Unit Test 12 — Aldehydes, Ketones & Carboxylic Acids

Answer Key · Std. 12th · Chemistry

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Unit Test 12 — Aldehydes, Ketones & Carboxylic Acids

⏱ Time: 90 Min ✎ Marks: 25 📘 Chapter: 12

Step 1 · Sec A

MCQ + Short

07 marks

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Step 2 · Sec B

Any Four

08 marks

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Step 3 · Sec C

Any Two

06 marks

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Step 4 · Sec D

Any One

04 marks

SECTION A Objective & Very Short Answer [07]

1iBenzaldehyde does NOT show a positive test with —›

• Schiff's reagent
• Tollens's reagent
• Sodium bisulphite solution
• Fehling solution

Why

Benzaldehyde is an aromatic aldehyde with a bulky phenyl group next to the carbonyl carbon. This steric hindrance blocks the bisulphite ion from adding on, so it does not form the usual white crystalline bisulphite addition product.

1iiIUPAC name of CH₃‑COOH is —›

• methanoic acid
• methyl formate
• methylmethanoate
• ethanoic acid

1iiiWhich one does NOT give the Cannizzaro reaction?›

• Benzaldehyde
• 2‑Methylpropanal
• p‑Methoxybenzaldehyde
• 2,2‑Dimethylpropanal

Why

Cannizzaro's reaction needs an aldehyde with no α‑hydrogen. Benzaldehyde, p‑methoxybenzaldehyde and 2,2‑dimethylpropanal (neopentanaldehyde) all lack an α‑H. 2‑Methylpropanal has an α‑H, so it undergoes aldol condensation instead.

1ivAldol condensation is a(n) —›

• electrophilic substitution reaction
• nucleophilic substitution reaction
• elimination reaction
• addition – elimination reaction

Nucleophilic addition of an enolate to a carbonyl gives the β‑hydroxy aldehyde/ketone (aldol); heating then causes elimination of water to give the α,β‑unsaturated product.

Q2 iWhat are aromatic ketones?›

Ketones in which the carbonyl group is directly attached to at least one aryl (benzene) ring.

e.g. C₆H₅‑CO‑CH₃ (acetophenone), C₆H₅‑CO‑C₆H₅ (benzophenone)

Q2 iiWhat is formalin?›

A 40% aqueous solution of formaldehyde (HCHO). It is used as a disinfectant, an antiseptic, and to preserve biological specimens because it hardens tissue proteins.

Q2 iiiReaction: conversion of ethanenitrile into ethanol›

Step 1 — Hydrolysis

CH₃CN + 2H₂O --H⁺, Δ--> CH₃COOH + NH₃

Step 2 — Reduction

CH₃COOH --LiAlH₄--> CH₃CH₂OH (ethanol)

SECTION B Answer any FOUR [08]

Q3Aldehydes are more reactive towards nucleophilic addition than ketones. Explain.›

Steric factor

An aldehyde carries only one alkyl/aryl group and one H on the carbonyl carbon, while a ketone carries two bulkier groups. Less crowding around the carbonyl carbon of an aldehyde lets the nucleophile approach more easily.

Electronic factor

Alkyl groups are electron‑releasing (+I effect). A ketone has two such groups pushing electron density onto the carbonyl carbon, lowering its positive character; an aldehyde has only one (or none), so its carbonyl carbon stays more electrophilic and reacts faster.

Q4Action of hydrogen cyanide (HCN) on (a) acetaldehyde and (b) acetone›

a) Acetaldehyde

CH₃CHO + HCN → CH₃‑CH(OH)‑CN (acetaldehyde cyanohydrin)

b) Acetone

(CH₃)₂CO + HCN → (CH₃)₂C(OH)CN (acetone cyanohydrin)

In both cases HCN adds across the C=O bond to give a hydroxynitrile (cyanohydrin).

Q5Write a note on the Stephen reduction reaction.›

Nitriles are reduced to aldehydes with stannous chloride and HCl gas, followed by hydrolysis of the intermediate imine.

RCN --SnCl₂ / HCl--> RCH=NH·HCl --H₂O--> RCHO + NH₄Cl

This is a valuable method for stopping the reduction exactly at the aldehyde stage.

Q6Action of (a) SOCl₂ and (b) PCl₅ on acetic acid›

a) Thionyl chloride

CH₃COOH + SOCl₂ → CH₃COCl (acetyl chloride) + SO₂ + HCl

b) Phosphorus pentachloride

CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl

Both reagents replace the ‑OH of the acid with ‑Cl, giving acetyl chloride.

Q7Conversions: (a) Benzoic acid → Benzaldehyde (b) Ethanal → 3‑Hydroxybutanal›

a) Benzoic acid → Benzaldehyde

C₆H₅COOH --SOCl₂--> C₆H₅COCl --H₂/Pd‑BaSO₄ (Rosenmund)--> C₆H₅CHO

b) Ethanal → 3‑Hydroxybutanal (Aldol addition)

2 CH₃CHO --dil. NaOH--> CH₃‑CH(OH)‑CH₂‑CHO

Q8Explain the Gatterman–Koch formylation of arenes.›

An aromatic ring is treated with carbon monoxide and hydrogen chloride under pressure, using anhydrous AlCl₃ (with a trace of CuCl) as catalyst, to introduce a ‑CHO group directly onto the ring.

C₆H₆ + CO + HCl --anhyd. AlCl₃ / CuCl, pressure--> C₆H₅CHO + HCl

It is essentially a Friedel–Crafts formylation and is the standard lab route to benzaldehyde from benzene.

SECTION C Answer any TWO [06]

Q9a) Action of hydroxylamine & hydrazine on acetaldehyde b) Rosenmund reduction›

a·i) Hydroxylamine

CH₃CHO + NH₂OH → CH₃CH=N‑OH (acetaldoxime) + H₂O

a·ii) Hydrazine

CH₃CHO + NH₂NH₂ → CH₃CH=N‑NH₂ (hydrazone) + H₂O

b) Rosenmund reduction

An acyl chloride is hydrogenated over palladium on barium sulphate (its activity deliberately "poisoned" with sulphur/quinoline) in boiling xylene. Poisoning stops the reaction at the aldehyde stage instead of going all the way to the alcohol.

RCOCl + H₂ --Pd/BaSO₄--> RCHO + HCl

Q10a) Wolff–Kishner reduction b) Preparing acetic acid from a Grignard reagent›

a) Wolff–Kishner reduction

The carbonyl group of an aldehyde/ketone is first converted to a hydrazone with hydrazine; heating the hydrazone with KOH (or NaOH) in a high‑boiling solvent like ethylene glycol reduces it all the way to a ‑CH₂‑ group, releasing nitrogen gas.

>C=O --NH₂NH₂--> >C=N‑NH₂ --KOH, Δ (ethylene glycol)--> ‑CH₂‑ + N₂ + H₂O

b) Acetic acid via Grignard reagent

CH₃MgI + CO₂ (dry ice) → CH₃COOMgI --H₃O⁺--> CH₃COOH

Q11Explain the Aldol condensation reaction.›

Two molecules of an aldehyde or ketone possessing an α‑hydrogen, treated with dilute base (e.g. NaOH), combine: the base removes an α‑H to form an enolate, which attacks the carbonyl carbon of a second molecule. This gives a β‑hydroxy aldehyde/ketone called the aldol. Gentle heating then eliminates a water molecule to give an α,β‑unsaturated carbonyl compound — the condensation product.

2 CH₃CHO --dil.NaOH--> CH₃CH(OH)CH₂CHO (aldol) --Δ, −H₂O--> CH₃CH=CHCHO (crotonaldehyde)

SECTION D Answer any ONE [04]

Q12a) Clemmensen reduction b) Tollens' test for acetaldehyde›

a) Clemmensen reduction

The carbonyl group of an aldehyde/ketone is reduced directly to a ‑CH₂‑ group using zinc amalgam and concentrated HCl. Being an acidic method, it is used for carbonyl compounds that are stable to acid (acid‑sensitive substrates instead use the base‑mediated Wolff–Kishner reduction).

>C=O + 4[H] --Zn‑Hg / conc. HCl--> ‑CH₂‑ + H₂O

b) Tollens' test

Acetaldehyde is warmed gently with ammoniacal silver nitrate (Tollens' reagent). The aldehyde reduces the diammine‑silver(I) complex to metallic silver, which deposits as a bright silver mirror on the walls of the clean test tube.

CH₃CHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → CH₃COO⁻ + 2Ag↓ (mirror) + 4NH₃ + 2H₂O

Q13a) Benzonitrile → Benzaldehyde b) Note on the Cannizzaro reaction›

a) Benzonitrile → Benzaldehyde (Stephen reduction)

C₆H₅CN --SnCl₂/HCl--> C₆H₅CH=NH·HCl --H₂O--> C₆H₅CHO + NH₄Cl

b) Cannizzaro reaction

Aldehydes that have no α‑hydrogen, when treated with concentrated alkali, undergo self oxidation–reduction (disproportionation): one molecule is oxidised to the carboxylate salt while a second is simultaneously reduced to the alcohol.

2 HCHO --conc. NaOH--> CH₃OH + HCOONa

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