Questions & explanations
1. An organic compound (C4H10O) does not react with Na metal but gives a violet colour with neutral FeCl3. What is the compound?
- Ethoxyethane
- Butan-2-ol
- Butan-1-ol
- 2-Methylpropan-2-ol
Answer: Ethoxyethane
No reaction with Na indicates absence of acidic hydrogen, so it is not an alcohol or phenol. Violet colour with FeCl3 is characteristic of phenols, but C4H10O cannot be a phenol (requires at least C6). Therefore, the compound must be an ether; ethoxyethane (diethyl ether) fits C4H10O and gives no Na reaction and no FeCl3 colour. The violet colour is a distractor: FeCl3 gives colour only with phenols, not ethers. So the correct answer is ethoxyethane.
2. Arrange the following in order of INCREASING acidity: p-cresol, phenol, m-nitrophenol, p-nitrophenol, 2,4-dinitrophenol.
- p-cresol < phenol < m-nitrophenol < p-nitrophenol < 2,4-dinitrophenol
- phenol < p-cresol < m-nitrophenol < p-nitrophenol < 2,4-dinitrophenol
- p-cresol < phenol < p-nitrophenol < m-nitrophenol < 2,4-dinitrophenol
- phenol < p-cresol < p-nitrophenol < m-nitrophenol < 2,4-dinitrophenol
Answer: p-cresol < phenol < m-nitrophenol < p-nitrophenol < 2,4-dinitrophenol
Acidity of phenols depends on stability of phenoxide ion. Methyl is electron-donating (+I and hyperconjugation), so p-cresol is least acidic. -NO2 is electron-withdrawing; at para it stabilises by resonance and induction, at meta only by induction. 2,4-dinitrophenol has two -NO2 groups, so most acidic. Correct order: p-cresol < phenol < m-nitrophenol < p-nitrophenol < 2,4-dinitrophenol.
3. Arrange the following in increasing order of acidity: phenol, p-nitrophenol, p-cresol, p-methoxyphenol.
- p-methoxyphenol < p-cresol < phenol < p-nitrophenol
- p-nitrophenol < phenol < p-cresol < p-methoxyphenol
- phenol < p-cresol < p-methoxyphenol < p-nitrophenol
- p-cresol < p-methoxyphenol < phenol < p-nitrophenol
Answer: p-methoxyphenol < p-cresol < phenol < p-nitrophenol
Acidity of phenols increases with electron-withdrawing groups (EWG) and decreases with electron-donating groups (EDG). p-Nitrophenol has strong EWG (-NO2), so most acidic. Phenol has no substituent. p-Cresol has weak EDG (-CH3), less acidic than phenol. p-Methoxyphenol has stronger EDG (-OCH3), least acidic. Thus order: p-methoxyphenol < p-cresol < phenol < p-nitrophenol.
4. Which of the following is the correct 4-step synthesis of aspirin from benzene?
- Benzene → Hock process → Friedel-Crafts alkylation → Kolbe reaction → acetylation
- Benzene → Friedel-Crafts alkylation → Hock process → Kolbe reaction → acetylation
- Benzene → Friedel-Crafts alkylation → Kolbe reaction → Hock process → acetylation
- Benzene → Kolbe reaction → Friedel-Crafts alkylation → Hock process → acetylation
Answer: Benzene → Friedel-Crafts alkylation → Hock process → Kolbe reaction → acetylation
First, benzene undergoes Friedel-Crafts alkylation with propene to give cumene. Cumene is then converted to phenol via the Hock process (oxidation then acid-catalysed rearrangement). Phenol is converted to sodium phenoxide, which undergoes Kolbe reaction with CO₂ to give salicylic acid after acidification. Finally, acetylation with acetic anhydride yields aspirin.
5. Arrange the following in increasing order of acidity: ethanol, phenol, p-nitrophenol, p-methoxyphenol.
- p-methoxyphenol < ethanol < phenol < p-nitrophenol
- ethanol < p-methoxyphenol < phenol < p-nitrophenol
- ethanol < phenol < p-methoxyphenol < p-nitrophenol
- p-nitrophenol < phenol < p-methoxyphenol < ethanol
Answer: ethanol < p-methoxyphenol < phenol < p-nitrophenol
Acidity depends on conjugate base stability. Ethoxide has no resonance, least stable. Phenoxide is resonance-stabilised. p-Methoxyphenol has electron-donating -OCH3 destabilising phenoxide, so less acidic than phenol. p-Nitrophenol has electron-withdrawing -NO2 stabilising phenoxide, most acidic. Thus order: ethanol < p-methoxyphenol < phenol < p-nitrophenol.
6. Starting from benzene, which sequence correctly synthesises anisole?
- Benzene → chlorobenzene (Cl2/FeCl3), then CH3ONa → anisole
- Benzene → phenol (cumene process), then NaOH → sodium phenoxide, then CH3I → anisole
- Benzene → phenol (Dow process), then CH3I/Ag2O → anisole
- Benzene → toluene (Friedel-Crafts), then KMnO4 → benzoic acid, then CH3OH/H+ → anisole
Answer: Benzene → phenol (cumene process), then NaOH → sodium phenoxide, then CH3I → anisole
Anisole (methyl phenyl ether) is prepared from benzene via three steps: (1) benzene to phenol by cumene process (air oxidation of cumene then acid rearrangement), (2) phenol to sodium phenoxide by NaOH, (3) Williamson ether synthesis with CH3I (SN2). This route works because phenol is first made, then the alkoxide attacks a primary alkyl halide.
7. Phenol is treated with dilute HNO3 at room temperature. The major product(s) is/are:
- Picric acid
- 2,4-Dinitrophenol
- o-Nitrophenol and p-Nitrophenol
- m-Nitrophenol
Answer: o-Nitrophenol and p-Nitrophenol
Dilute HNO3 at room temperature nitrates phenol only once, giving a mixture of ortho- and para-nitrophenol. The -OH group activates ortho and para positions. Picric acid requires concentrated HNO3/H2SO4. 2,4-Dinitrophenol requires further nitration. m-Nitrophenol is not formed directly because -OH is ortho/para directing.
8. Excess HI is heated with each ether. Which option correctly lists the products?
- Diethyl ether: EtOH + EtI; tert-butyl methyl ether: tBuI + MeOH; anisole: PhI + MeOH; diphenyl ether: no reaction
- Diethyl ether: EtOH + EtI; tert-butyl methyl ether: tBuOH + MeI; anisole: PhI + MeOH; diphenyl ether: PhOH + PhI
- Diethyl ether: 2 EtI; tert-butyl methyl ether: tBuOH + MeI; anisole: PhOH + MeI; diphenyl ether: PhOH + PhI
- Diethyl ether: 2 EtI; tert-butyl methyl ether: tBuI + MeOH; anisole: PhOH + MeI; diphenyl ether: no reaction
Answer: Diethyl ether: 2 EtI; tert-butyl methyl ether: tBuI + MeOH; anisole: PhOH + MeI; diphenyl ether: no reaction
Excess HI cleaves ethers via SN1/SN2. Diethyl ether (both primary) undergoes SN2 to give 2 EtI. tert-Butyl methyl ether: tertiary carbocation forms tBuI (SN1) + MeOH. Anisole: aryl-O bond strong, so SN2 on methyl gives PhOH + MeI. Diphenyl ether: both aryl-O bonds resist cleavage, so no reaction.
9. Which of the following statements about commercial alcohols is correct?
- Methanol is prepared by fermentation of sugars using yeast.
- Absolute alcohol is 100% ethanol obtained by simple distillation of rectified spirit.
- Rectified spirit contains 95.6% ethanol and is obtained by distillation of fermented broth.
- Denatured alcohol is pure ethanol with no additives.
Answer: Rectified spirit contains 95.6% ethanol and is obtained by distillation of fermented broth.
Rectified spirit is obtained by distillation of fermented broth and contains 95.6% ethanol (azeotrope with water). Absolute alcohol requires drying over CaO, not simple distillation. Methanol is made from CO and H2, not fermentation. Denatured alcohol has additives like methanol or pyridine.
10. When phenol is treated with bromine water, the product is a white precipitate. What is the structure of this precipitate?
- p-Bromophenol
- 2,4,6-Tribromophenol
- o-Bromophenol
- 2,4-Dibromophenol
Answer: 2,4,6-Tribromophenol
Bromine water (Br2 in H2O) is a strong electrophile due to polarisation by water. Phenol, with its -OH group activating the ring strongly at ortho and para positions, undergoes rapid tribromination to give 2,4,6-tribromophenol as a white precipitate. This is a characteristic test for phenol.
11. Arrange the following in increasing order of acidity: p-cresol, phenol, p-chlorophenol, p-nitrophenol, picric acid.
- p-cresol < phenol < p-chlorophenol < p-nitrophenol < picric acid
- phenol < p-cresol < p-chlorophenol < p-nitrophenol < picric acid
- p-cresol < phenol < p-nitrophenol < p-chlorophenol < picric acid
- p-cresol < p-chlorophenol < phenol < p-nitrophenol < picric acid
Answer: p-cresol < phenol < p-chlorophenol < p-nitrophenol < picric acid
Electron-donating groups (methyl) decrease acidity, so p-cresol is least acidic. Electron-withdrawing groups (Cl, NO2) increase acidity; NO2 is stronger than Cl. Multiple NO2 groups in picric acid make it most acidic. Order: p-cresol < phenol < p-chlorophenol < p-nitrophenol < picric acid.
12. A mixture contains benzoic acid, phenol, and ethanol. Which reagent will selectively extract only benzoic acid?
- Aqueous NaOH
- Aqueous NaHCO3
- Aqueous Na2CO3
- Water
Answer: Aqueous NaHCO3
Benzoic acid (pKa ~4.2) is stronger than carbonic acid (pKa ~6.4), so it reacts with NaHCO3 to form sodium benzoate and CO2. Phenol (pKa ~10) is weaker than carbonic acid, so it does not react with NaHCO3. Ethanol (pKa ~16) does not react. Thus NaHCO3 selectively extracts benzoic acid.