Problem 56 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reagents cannot be used to distinguish between phenol benzyl alcohol? (a) \(\mathrm{NaOH}\) (b) Bromine/ \(\mathrm{CCl}_{4}\) (c) \(\mathrm{NaHCO}_{3}\) (d) Both (b) and (c)

Step-by-Step Solution

Answer

Option (d) Both (b) and (c).

Step 1: Understanding the Chemical Properties

To distinguish between phenol and benzyl alcohol, we can use their distinct chemical properties. Phenol is aromatic and more acidic due to the resonance stabilization of its anion, whereas benzyl alcohol is a primary alcohol and lacks this aromatic acidity.

Step 2: Analyzing Option (a) NaOH

Phenol reacts with NaOH to form sodium phenoxide due to its acidic nature, while benzyl alcohol does not react as it is a neutral alcohol. Hence, NaOH can distinguish between phenol and benzyl alcohol.

Step 3: Analyzing Option (b) Bromine/CCl4

Phenol reacts with bromine water to give a white precipitate of 2,4,6-tribromophenol, but in carbon tetrachloride (CCl4), the reaction might not result in distinguishable products under all conditions for both compounds.

Step 4: Analyzing Option (c) NaHCO3

Phenol is not acidic enough to react with NaHCO3 to release carbon dioxide, whereas a carboxylic acid would. Benzyl alcohol also does not react with NaHCO3. Thus, NaHCO3 cannot distinguish between phenol and benzyl alcohol.

Step 5: Conclusion

NaOH can be used to distinguish between the two, while neither bromine/CCl4 nor NaHCO3 can effectively distinguish the two under all scenarios. Thus, options (b) and (c) cannot be used.

Key Concepts

Phenol

Phenol is an organic compound with the formula \( C_6H_5OH \). It's a unique type of alcohol because the hydroxyl group \(-OH\) is directly bonded to an aromatic benzene ring. This structure gives phenol distinct properties compared to other alcohols.

One of the most notable features of phenol is its acidity. Although not as strong as carboxylic acids, phenol is significantly more acidic than aliphatic alcohols. This is because the hydroxyl group's proton can be released, forming the phenoxide ion, \( C_6H_5O^- \). This ion is resonance stabilized, meaning the negative charge is distributed over the benzene ring, lowering the energy and increasing the stability of the ion.

Phenol reacts with strong bases like \( ext{NaOH} \) to form water and sodium phenoxide. This reaction highlights a clear difference between phenol and other alcohols, which do not show this behavior.

• Aromatic Ring: Phenol's acidic property is due to the resonance in the aromatic ring.
• Hydroxyl Group: The hydroxyl group is directly attached to the benzene ring.
• Acidity: Higher than typical alcohols.

Benzyl Alcohol

Benzyl alcohol, \( C_6H_5CH_2OH \), is an aromatic alcohol but distinctly different from phenol. The key difference lies in how the hydroxyl group is attached. In benzyl alcohol, the \(-OH\) group is bonded to a \(\text{CH}_2\) group, which is then attached to a benzene ring.

This structure means that benzyl alcohol does not exhibit the same acidity as phenol because the hydroxyl group is not directly connected to the aromatic system. As a primary alcohol, benzyl alcohol behaves more like aliphatic alcohols, lacking the resonance stabilization that phenoxides have.

Because of these properties, benzyl alcohol does not react with strong bases like phenol does. In chemical tests, benzyl alcohol does not form ionic salts with \( ext{NaOH} \), making it quite neutral.

• Hydroxyl Attachment: The \(-OH\) group is connected through a \( \text{CH}_2 \) group.
• Neutral Character: Does not exhibit phenol's acidity.
• Primary Alcohol: Reacts like typical aliphatic alcohols.

Chemical Properties

The chemical properties help us understand the behavior of compounds in reactions. When comparing phenol and benzyl alcohol, their different structures lead to varying chemical reactivity:

• Distinguishing with Bases: Since phenol is more acidic, it reacts with bases like \( ext{NaOH} \), forming sodium phenoxide. Benzyl alcohol, being neutral, won't react with \( ext{NaOH} \).


• Bromine Reaction: Phenol can react with bromine in water to form a white precipitate, but not necessarily in \( ext{CCl}_4 \). This behavior is a chemical property that reflects phenol's reactivity.


• Non-reaction with \( ext{NaHCO}_3 \): Neither phenol nor benzyl alcohol is acidic enough to evolve \( ext{CO}_2 \) when reacted with sodium bicarbonate.

These unique chemical properties are methods used in chemistry to identify and differentiate between compounds. Understanding these reactions is crucial for distinguishing phenol and benzyl alcohol in experimental setups.