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Preparation of acetanilide, p-nitroacetanilide, aniline yellow and iodoform: NEET MCQs with answers

50 questions on this topic of Principles Related to Practical Chemistry (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following reagent combinations can be successfully used in place of NaOH and I₂ to produce iodoform from acetone?

  1. KI and concentrated H₂SO₄
  2. NaHCO₃ and HI
  3. Na₂CO₃ and I₂
  4. NaIO₃ and dilute HCl
Show answer and explanation

Answer: (C) Na₂CO₃ and I₂

Sodium carbonate (Na₂CO₃) hydrolyses in aqueous solution to furnish an alkaline medium (OH⁻ ions) capable of reacting with iodine to generate hypoiodite (IO⁻), which oxidizes and iodinates methyl ketones to form iodoform.

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2. In the synthesis of the azo dye aniline yellow, what is the chemical nature of the intermediate diazoaminobenzene and the final dye p-aminoazobenzene, respectively?

  1. Both compounds are identical tautomers that remain in a 1:1 dynamic equilibrium
  2. Diazoaminobenzene is a tertiary amine; aniline yellow is a primary aliphatic amine
  3. Diazoaminobenzene contains a nitroso group; aniline yellow contains an amide group
  4. Diazoaminobenzene has an -N=N-NH- linkage; aniline yellow has an -N=N- linkage between two aromatic rings
Show answer and explanation

Answer: (D) Diazoaminobenzene has an -N=N-NH- linkage; aniline yellow has an -N=N- linkage between two aromatic rings

Benzene diazonium chloride couples initially at the nitrogen of aniline in weakly acidic/neutral conditions to yield diazoaminobenzene (C₆H₅-N=N-NH-C₆H₅). Upon warming with aniline hydrochloride, it rearranges to the C-coupled product aniline yellow (p-aminoazobenzene, C₆H₅-N=N-C₆H₄-NH₂).

hard

3. During the recrystallization of crude p-nitroacetanilide containing minor traces of o-nitroacetanilide, why is ethanol used as the recrystallizing solvent?

  1. o-Nitroacetanilide is completely insoluble in ethanol at all temperatures
  2. Ethanol forms an insoluble crystalline coordination complex exclusively with p-nitroacetanilide
  3. Ethanol chemically hydrolyzes o-nitroacetanilide into water-soluble o-nitroaniline
  4. p-Nitroacetanilide is sparingly soluble in cold ethanol, while o-nitroacetanilide is more soluble and remains in the mother liquor
Show answer and explanation

Answer: (D) p-Nitroacetanilide is sparingly soluble in cold ethanol, while o-nitroacetanilide is more soluble and remains in the mother liquor

Due to intramolecular hydrogen bonding in o-nitroacetanilide, it has greater solubility in organic solvents like ethanol compared to the symmetrical, highly lattice-stabilized p-isomer. Thus, on cooling the hot ethanolic solution, pure p-nitroacetanilide crystallizes out while the ortho-isomer remains dissolved.

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4. Given below are two statements regarding organic preparations: Statement I: In the haloform reaction, acetone consumes 3 moles of iodine (I₂) and 4 moles of sodium hydroxide (NaOH) per mole of acetone to form iodoform. Statement II: Ethanol consumes 4 moles of iodine (I₂) and 6 moles of sodium hydroxide (NaOH) per mole of ethanol to produce one mole of iodoform. In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct

For acetone: CH₃COCH₃ + 3I₂ + 4NaOH → CHI₃ + CH₃COONa + 3NaI + 3H₂O (consumes 3 I₂ and 4 NaOH). For ethanol: CH₃CH₂OH + 4I₂ + 6NaOH → CHI₃ + HCOONa + 5NaI + 5H₂O (first oxidized to acetaldehyde consuming 1 I₂ and 2 NaOH, then triiodinated and cleaved consuming 3 I₂ and 4 NaOH, totaling 4 I₂ and 6 NaOH). Both statements are correct.

easy

5. What is the melting point of pure dry acetanilide when prepared properly in the practical laboratory?

  1. 80 °C
  2. 114 °C
  3. 166 °C
  4. 212 °C
Show answer and explanation

Answer: (B) 114 °C

Pure acetanilide is a white crystalline solid with a sharp, characteristic melting point of 113–114 °C, commonly used as a calibration standard in melting point determinations.

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6. In a laboratory practical, 5.80 g of pure acetone (molar mass = 58.0 g mol⁻¹) is reacted with an excess of iodine and aqueous sodium hydroxide. If the isolated and dried yield of pure iodoform (molar mass = 394 g mol⁻¹) is 27.58 g, what is the percentage yield of the reaction?

  1. 70.0%
  2. 65.0%
  3. 85.0%
  4. 75.5%
Show answer and explanation

Answer: (A) 70.0%

Moles of acetone used = 5.80 g / 58.0 g mol⁻¹ = 0.100 mol. Theoretical moles of CHI₃ = 0.100 mol (1:1 stoichiometry). Theoretical mass of CHI₃ = 0.100 mol × 394 g mol⁻¹ = 39.4 g. Percentage yield = (27.58 g / 39.4 g) × 100% = 70.0%.

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7. Which of the following gives a positive iodoform test?

  1. Ethanol
  2. Methanol
  3. Propan-1-ol
  4. Benzaldehyde
Show answer and explanation

Answer: (A) Ethanol

Ethanol has the CH₃CH(OH)- group, which is oxidised to CH₃CO- and gives iodoform. Methanol, propan-1-ol and benzaldehyde lack this group.

NCERT Class 11

easy

8. In the preparation of p-nitroacetanilide, why is aniline first converted to acetanilide rather than being nitrated directly with a nitrating mixture?

  1. Direct nitration of aniline predominantly gives m-nitroaniline accompanied by significant oxidative degradation
  2. The acetyl group increases the electron density of the benzene ring, speeding up nitration
  3. Direct nitration of aniline produces an explosive diazonium intermediate at room temperature
  4. The acetyl group directs the incoming nitronium ion exclusively to the meta position
Show answer and explanation

Answer: (A) Direct nitration of aniline predominantly gives m-nitroaniline accompanied by significant oxidative degradation

In strongly acidic nitrating medium, aniline gets protonated to form the anilinium ion, which is meta-directing and leads to a large amount of m-nitroaniline along with tarry oxidation products. Acetylation protects the -NH₂ group and moderates ring activation, cleanly directing nitration to the para position.

easy

9. What is the color and crystalline morphology of pure iodoform crystals obtained in the laboratory?

  1. White needle-like crystals
  2. Bright yellow hexagonal or needle-shaped crystals
  3. Dark brown prismatic crystals
  4. Orange-red amorphous flakes
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Answer: (B) Bright yellow hexagonal or needle-shaped crystals

Pure iodoform (CHI₃) precipitates as characteristic bright yellow, crystalline solids that typically exhibit hexagonal plates or small needle-shaped morphology and possess a distinctive medicinal odor.

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10. Iodoform is prepared by warming ethanol or acetone with:

  1. Conc. H₂SO₄
  2. Tollens' reagent
  3. Zinc and HCl
  4. Iodine and sodium hydroxide (or KI and sodium hypochlorite)
Show answer and explanation

Answer: (D) Iodine and sodium hydroxide (or KI and sodium hypochlorite)

Compounds with CH₃CO- or CH₃CH(OH)- groups react with iodine in alkali to form yellow crystals of iodoform (CHI₃) with a characteristic smell.

NCERT Class 11

easy

11. During the synthesis of acetanilide from aniline using acetic anhydride, why is a small amount of zinc dust often added to the reaction mixture containing aniline?

  1. To prevent the oxidation of aniline to colored impurities during heating
  2. To act as a Lewis acid catalyst for the acetylation reaction
  3. To hydrolyze excess acetic anhydride into acetic acid
  4. To increase the nucleophilicity of the amino group by complexation
Show answer and explanation

Answer: (A) To prevent the oxidation of aniline to colored impurities during heating

Commercial aniline is often dark-colored due to atmospheric oxidation products. Adding a pinch of zinc dust reduces these colored oxidation impurities and prevents further oxidation of aniline during the acetylation process, yielding colorless acetanilide crystals.

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12. During the preparation of acetanilide, a student washes the filtered crude solid on the Buchner funnel with ice-cold water rather than warm water. What is the primary practical rationale for this step?

  1. To hydrolyze any unreacted acetic anhydride rapidly
  2. To convert the ortho-isomer into the more stable para-isomer
  3. To oxidize remaining traces of aniline to prevent discoloration
  4. To prevent the dissolution of acetanilide due to its moderate solubility in hot water
Show answer and explanation

Answer: (D) To prevent the dissolution of acetanilide due to its moderate solubility in hot water

Acetanilide is significantly soluble in warm and hot water but sparingly soluble in cold water. Washing the crude product with ice-cold water removes adhering water-soluble impurities (such as acetic acid) without causing substantial loss of the product through dissolution.

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