Which one of the following, with HBr will give a phenol?

Core Logic

Let's track the conversions row by row based on the description:

• Toxic compound A is methanol (CH₃OH$CH_3OH$). Reacting it via acidic NaCN sequences effectively converts it eventually into acetic acid (CH₃COOH$CH_3COOH$), which is an edible cooking component and food preservative (vinegar) B.
• Reduction of acetic acid (B) with diborane (B₂H₆$B_2H_6$) yields ethanol (CH₃CH₂OH$CH_3CH_2OH$) C. Ethanol is used as a biofuel additive in petrol.
• Ethanol (C) upon heating with oleum/concentrated sulfuric acid at 140°C$140^{\circ}\mathrm{C}$ undergoes intermolecular dehydration to yield diethyl ether (C₂H₅-O-C₂H₅$C_2H_5-O-C_2H_5$) D, which functions as an inhalable anesthetic.

Step 1: Chemical Reaction Sequence Flow

The full sequential progression corresponds exactly to:

Pattern Recognition

Key temperature indicator: Dehydration of ethanol at 140°C$140^{\circ}\mathrm{C}$ yields diethyl ether (ether synthesis), whereas 170°C$170^{\circ}\mathrm{C}$ yields ethene gas. Combined with the food preservative clue (acetic acid), the sequence locks instantly to option (3).

Chapter Mix

Class 12 Chemistry: Alcohols, Phenols and Ethers

Core Logic

An electrophile seeks electrons. Higher electron density in the benzene ring makes it more susceptible (reactive) to electrophilic attack. The ring's electron density is governed by the inductive (I) and mesomeric/resonance (M) effects of the attached groups.

Step 1: Evaluate Substituent Effects

• Cl$Cl$ (in chlorobenzene): shows weak +M effect but strong -I effect, causing net deactivation.
• CH₃$CH_3$ (in toluene): shows +I effect and hyperconjugation, slightly activating the ring.
• COOH$COOH$ (in benzoic acid): shows strong -M and -I effect, highly deactivating.
• OH$OH$ (in phenol): shows strong +M effect which dominates its weak -I effect, strongly activating the ring.

Step 2: Conclusion

Phenol has the highest electron density in the ring among the given options due to the strong +M effect of the -OH group. Thus, it is most easily attacked by an electrophile.

Pattern Recognition

Reactivity towards Electrophilic Aromatic Substitution (EAS): Strong +M (-OH, -NH2) > Weak +I/Hyperconjugation (-CH3) > Halogens (-Cl, net deactivating) > Strong -M (-COOH, -NO2).

Chapter Mix

Class 11 Chemistry: Hydrocarbons Class 12 Chemistry: Alcohols, Phenols and Ethers

Core Logic

Aliphatic alcohols (Bu-OH$\text{Bu-OH}$) are least acidic due to +I$+I$ effects. Among phenols, electron-donating groups like -OMe$-\text{OMe}$ (+M$+M$ effect) reduce acidity relative to plain phenol, while electron-withdrawing groups (-NO₂$-\text{NO}_2$, -M$-M$ and -I$-I$ effects) significantly enhance stability of the conjugate phenoxide base. Two -NO₂$-\text{NO}_2$ groups heighten acidity maximally.

Step 1: Order verification

Pattern Recognition

+M$+M$ groups lower acidity; -M$-M$ groups raise it. Alcohols are always less acidic than resonant phenols.

Chapter Mix

Class 12 Chemistry: Alcohols, Phenols and Ethers

Core Logic

Statement I is correct: At the para-position, the -NO₂$-\text{NO}_2$ group exerts both powerful -I$-I$ and -M$-M$ effects, maximizing electron withdrawal from the phenoxide ion. Intramolecular hydrogen bonding reduces the acidity of o-nitrophenol.

Statement II is incorrect: Lucas reagent (conc. HCl + anhydrous ZnCl₂$\text{conc. HCl} + \text{anhydrous ZnCl}_2$) reacts instantly with tertiary alcohols to give immediate turbidity. Primary alcohols like ethanol do not show turbidity at room temperature without prolonged heating.

Pattern Recognition

Para-nitrophenol acidity maximization vs Lucas test thresholds (3° > 2° > 1°$3^{\circ} > 2^{\circ} > 1^{\circ}$). Primary alcohols react very slowly.

Chapter Mix

Class 12 Chemistry: Alcohols, Phenols and Ethers