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CBSE Class 12 Chemistry Chapter 8 Aldehydes, Ketones and Carboxylic Acids — 20 MCQs with Answers

Chapter 8 of NCERT Class 12 Chemistry — Aldehydes, Ketones and Carboxylic Acids — accounts for roughly 4–6 marks in the CBSE board paper, appearing both as standalone MCQs and in assertion-reason format. The chapter hinges on understanding the carbonyl group's electrophilic nature, nucleophilic addition versus substitution pathways, and chemical tests that distinguish aldehyde from ketone. Below are 20 multiple-choice questions organised by sub-topic, each with four options and a concise explanation to reinforce NCERT concepts and sharpen your exam accuracy.

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Key takeaways

  • ✓20 exam-style MCQs mirror the CBSE Class 12 Chemistry board paper pattern, mixing one-mark recall and assertion-reason questions on Aldehydes, Ketones and Carboxylic Acids.
  • ✓Questions cover NCERT Chapter 8 topics: IUPAC nomenclature, preparation (oxidation of alcohols, ozonolysis, hydration of alkynes), nucleophilic addition mechanisms, and distinction tests.
  • ✓Each MCQ includes four options, the correct answer clearly marked, and a one-line reason citing NCERT concepts or mechanisms.
  • ✓Difficulty ranges from easy (name-to-structure, reagent recall) to HOTS (mechanism prediction, assertion-reason pairing) for complete board-exam readiness.
  • ✓A dedicated strategy section explains how to tackle MCQs in the CBSE Chemistry paper — elimination technique, keyword spotting, and time allocation per question.

Nomenclature and Structure of Aldehydes and Ketones

IUPAC nomenclature for aldehydes uses the suffix '-al' (e.g. methanal for HCHO), while ketones take '-one' with the carbonyl carbon numbered to give the lowest locant. Common names persist in board exams: formaldehyde (methanal), acetaldehyde (ethanal), acetone (propanone), benzophenone (diphenyl ketone). Structure questions test whether you can draw correct bond angles (~120° sp² hybridised carbonyl carbon) and identify alpha-hydrogens adjacent to C=O. The four MCQs below drill naming conventions and structural recognition, mirroring the one-mark questions that appear in Section A of the CBSE paper.
  • Q1. The IUPAC name of CH₃CH₂CH₂CHO is (A) Butanal (B) Butanone (C) Butan-2-one (D) Propanal | Answer: (A) Butanal. The aldehyde group (-CHO) receives suffix '-al' and the longest chain is four carbons.
  • Q2. Which compound does NOT contain a carbonyl group? (A) Acetone (B) Formaldehyde (C) Diethyl ether (D) Acetophenone | Answer: (C) Diethyl ether. Ethers have C–O–C linkage; no C=O group.
  • Q3. The number of alpha-hydrogens in propanone (CH₃COCH₃) is (A) 2 (B) 4 (C) 6 (D) 0 | Answer: (C) 6. Each methyl has three alpha-H atoms adjacent to carbonyl; 3 + 3 = 6.
  • Q4. Assertion (A): Benzaldehyde is named as benzenecarbaldehyde in IUPAC. Reason (R): The aldehyde carbon is not counted in the benzene ring numbering. (A) Both A and R true, R explains A (B) Both true, R does NOT explain A (C) A true, R false (D) A false, R true | Answer: (A). IUPAC treats –CHO as a substituent 'carbaldehyde' on benzene; carbonyl carbon is extra, so R correctly explains A.

Preparation of Aldehydes and Ketones

NCERT Chapter 8 details multiple synthetic routes: oxidation of primary alcohols yields aldehydes (controlled with PCC or by distillation to prevent over-oxidation to carboxylic acid), secondary alcohols oxidise to ketones; ozonolysis of alkenes cleaves the double bond producing two carbonyl compounds; hydration of alkynes (Hg²⁺/H₂SO₄) gives ketones via enol tautomerism, except terminal alkynes which yield aldehydes; Friedel–Crafts acylation introduces an acyl group onto benzene forming aromatic ketones. These preparation methods form 15–20 per cent of Chapter 8 board questions. The MCQs below test reagent choice and product prediction, critical for three-mark 'name the reagent' questions in Section B.
  • Q5. Which reagent converts 1-propanol to propanal without further oxidation? (A) KMnO₄/H⁺ (B) PCC in CH₂Cl₂ (C) K₂Cr₂O₇/H₂SO₄ reflux (D) Conc. H₂SO₄ | Answer: (B) PCC in CH₂Cl₂. Pyridinium chlorochromate oxidises 1° alcohols to aldehydes in anhydrous medium, preventing over-oxidation.
  • Q6. Ozonolysis of 2-butene (CH₃CH=CHCH₃) followed by Zn/H₂O gives (A) Two molecules of ethanal (B) Propanal + methanal (C) Butanone (D) Butanal | Answer: (A) Two molecules of ethanal. Symmetrical alkene cleaves to identical carbonyl fragments.
  • Q7. Hydration of propyne (CH₃C≡CH) in the presence of Hg²⁺/H₂SO₄ produces (A) Propanal (B) Propanone (C) Propan-1-ol (D) Propan-2-ol | Answer: (B) Propanone. Terminal alkyne adds water forming enol (CH₃C(OH)=CH₂) which tautomerises to ketone (CH₃COCH₃).
  • Q8. Friedel–Crafts acylation of benzene with acetyl chloride in presence of AlCl₃ yields (A) Benzaldehyde (B) Acetophenone (C) Benzoic acid (D) Toluene | Answer: (B) Acetophenone (C₆H₅COCH₃). Acyl chloride introduces –COCH₃ group onto benzene ring.

Chemical Reactions — Nucleophilic Addition Mechanisms

The carbonyl carbon in aldehydes and ketones is electrophilic (δ⁺) due to oxygen's electronegativity. Nucleophiles attack this carbon, and the π-bond oxygen accepts a proton. Key NCERT reactions include addition of HCN (cyanohydrin formation), NaHSO₃ (bisulfite adduct), NH₂–NH₂ (hydrazone), NH₂OH (oxime), and alcohols in acid (hemiacetal/acetal). Aldehydes are more reactive than ketones because of lower steric hindrance and weaker +I effect. These mechanisms underpin three-mark 'write mechanism' questions and one-mark MCQs on product identification. The four questions below test your ability to predict products and recall reagent names.
  • Q9. Propanal reacts with HCN to form (A) 2-hydroxybutanenitrile (B) 2-hydroxypropanenitrile (C) Propanoic acid (D) Propanone | Answer: (B) 2-hydroxypropanenitrile (CH₃CH(OH)CN). Nucleophilic CN⁻ adds to carbonyl carbon; product is a cyanohydrin.
  • Q10. Which reagent forms a crystalline bisulfite adduct with aldehydes and methyl ketones? (A) NaOH (B) NaHSO₃ (C) Na₂CO₃ (D) NaCl | Answer: (B) NaHSO₃. Sodium bisulfite adds to carbonyl forming a solid adduct, used for purification.
  • Q11. Assertion (A): Acetaldehyde reacts with phenylhydrazine to give a phenylhydrazone. Reason (R): The reaction involves nucleophilic addition followed by elimination of water. (A) Both A and R true, R explains A (B) Both true, R does NOT explain A (C) A true, R false (D) A false, R true | Answer: (A). Phenylhydrazine (C₆H₅NHNH₂) attacks carbonyl, then –OH and –H eliminate as H₂O forming C=N–NHC₆H₅, so R explains A.
  • Q12. Treatment of benzaldehyde with excess ethanol and dry HCl gas produces (A) Hemiacetal (B) Acetal (C) Ester (D) Ether | Answer: (B) Acetal. First molecule of ethanol forms hemiacetal; second molecule (in excess + acid) replaces –OH to give acetal (C₆H₅CH(OC₂H₅)₂).

Oxidation and Reduction Reactions of Aldehydes and Ketones

Aldehydes oxidise easily to carboxylic acids with mild oxidants (Tollens' reagent Ag(NH₃)₂⁺ deposits silver mirror; Fehling's solution Cu²⁺ reduces to brick-red Cu₂O precipitate). Ketones resist oxidation under mild conditions; strong oxidants break C–C bonds. Reduction reactions convert carbonyl to alcohol: LiAlH₄ or NaBH₄ reduce aldehydes to primary alcohols and ketones to secondary alcohols; Clemmensen reduction (Zn-Hg/HCl) or Wolff–Kishner reduction (NH₂NH₂/KOH) convert carbonyl directly to –CH₂– (complete reduction). Distinguishing aldehyde from ketone via Tollens' or Fehling's test is a favourite CBSE one-mark MCQ. The questions below cover oxidation, reduction and chemical tests.
  • Q13. Which test distinguishes aldehyde from ketone? (A) Iodoform test (B) Lucas test (C) Tollens' test (D) Carbylamine test | Answer: (C) Tollens' test. Aldehydes reduce Ag⁺ to Ag mirror; ketones do not (except α-hydroxy ketones).
  • Q14. Fehling's solution oxidises (A) All aldehydes (B) Only aromatic aldehydes (C) Only aliphatic aldehydes (D) Ketones | Answer: (C) Only aliphatic aldehydes. Aromatic aldehydes (e.g. benzaldehyde) do not reduce Fehling's due to resonance stabilisation.
  • Q15. Reduction of propanone with LiAlH₄ followed by hydrolysis gives (A) Propan-1-ol (B) Propan-2-ol (C) Propanal (D) Propanoic acid | Answer: (B) Propan-2-ol (CH₃CH(OH)CH₃). Ketone reduces to secondary alcohol.
  • Q16. Wolff–Kishner reduction of acetophenone (C₆H₅COCH₃) yields (A) Ethylbenzene (B) Styrene (C) Benzyl alcohol (D) Benzoic acid | Answer: (A) Ethylbenzene (C₆H₅CH₂CH₃). Carbonyl group is completely reduced to –CH₂– with NH₂NH₂/KOH at high temperature.

Carboxylic Acids — Nomenclature, Acidity and Preparation

Carboxylic acids (–COOH) are named with suffix '-oic acid' in IUPAC (e.g. ethanoic acid for CH₃COOH). They are weak acids (pKₐ ~ 4–5) because the carboxylate anion (RCOO⁻) is resonance-stabilised. Electron-withdrawing groups (–Cl, –NO₂) on the α-carbon increase acidity by stabilising the anion further; electron-donating groups (–CH₃) decrease acidity. Preparation methods include oxidation of primary alcohols or aldehydes with strong oxidants (KMnO₄, K₂Cr₂O₇), hydrolysis of nitriles (RCN + H₂O/H⁺ → RCOOH), and Grignard reagent reaction with CO₂ followed by hydrolysis. The four MCQs below test acidity trends, nomenclature and synthetic routes, all high-yield topics for CBSE board one-markers.
  • Q17. Which carboxylic acid is MOST acidic? (A) CH₃COOH (B) ClCH₂COOH (C) Cl₂CHCOOH (D) Cl₃CCOOH | Answer: (D) Cl₃CCOOH (trichloroacetic acid). Three –Cl groups exert strong –I effect, stabilising carboxylate ion maximally.
  • Q18. IUPAC name of HOOC–CH₂–CH₂–COOH is (A) Succinic acid (B) Butanedioic acid (C) Adipic acid (D) Malonic acid | Answer: (B) Butanedioic acid. Four-carbon chain with carboxyl groups at C1 and C4; '-dioic acid' indicates two –COOH.
  • Q19. Oxidation of ethanol with alkaline KMnO₄ finally yields (A) Ethanal (B) Ethanoic acid (C) Ethane (D) Ethene | Answer: (B) Ethanoic acid (CH₃COOH). Primary alcohol oxidises via aldehyde intermediate to carboxylic acid under strong oxidising conditions.
  • Q20. Hydrolysis of ethanenitrile (CH₃CN) with dilute HCl gives (A) Ethanoic acid (B) Ethylamine (C) Ethanal (D) Ethanol | Answer: (A) Ethanoic acid. Nitrile (–C≡N) hydrolyses to carboxylic acid (–COOH) in acidic medium.

Reactions of Carboxylic Acids — Esterification, Acyl Chloride and Decarboxylation

Carboxylic acids undergo nucleophilic acyl substitution. Esterification (Fischer esterification) reacts acid with alcohol in presence of conc. H₂SO₄ catalyst to form ester (RCOOR') and water. Conversion to acyl chloride (RCOCl) uses reagents like SOCl₂, PCl₃ or PCl₅; acyl chlorides are more reactive than acids and are used to prepare amides, esters and ketones. Decarboxylation (loss of CO₂) occurs when carboxylic acid is heated with soda-lime (NaOH + CaO) yielding alkane with one fewer carbon. Aromatic acids decarboxylate at high temperature. Hell–Volhard–Zelinsky reaction (Cl₂/P) chlorinates the α-carbon. These transformations frequently appear in three-mark 'write equations' questions and one-mark product-prediction MCQs.

How to Attempt MCQs in the CBSE Chemistry Paper — Strategy and Time Management

Section A of the CBSE Class 12 Chemistry paper carries 20 one-mark MCQs (including four assertion-reason questions). Chapters like Aldehydes, Ketones and Carboxylic Acids typically contribute 1–2 MCQs. To maximise marks: (1) Read the question stem fully before scanning options — keyword spotting ('oxidation', 'aldehyde', 'acidity') narrows choices instantly. (2) Eliminate obviously wrong options first; if two remain, recall NCERT line diagrams or reaction conditions to choose. (3) For assertion-reason MCQs, evaluate assertion truth first, then reason truth, then check if reason explains assertion — this logical sequence prevents confusion. (4) Do not spend more than 30–40 seconds per one-marker; flag tough questions, solve easier ones first, return if time permits. (5) In calculation-based MCQs (rare in Chapter 8 but possible in isomer counting), do back-of-the-envelope checks. (6) Mark your answer on the OMR carefully — one wrong bubble costs the full mark. Practicing 50+ MCQs from past-year papers and NCERT Exemplar builds speed and pattern recognition, both critical for scoring full marks in Section A.
  • Read each MCQ stem completely; underline keywords like 'IUPAC', 'reagent', 'product' to focus your recall.
  • Use elimination: cross out one or two distractors (e.g. if question asks for aldehyde product, eliminate ketone options immediately).
  • For assertion-reason questions, follow the four-step check: A true? R true? Does R explain A? Match the logic to option codes (A), (B), (C), (D).
  • Allocate ~15 minutes for all 20 MCQs; spend saved time on Section-B short answers where part-marks are available.
  • If unsure between two options, trust NCERT text or your class notes over intuition — board MCQs rarely include 'trick' distractors.

Frequently asked questions

How many MCQs from Chapter 8 Aldehydes, Ketones and Carboxylic Acids appear in the CBSE Class 12 board exam?+
Typically 1–2 one-mark MCQs appear in Section A (out of 20 total). Additionally, one assertion-reason question may come from this chapter, raising the MCQ contribution to 2–3 marks. Chapter 8 also features in Section-B short answers and Section-C long answers.
Which topics in Chapter 8 are most frequently tested as MCQs in the board paper?+
IUPAC nomenclature (aldehyde/ketone/acid naming), chemical tests (Tollens', Fehling's, iodoform), preparation methods (oxidation of alcohols, ozonolysis), nucleophilic addition products (cyanohydrin, oxime), and acidity comparison of carboxylic acids (effect of electron-withdrawing groups) dominate MCQ questions.
How do I distinguish between aldehyde and ketone in an MCQ if reagents are not mentioned?+
Look for structural clues: aldehydes have –CHO at terminal carbon; ketones have carbonyl within the chain. Aldehydes reduce Tollens' and Fehling's reagents; ketones do not (except α-hydroxy ketones). Aldehydes oxidise to carboxylic acids; ketones resist mild oxidation. These distinctions resolve most MCQ options.
What is the best way to remember whether an assertion-reason answer is (A), (B), (C) or (D)?+
Use a checklist: First verify assertion truth (true/false), then reason truth (true/false). If both true, ask 'Does reason explain or justify the assertion?' If yes → (A); if both true but unconnected → (B). If assertion true but reason false → (C). If assertion false (reason truth irrelevant) → (D). Write this checklist on your rough sheet during the exam.
Are mechanisms of nucleophilic addition asked in MCQ format or only in descriptive answers?+
Mechanisms rarely appear as full diagrams in MCQs. However, MCQs may ask 'What is the first step in cyanohydrin formation?' or 'Which species attacks the carbonyl carbon?' — testing mechanism knowledge indirectly. Focus on recognising nucleophiles (CN⁻, NH₂OH, ROH) and the sequence: nucleophilic attack → protonation.
Why is trichloroacetic acid more acidic than acetic acid?+
Three chlorine atoms on the α-carbon exert a strong –I (electron-withdrawing inductive) effect, pulling electron density away from the carboxylate anion and stabilising the negative charge by delocalisation. Greater anion stability means the acid dissociates more readily, hence lower pKₐ (higher acidity) for CCl₃COOH compared to CH₃COOH.
Can I use CBSETUTOR.ai to practice unlimited Chapter 8 MCQs and get instant feedback?+
Yes. CBSETUTOR.ai offers a 24×7 AI tutor at ₹999/month (one price for Classes 6–12) with a 3-day free trial. Upload a photo of any NCERT exercise, past-year MCQ set or your own doubt — the AI delivers step-by-step solutions, suggests similar practice questions, and tracks weak areas. Ideal for self-paced MCQ drills before board exams.
Which reagent converts a primary alcohol to an aldehyde without over-oxidising to carboxylic acid?+
Pyridinium chlorochromate (PCC) in anhydrous dichloromethane (CH₂Cl₂) selectively oxidises 1° alcohols to aldehydes. Alternatively, use Collins reagent (CrO₃·2C₅H₅N) or perform distillation during oxidation with K₂Cr₂O₇/H₂SO₄ to remove aldehyde as it forms, preventing further oxidation.
What is the iodoform test and which carbonyl compounds give a positive result?+
Iodoform test involves treating the compound with I₂ and NaOH; a positive result is the formation of a yellow precipitate of iodoform (CHI₃). Compounds containing CH₃CO– (methyl ketones) or CH₃CH(OH)– (secondary alcohols with methyl adjacent to –OH) give positive iodoform test. Aldehydes except ethanal do not.
How much time should I spend on each one-mark MCQ in the board exam?+
Aim for 30–40 seconds per MCQ. Section A has 20 MCQs worth 20 marks; spending 10–12 minutes total leaves ample time for Sections B and C where part-marking is possible. If an MCQ takes longer than 60 seconds, mark your best guess, flag it on the question paper, and return if time remains after completing other sections.

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