Chemistry Objective Questions and Answers: Aldehydes Ketones and Carboxylic Acids
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Multiple Choice Questions
(a) Lucas reagent
(b) 2,4-DNP
(c) Fehling's solution
(d) \( \ce{NaHSO3} \)
Show Answer & Explanation
Answer: (c) Fehling's solution
Explanation:
1. P gives acetone and Q gives propanal.
2. Only propanal reduces Fehling's solution; both give 2,4-DNP derivatives and bisulphite adducts.
Teacher's Note:
1. Fehling's solution is reduced by aliphatic aldehydes (red Cu2O) but not by ketones or aromatic aldehydes.
2. Check whether a reagent attacks the carbonyl, an alcohol or a C=C bond first.
(a) \( \ce{(CH3)2C(OH)-CH(OH)-CH2CH2CH3} \)
(b) 2-Methylpropanoic acid and ethanoic acid
(c) Propan-2-ol and propan-1-ol
(d) Acetone and butanoic acid
Show Answer & Explanation
Answer: (d) Acetone and butanoic acid
Explanation:
1. Cleavage at the double bond gives acetone from the CMe2 end and an aldehyde, oxidised on to butanoic acid, from the other end.
Teacher's Note:
1. Draw each structure and mark alpha-hydrogens before choosing a named reaction.
2. For multi-step conversions write the intermediate after each reagent.
(a) 1 and 2
(b) 2 and 4
(c) 3 and 4
(d) 1 and 3
Show Answer & Explanation
Answer: (c) 3 and 4
Explanation:
1. Acetophenone is nitrated at the meta position because the acyl group is meta-directing, so statement 3 is wrong.
2. It has a CH3CO- group and gives iodoform, so statement 4 is wrong.
3. Statements 1 and 2 are correct.
Teacher's Note:
1. The iodoform test is positive for CH3CO- groups and CH3CH(OH)- groups (ethanol, propan-2-ol, ethanal, methyl ketones); methanal, benzophenone and 3-pentanone do not respond.
2. For multi-step conversions write the intermediate after each reagent.
(a) \( \ce{CH3CH2COCH3} \)
(b) \( \ce{CH3CH2CH2CHO} \)
(c) \( \ce{CH3CH2CHO + HCHO} \)
(d) \( \ce{CH3CH2COOH + HCOOH} \)
Show Answer & Explanation
Answer: (a) \( \ce{CH3CH2COCH3} \)
Explanation:
1. Markovnikov hydration of the terminal alkyne gives the enol \( \ce{CH3CH2C(OH)=CH2} \), which tautomerises to butan-2-one.
Teacher's Note:
1. Check whether a reagent attacks the carbonyl, an alcohol or a C=C bond first.
2. Draw each structure and mark alpha-hydrogens before choosing a named reaction.
(a) Proton
(b) Hydride ion
(c) Hydronium ion
(d) Hydrogen bond
Show Answer & Explanation
Answer: (b) Hydride ion
Explanation:
1. A tetrahedral alkoxide intermediate transfers a hydride ion to the carbonyl carbon of a second aldehyde molecule.
Teacher's Note:
1. Cannizzaro reaction: an aldehyde with no alpha-hydrogen, in strong alkali, gives equal amounts of alcohol and carboxylate through hydride transfer.
2. Draw each structure and mark alpha-hydrogens before choosing a named reaction.
(a) \( \ce{KMnO4} \)
(b) \( \ce{K2Cr2O7} \)
(c) \( \ce{CrO3} \)
(d) PCC (pyridinium chlorochromate)
Show Answer & Explanation
Answer: (d) PCC (pyridinium chlorochromate)
Explanation:
1. PCC oxidises primary alcohols and stops at the aldehyde.
2. The other reagents oxidise further to the acid.
Teacher's Note:
1. For multi-step conversions write the intermediate after each reagent.
2. Check whether a reagent attacks the carbonyl, an alcohol or a C=C bond first.
(a) Beckmann rearrangement
(b) Curtius rearrangement
(c) Lossen rearrangement
(d) Hofmann rearrangement
Show Answer & Explanation
Answer: (a) Beckmann rearrangement
Explanation:
1. Acetophenone gives its oxime, and Beckmann rearrangement (acid) moves the phenyl group to nitrogen, giving \( \ce{CH3CONHC6H5} \).
Teacher's Note:
1. Ammonia derivatives (NH2OH, NH2NH2, PhNHNH2, semicarbazide, 2,4-DNP) condense with C=O to give C=N products by addition then loss of water.
2. Check whether a reagent attacks the carbonyl, an alcohol or a C=C bond first.
(a) \( \ce{CH3COOH} \)
(b) \( \ce{BrCH2-COOH} \)
(c) \( \ce{(CH3CO)2O} \)
(d) \( \ce{CHO-COOH} \)
Show Answer & Explanation
Answer: (c) \( \ce{(CH3CO)2O} \)
Explanation:
1. This is a Perkin reaction: an aromatic aldehyde, acetic anhydride and sodium acetate give a cinnamic acid.
Teacher's Note:
1. Perkin reaction: aromatic aldehyde, acid anhydride and the sodium salt of the acid give a cinnamic acid.
2. For multi-step conversions write the intermediate after each reagent.
(a) \( \ce{CH3CHO + CH3CH2CHO} \)
(b) \( \ce{CH3COOH + CH3CH2CHO} \)
(c) \( \ce{CH3COOH + CH3CH2COOH} \)
(d) \( \ce{CH3COOH + CO2} \)
Show Answer & Explanation
Answer: (c) \( \ce{CH3COOH + CH3CH2COOH} \)
Explanation:
1. Ozonolysis of an alkyne cleaves the triple bond and gives two carboxylic acids, ethanoic acid and propanoic acid.
Teacher's Note:
1. Check whether a reagent attacks the carbonyl, an alcohol or a C=C bond first.
2. Draw each structure and mark alpha-hydrogens before choosing a named reaction.
(a) An aldehyde
(b) An acid
(c) An alcohol
(d) An ether
Show Answer & Explanation
Answer: (a) An aldehyde
Explanation:
1. M = 44 fits acetaldehyde, \( \ce{C2H4O} \)
2. Its oxidation gives acetic acid, M = 60.
Teacher's Note:
1. Draw each structure and mark alpha-hydrogens before choosing a named reaction.
2. For multi-step conversions write the intermediate after each reagent.
Assertion (A): Acetic acid does not undergo haloform reaction.
Reason (R): Acetic acid has no alpha hydrogens.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. Acetic acid has no CH3CO- group (the carbonyl is attached to OH), so the haloform reaction fails and the assertion is true.
2. The reason is false because the methyl group of acetic acid has three alpha-hydrogens.
Teacher's Note:
1. The iodoform test is positive for CH3CO- groups and CH3CH(OH)- groups (ethanol, propan-2-ol, ethanal, methyl ketones); methanal, benzophenone and 3-pentanone do not respond.
2. Judge the assertion and the reason separately, then ask whether the reason explains the assertion.
Assertion (A): Benzonitrile is prepared by the reaction of chlorobenzene with potassium cyanide.
Reason (R): Cyanide (CN) is a strong nucleophile.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (d) A is false, but R is true.
Explanation:
1. The C-Cl bond in chlorobenzene has partial double bond character, so simple nucleophilic substitution does not occur and the assertion is false.
2. Cyanide is indeed a strong nucleophile, so the reason is true.
Teacher's Note:
1. Look out for alpha-hydrogen, resonance and steric hindrance as the usual explanations.
2. Judge the assertion and the reason separately, then ask whether the reason explains the assertion.
Assertion (A): Lower aldehydes and ketones are soluble in water but the solubility decreases as molecular mass increases.
Reason (R): Aldehydes and ketones can be distinguished by Tollens' reagent.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. Hydrogen bonding with water and a growing hydrophobic alkyl part explain the assertion, so it is true.
2. The reason is also true, but it has no bearing on solubility.
Teacher's Note:
1. Judge the assertion and the reason separately, then ask whether the reason explains the assertion.
2. Look out for alpha-hydrogen, resonance and steric hindrance as the usual explanations.
Assertion (A): Acetaldehyde on treatment with alkali gives aldol.
Reason (R): Acetaldehyde molecules contain alpha-hydrogen atoms.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Alpha-hydrogens give the enolate needed for the aldol addition, so the reason explains the assertion.
Teacher's Note:
1. Aldol addition needs at least one alpha-hydrogen; dilute alkali gives the beta-hydroxy carbonyl compound, and heating dehydrates it to an enone.
2. Look out for alpha-hydrogen, resonance and steric hindrance as the usual explanations.
Assertion (A): Acetylene on treatment with alkaline \( \ce{KMnO4} \) produces acetaldehyde.
Reason (R): Alkaline \( \ce{KMnO4} \) is an oxidising agent.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (d) A is false, but R is true.
Explanation:
1. Alkaline permanganate oxidises ethyne to oxalic acid or carbon dioxide, not acetaldehyde, so the assertion is false.
2. The reason is true.
Teacher's Note:
1. Judge the assertion and the reason separately, then ask whether the reason explains the assertion.
2. Look out for alpha-hydrogen, resonance and steric hindrance as the usual explanations.
Assertion (A): Acetophenone and benzophenone can be distinguished by the iodoform test.
Reason (R): Acetophenone and benzophenone both are carbonyl compounds.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. Acetophenone has a CH3CO- group and gives iodoform; benzophenone does not, so the assertion is true.
2. Being carbonyl compounds is true of both but does not explain the difference.
Teacher's Note:
1. The iodoform test is positive for CH3CO- groups and CH3CH(OH)- groups (ethanol, propan-2-ol, ethanal, methyl ketones); methanal, benzophenone and 3-pentanone do not respond.
2. Look out for alpha-hydrogen, resonance and steric hindrance as the usual explanations.
Assertion (A): Isobutanal does not give the iodoform test.
Reason (R): It does not have an alpha-hydrogen.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. Isobutanal, \( \ce{(CH3)2CHCHO} \), has no CH3CO- or CH3CH(OH)- group, so the assertion is true.
2. It has one alpha-hydrogen, so the reason is false.
Teacher's Note:
1. The iodoform test is positive for CH3CO- groups and CH3CH(OH)- groups (ethanol, propan-2-ol, ethanal, methyl ketones); methanal, benzophenone and 3-pentanone do not respond.
2. Judge the assertion and the reason separately, then ask whether the reason explains the assertion.
Assertion (A): Aldol condensation can be catalysed both by acids and bases.
Reason (R): Beta-hydroxy aldehydes or ketones readily undergo acid catalysed dehydration.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Under acid catalysis the aldol addition product loses water easily to form the conjugated enone, which drives the condensation, so the reason explains the assertion.
Teacher's Note:
1. Aldol addition needs at least one alpha-hydrogen; dilute alkali gives the beta-hydroxy carbonyl compound, and heating dehydrates it to an enone.
2. Look out for alpha-hydrogen, resonance and steric hindrance as the usual explanations.
Assertion (A): Ketones are less reactive than aldehydes.
Reason (R): Ketones do not give Schiff's test.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. Two alkyl groups reduce the electrophilicity and add steric hindrance, so the assertion is true.
2. The reason is true but is a consequence, not the cause.
Teacher's Note:
1. Judge the assertion and the reason separately, then ask whether the reason explains the assertion.
2. Look out for alpha-hydrogen, resonance and steric hindrance as the usual explanations.
Assertion (A): Oximes are less acidic than hydroxylamine.
Reason (R): Oximes of aldehydes and ketones show geometrical isomerism.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (d) A is false, but R is true.
Explanation:
1. The sp2 nitrogen of an oxime is more electron-withdrawing, so oximes are slightly more acidic than hydroxylamine, making the assertion false.
2. Aldoximes and unsymmetrical ketoximes do show syn-anti isomerism, so the reason is true.
Teacher's Note:
1. Ammonia derivatives (NH2OH, NH2NH2, PhNHNH2, semicarbazide, 2,4-DNP) condense with C=O to give C=N products by addition then loss of water.
2. Look out for alpha-hydrogen, resonance and steric hindrance as the usual explanations.
Assertion (A): The bond energy of the C=O bond is less than that of the C=C bond in alkenes.
Reason (R): The carbon atom in a carbonyl group is sp2 hybridised.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (d) A is false, but R is true.
Explanation:
1. The C=O bond (about 745 kJ/mol) is stronger than C=C (about 614 kJ/mol), so the assertion is false.
2. The carbonyl carbon is sp2, so the reason is true.
Teacher's Note:
1. Judge the assertion and the reason separately, then ask whether the reason explains the assertion.
2. Look out for alpha-hydrogen, resonance and steric hindrance as the usual explanations.
Assertion (A): Formaldehyde cannot be prepared by Rosenmund's reduction.
Reason (R): Acid chlorides can be reduced into aldehydes with hydrogen in boiling xylene using palladium on barium sulphate. This is known as Rosenmund's reduction.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. Formyl chloride is unstable and decomposes to CO and HCl, so the assertion is true.
2. The reason correctly defines Rosenmund reduction but does not state why HCHO fails.
Teacher's Note:
1. Aldehyde syntheses: Rosenmund (acid chloride, H2, Pd/BaSO4), Stephen (nitrile, SnCl2/HCl), Etard (toluene, CrO2Cl2), Gattermann-Koch (benzene, CO/HCl, AlCl3).
2. Look out for alpha-hydrogen, resonance and steric hindrance as the usual explanations.
Assertion (A): \( \ce{CH3CHO} \) reacts with \( \ce{NH3} \) to form urotropine.
Reason (R): Urotropine is used as a medicine in case of urinary troubles.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (d) A is false, but R is true.
Explanation:
1. Urotropine comes from methanal, not ethanal (ethanal gives aldehyde ammonia), so the assertion is false.
2. Urotropine is a urinary antiseptic, so the reason is true.
Teacher's Note:
1. Urotropine, (CH2)6N4, comes from methanal and ammonia; it is a urinary antiseptic.
2. Judge the assertion and the reason separately, then ask whether the reason explains the assertion.
Assertion (A): Alpha-hydrogen atoms in aldehydes and ketones are acidic.
Reason (R): The anion left after the removal of an alpha-hydrogen is stabilised by the inductive effect.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. The conjugate base is an enolate, stabilised by resonance with the carbonyl, so the assertion is true.
2. Resonance, not the inductive effect, is the main stabilising factor, so the reason is false.
Teacher's Note:
1. Look out for alpha-hydrogen, resonance and steric hindrance as the usual explanations.
2. Judge the assertion and the reason separately, then ask whether the reason explains the assertion.
Assertion (A): 2,2-Dimethylpropanal undergoes Cannizzaro reaction with concentrated NaOH.
Reason (R): Cannizzaro reaction is a disproportionation reaction.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. The aldehyde has no alpha-hydrogen, so it undergoes Cannizzaro reaction.
2. The reason is a true description of the reaction type but does not explain why this aldehyde reacts.
Teacher's Note:
1. Cannizzaro reaction: an aldehyde with no alpha-hydrogen, in strong alkali, gives equal amounts of alcohol and carboxylate through hydride transfer.
2. Judge the assertion and the reason separately, then ask whether the reason explains the assertion.
Assertion (A): Benzaldehyde undergoes aldol condensation.
Reason (R): Aldehydes that have no alpha-hydrogen do not undergo aldol condensation.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (d) A is false, but R is true.
Explanation:
1. Benzaldehyde has no alpha-hydrogen, so it cannot form an enolate and does not give aldol condensation by itself: the assertion is false.
2. The reason is a true statement.
Teacher's Note:
1. Aldol addition needs at least one alpha-hydrogen; dilute alkali gives the beta-hydroxy carbonyl compound, and heating dehydrates it to an enone.
2. Look out for alpha-hydrogen, resonance and steric hindrance as the usual explanations.
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