JEE Chemistry Aldehydes Ketones and Carboxylic Acids MCQs Set 05

Answer: (b) an aldehyde

Explanation:
1. Acetaldol (3-hydroxybutanal) dehydrates easily to crotonaldehyde, an alpha,beta-unsaturated aldehyde.

Teacher's Note:
1. Dehydration is driven by conjugation.

Answer: (a) \( \ce{NaHSO3} \)

Explanation:
1. Acetaldehyde forms a bisulphite adduct, but acetophenone (hindered aryl methyl ketone) does not.
2. The adduct can be separated and hydrolysed.

Teacher's Note:
1. Iodoform is positive for both.

Answer: (d) PCC (pyridinium chlorochromate)

Explanation:
1. PCC oxidises primary alcohols to aldehydes without further oxidation to acids.
2. The other reagents are strong oxidising agents.

Teacher's Note:
1. PCC is used in dry CH2Cl2.

Answer: (d) none of these is incorrect

Explanation:
1. The -NH2 attached to NH adds to the carbonyl carbon, and water is then eliminated.
2. The product is a semicarbazone, and the reaction is a condensation. All three statements are correct.

Teacher's Note:
1. Condensation = two molecules join, losing a small molecule.

Answer: (b) (1-methylcyclohexyl)methanol

Explanation:
1. P is 1-methylcyclohexanol and Q is 1-bromo-1-methylcyclohexane.
2. R is the Grignard reagent on the same carbon; formaldehyde adds one CH2OH group to that carbon.
3. So S is (1-methylcyclohexyl)methanol.

Teacher's Note:
1. Grignard + HCHO gives a primary alcohol with one extra carbon.

Answer: (b) cyclopent-1-ene-1-carbaldehyde

Explanation:
1. Intramolecular aldol reaction forms a five-membered ring (the enolate at C2 attacks the C6 aldehyde).
2. Dehydration gives cyclopentene carbaldehyde.

Teacher's Note:
1. Five- and six-membered rings form easily.

Answer: (d) the bicarbonate

Explanation:
1. The acid protonates the bicarbonate ion to form carbonic acid, which decomposes to \( \ce{CO2} \) and water.
2. So the carbon comes from bicarbonate.

Teacher's Note:
1. The acid's own carbon stays in the carboxylate salt.

Answer: (a) 2-hydroxy-2-phenylpropanoic acid

Explanation:
1. A is acetyl chloride and B is acetophenone (Friedel-Crafts acylation).
2. HCN gives the cyanohydrin, and hydrolysis converts -CN to -COOH, giving the alpha-hydroxy acid.

Teacher's Note:
1. Cyanohydrin hydrolysis gives alpha-hydroxy acids.

Answer: (b) \( \ce{CH3CO-O-COCH3} \), \( \ce{C6H5OCOCH3} \)

Explanation:
1. Acetyl chloride with sodium acetate gives acetic anhydride (A).
2. With phenol it gives phenyl acetate (B).

Teacher's Note:
1. Acid chlorides are strong acylating agents.

Answer: (a) ortho > meta > para

Explanation:
1. The strong -I effect and steric effect of the nitro group at the ortho position destabilise the carboxylate and help carbon dioxide loss.
2. The effect decreases with distance: ortho > meta > para.

Teacher's Note:
1. Distance weakens the inductive effect.

Answer: (b) N-methyl-4-nitrobenzamide

Explanation:
1. The -NO2 group withdraws electrons and makes the carbonyl carbon more electrophilic.
2. So hydrolysis is fastest for the 4-nitro compound.

Teacher's Note:
1. Electron-withdrawing groups speed up nucleophilic acyl substitution.

Answer: (b) nucleophilic substitution reactions

Explanation:
1. A nucleophile adds to the acyl carbon and chloride is lost as the leaving group.
2. The net change is nucleophilic acyl substitution.

Teacher's Note:
1. Addition-elimination mechanism.

Answer: (a) \( \ce{CH3CH2NH2} \)

Explanation:
1. LiAlH4 reduces the amide C=O to CH2.
2. So \( \ce{CH3CONH2} \) becomes \( \ce{CH3CH2NH2} \).

Teacher's Note:
1. Amide reduction keeps the carbon count.

Answer: (c) acid halide

Explanation:
1. Rosenmund reduction is the partial hydrogenation of an acid chloride over Pd/BaSO4 (poisoned catalyst) to the aldehyde.

Teacher's Note:
1. The poison prevents further reduction to alcohol.

Answer: (c) \( \ce{CH3COCl} \)

Explanation:
1. Chloride is the best leaving group among these and Cl also withdraws electrons strongly.
2. The order is acid chloride > anhydride > ester > amide.

Teacher's Note:
1. Remember the reactivity order of acid derivatives.

Answer: (b) \( \ce{CH3-CN} \)

Explanation:
1. \( \ce{P2O5} \) is a dehydrating agent and removes a molecule of water from the primary amide to give the nitrile.

Teacher's Note:
1. Dehydration of amide gives nitrile.

Answer: (b) phthalide (a five-membered lactone)

Explanation:
1. An intramolecular Cannizzaro reaction gives 2-(hydroxymethyl)benzoate.
2. On acidification and heating, the hydroxy acid forms a five-membered lactone, phthalide.

Teacher's Note:
1. Gamma-hydroxy acids lactonise readily.

Answer: (c) 1,3-diphenylbut-2-en-1-one

Explanation:
1. Aldol condensation of two molecules of acetophenone gives the beta-hydroxy ketone, which dehydrates to the conjugated enone (dypnone).

Teacher's Note:
1. Conjugation with two phenyl rings makes the enone stable.

Answer: (a) ketone

Explanation:
1. The alpha-hydrogen of a ketone is acidic because the enolate formed is stabilised by resonance with C=O.

Teacher's Note:
1. Alpha-H of carbonyls pKa is about 19-20.

Answer: (a) \( \ce{CH3-CBr2-COOH} \)

Explanation:
1. In Hell-Volhard-Zelinsky bromination, only the alpha carbon is substituted.
2. With excess bromine both alpha-H atoms are replaced, giving 2,2-dibromopropanoic acid.

Teacher's Note:
1. Substitution occurs only at the alpha carbon.

Answer: (a) ethyl 3-bromobenzoate

Explanation:
1. Side-chain oxidation gives benzoic acid (B).
2. The -COOH group is meta directing, so bromination gives 3-bromobenzoic acid (C).
3. Fischer esterification gives ethyl 3-bromobenzoate (D).

Teacher's Note:
1. Remember -COOH directs meta.

Answer: (c) Cannizzaro reaction

Explanation:
1. Friedel-Crafts, Reimer-Tiemann and Wurtz reactions all form new C-C bonds.
2. Cannizzaro reaction is a hydride transfer and forms no C-C bond.

Teacher's Note:
1. Check for new C-C bond in the product.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Both are derivatives of benzoic acid.
2. The amide is less reactive towards hydrolysis than the ester because nitrogen donates its lone pair more strongly. Both are true, but R does not explain A.

Teacher's Note:
1. Reactivity order: ester > amide.

Answer: (d) A is false, but R is true.

Explanation:
1. Dehydration of alcohols needs an acid catalyst, which converts -OH into a better leaving group, water.
2. Hydroxide is a poor leaving group. Both statements are false.

Teacher's Note:
1. Acid protonates the OH group.