and Esterification

Carboxylic acids are weak acids (pKa ~4-5) that donate a proton from the -COOH group, forming carboxylate anions RCOO⁻ stabilised by resonance. They react with bases (NaOH, NaHCO₃, Na₂CO₃) to form salts and water; effervescence with carbonates/bicarbonates distinguishes them from phenols and alcohols. Esterification (Fischer esterification) occurs when RCOOH reacts with an alcohol R'OH in the presence of conc. H₂SO₄ as catalyst, yielding ester RCOOR' and water. The reaction is reversible and driven to completion by removing water or using excess alcohol. Esters have pleasant fruity odours and are named as alkyl alkanoates. Reaction with PCl₃, PCl₅ or SOCl₂ converts -COOH to acyl halides RCOCl, which are highly reactive intermediates for further synthesis.

RCOOH + NaOH → RCOONa (sodium salt) + H₂O
RCOOH + NaHCO₃ → RCOONa + H₂O + CO₂ (effervescence test)
RCOOH + R'OH / H₂SO₄ ⇌ RCOOR' (ester) + H₂O (esterification)
RCOOH + PCl₅ → RCOCl (acyl chloride) + POCl₃ + HCl

α-Hydrogen Reactions — Aldol Condensation and Hell-Volhard-Zelinsky

Aldehydes and ketones with at least one α-hydrogen (hydrogen on the carbon next to C=O) undergo base-catalysed aldol condensation. The α-hydrogen is acidic; base abstracts it to form an enolate ion, which acts as a nucleophile attacking the carbonyl of another molecule. The initial product is a β-hydroxy aldehyde or ketone (aldol), which on heating dehydrates to form an α,β-unsaturated carbonyl compound. For example, two molecules of ethanal yield 3-hydroxybutanal, which dehydrates to but-2-enal (crotonaldehyde). The Hell-Volhard-Zelinsky reaction halogenates the α-carbon of carboxylic acids: RCOOH + Cl₂ (or Br₂) with catalytic red phosphorus gives RCH(X)COOH. This is a key step in synthesising α-amino acids and α-hydroxy acids. Cross-aldol reactions between different carbonyl compounds are possible but give mixtures unless one lacks α-hydrogens.

2 CH₃CHO + dil. NaOH → CH₃CH(OH)CH₂CHO (aldol), then heat → CH₃CH=CHCHO + H₂O
Aldol condensation requires α-H; formaldehyde and benzaldehyde lack α-H so act only as electrophiles
Hell-Volhard-Zelinsky: CH₃CH₂COOH + Br₂/red P → CH₃CHBrCOOH (2-bromopropanoic acid)
α-Haloacids are intermediates for preparing α-amino acids and α-hydroxy acids

Memory Tricks, Mnemonics and Common Mistakes

Students often confuse aldehyde and ketone preparation methods or mix up oxidation test results. Remember 'PCC Stops At Aldehyde' — PCC oxidises primary alcohols only to aldehydes, not further to acids. For tests, 'Tollen Twins Silver' and 'Fehling's Red Brick' link reagents with their positive test colours. In IUPAC nomenclature, the mnemonic 'Al for Aldehyde, One for Ketone, Oic Acid for Carboxylic Acid' helps recall suffixes. A frequent error is writing Fehling's test positive for all aldehydes; aromatic aldehydes like benzaldehyde do not reduce Fehling's but do reduce Tollen's. Another pitfall: students forget that iodoform test is positive for methyl ketones and ethanal, not all aldehydes. In reaction mechanisms, always show the nucleophile attacking the δ+ carbonyl carbon, not the oxygen. Units and signs: pKa values are dimensionless; carboxylate ion RCOO⁻ carries a negative charge, not neutral. Practise writing condensed structures versus skeletal structures; examiners expect clarity.

'PCC Stops At Aldehyde' — no over-oxidation to carboxylic acid
'Tollen = Silver mirror; Fehling = Red Cu₂O' — colour association mnemonic
Iodoform test: 'Methyl Ketone & Ethanal' → yellow CHI₃ precipitate
Aldol requires α-H; no α-H means no enolate, no aldol condensation

Three Solved Mini-Examples Applying Chapter Formulas

These worked problems mirror CBSE board exam question styles and apply the formulas and reaction schemes from earlier sections. Example 1 tests preparation methods and nomenclature. Example 2 combines nucleophilic addition with subsequent reduction, typical of multi-step synthesis questions. Example 3 applies chemical tests to differentiate isomeric carbonyl compounds. Each solution shows reagent selection, intermediate structures, and final product with IUPAC naming. Practise these patterns to build speed and accuracy. The 2024 CBSE Class 12 Chemistry board paper carried 5 marks on a similar aldol-condensation mechanism question, so mastering reaction pathways pays off directly in marks. For doubt-clearing and step-by-step photo solutions, CBSETUTOR.ai offers 24×7 AI tutoring at ₹999/month for all classes 6-12, with a 3-day free trial to test the platform.

One-Glance Last-Minute Revision Box

This condensed checklist captures every formula, reagent and key concept from Chapter 8 in bullet form for rapid revision the night before your exam or during the 15-minute reading time. Cover functional group structures, IUPAC suffix rules, all preparation routes (oxidation, dry distillation, ozonolysis, nitrile hydrolysis, Grignard + CO₂), nucleophilic addition reactions (HCN, NaHSO₃, ROH, NH₂OH, RMgX), oxidation tests (Tollen's, Fehling's, iodoform), reduction methods (H₂/Ni, NaBH₄, Clemmensen, Wolff-Kishner), carboxylic acid reactions (salt formation, esterification, acyl chloride formation), aldol condensation criteria and product, Hell-Volhard-Zelinsky halogenation, and common mistakes. Keep this box printed or saved as a phone screenshot for quick glances between other chapter revisions. Combine with NCERT intext questions and past three years' board papers for full coverage.

Aldehydes R-CHO (suffix -al); Ketones R-CO-R' (suffix -one); Carboxylic acids R-COOH (suffix -oic acid)
Preparation: 1° alcohol + PCC → aldehyde; 2° alcohol + K₂Cr₂O₇ → ketone; R-CN + H₂O → RCOOH
Nucleophilic addition: + HCN → cyanohydrin; + NH₂OH → oxime; + RMgX then H₃O⁺ → alcohol
Tollen's test (aldehyde +ve, silver mirror); Fehling's (aldehyde +ve except aromatic, red ppt); Iodoform (CH₃COR or CH₃CHO, yellow ppt)
Reduction: + H₂/Ni → alcohol; Clemmensen (Zn-Hg/HCl) or Wolff-Kishner (NH₂NH₂/KOH) → alkane
RCOOH + NaHCO₃ → CO₂ effervescence; + ROH/H₂SO₄ → ester; + PCl₅ → RCOCl
Aldol: 2 RCHO (with α-H) + OH⁻ → aldol, heat → α,β-unsaturated carbonyl
Hell-Volhard-Zelinsky: RCOOH + X₂/red P → α-halo acid

Frequently asked questions

What is the IUPAC name of acetone and how do you write its structure?+

The IUPAC name of acetone is propanone. Its structure is CH₃-CO-CH₃, a three-carbon ketone with the carbonyl group at the second carbon (position 2). The common name acetone is widely used in labs and industry.

How do you distinguish between an aldehyde and a ketone using a chemical test?+

Use Tollen's reagent (ammoniacal AgNO₃) or Fehling's solution. Aldehydes reduce both reagents — Tollen's gives a silver mirror, Fehling's gives a red precipitate of Cu₂O. Ketones show no reaction with either test under normal conditions.

Why does benzaldehyde not reduce Fehling's solution but reduces Tollen's reagent?+

Benzaldehyde is less reactive than aliphatic aldehydes due to electron-withdrawing resonance from the benzene ring, which stabilises the carbonyl. Fehling's reagent is a weaker oxidant than Tollen's, so benzaldehyde does not reduce it, but the stronger Tollen's reagent can still oxidise it to benzoate ion and deposit metallic silver.

What reagent converts a primary alcohol to an aldehyde without further oxidation to a carboxylic acid?+

PCC (pyridinium chlorochromate) in anhydrous dichloromethane (CH₂Cl₂) is the reagent of choice. It selectively oxidises primary alcohols to aldehydes and stops there, preventing over-oxidation to carboxylic acids. This is a standard NCERT Class 12 preparation method.

What is the product when acetaldehyde reacts with HCN, and what is this compound called?+

Acetaldehyde (CH₃CHO) reacts with HCN in the presence of a base catalyst to form CH₃CH(OH)CN, known as lactonitrile or 2-hydroxypropanenitrile. This is an example of nucleophilic addition forming a cyanohydrin.

Which compounds give a positive iodoform test?+

Compounds with the structure CH₃CO- (methyl ketones) or CH₃CH(OH)- (secondary alcohols oxidisable to methyl ketones) give a positive iodoform test, yielding a yellow precipitate of CHI₃. Ethanal (CH₃CHO) also tests positive. Ethanol is positive because it oxidises to ethanal in situ.

How is benzoic acid prepared from toluene in the laboratory?+

Toluene (methylbenzene, C₆H₅CH₃) is oxidised with alkaline potassium permanganate (KMnO₄) under reflux. The methyl group is oxidised completely to a carboxyl group, yielding benzoic acid C₆H₅COOH. This is a standard CBSE Class 12 preparation of carboxylic acids from aromatic hydrocarbons.

What is the Hell-Volhard-Zelinsky reaction and where is it used?+

The Hell-Volhard-Zelinsky reaction halogenates the α-carbon (carbon adjacent to -COOH) of a carboxylic acid using Cl₂ or Br₂ with red phosphorus as catalyst, yielding α-haloacids. It is important for synthesising α-amino acids and α-hydroxy acids in organic chemistry.

What is aldol condensation and what type of compounds undergo it?+

Aldol condensation is a base-catalysed reaction where two carbonyl compounds with at least one α-hydrogen combine to form a β-hydroxy aldehyde or ketone (aldol), which upon heating loses water to form an α,β-unsaturated carbonyl compound. Only aldehydes or ketones with α-H participate as nucleophiles.

How does CBSETUTOR.ai help students master organic chemistry reactions and mechanisms for board exams?+

CBSETUTOR.ai provides 24×7 AI-powered tutoring for CBSE Classes 6-12 at a flat ₹999/month. Students can upload photos of tricky reaction mechanisms or nomenclature problems and receive step-by-step worked solutions instantly. A 3-day free trial lets you test the platform before committing, making last-minute Chapter 8 doubts easy to resolve.

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