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/CCl \(_{4}\) (c) \(\mathrm{NaHCO}_{3}\) (d) Both (b) and (c)

Step-by-Step Solution

Answer

(c) \(\text{NaHCO}_3\) cannot distinguish as it does not react with either phenol or benzyl alcohol.

Step 1: Understanding the Compounds

Phenol is an aromatic compound with a hydroxyl group directly attached to a benzene ring, whereas benzyl alcohol has a hydroxyl group attached to a benzyl group (which is not directly attached to the benzene ring).

Step 1: Reaction with NaOH

Phenol, being a weakly acidic compound, can react with \( ext{NaOH} \) to form a phenoxide ion, reacting to form a water-soluble salt. Benzyl alcohol is not acidic and will not react with \( ext{NaOH} \). Thus, \( ext{NaOH} \) can be used to distinguish between phenol and benzyl alcohol.

Step 2: Reaction with Bromine/CCl4

In the presence of \( ext{Br}_2/ ext{CCl}_4 \), electrophilic aromatic substitution can occur with phenol, leading to the formation of a brominated phenol product. However, benzyl alcohol will not react under these conditions similarly. Therefore, \( ext{Br}_2/ ext{CCl}_4 \) can also distinguish between the two.

Step 3: Reaction with NaHCO3

Phenol does not react with \( ext{NaHCO}_3 \) since it is not acidic enough to react with bicarbonate to produce \( ext{CO}_2 \). Benzyl alcohol, being non-acidic, will also not react with \( ext{NaHCO}_3 \). Therefore, \( ext{NaHCO}_3 \) cannot distinguish between the two.

Step 5: Identify the Correct Answer

Since both \( ext{Br}_2/ ext{CCl}_4 \) and \( ext{NaOH} \) can be used to distinguish phenol from benzyl alcohol, and \( ext{NaHCO}_3 \) cannot, the correct choice is that \( ext{NaHCO}_3 \) alone cannot distinguish between the two, matching option (c).

Key Concepts

Phenol

Phenol is an organic compound characterized by a hydroxyl group (-OH) directly bonded to an aromatic benzene ring. This unique structure gives phenol specific properties, distinct from other alcohols. Phenol is a weak acid, stronger than alcohols in terms of donating a hydrogen ion.
Some of phenol's key properties include:

• Strong antiseptic qualities
• Ability to form salts with bases like \( \mathrm{NaOH} \)
• Involvement in electrophilic aromatic substitution reactions

These characteristics enable phenol's reactions with certain reagents while providing no reaction with others. For example, phenol reacts with \( \mathrm{NaOH} \) to form a phenoxide ion, a water-soluble salt. However, it does not react with \( \mathrm{NaHCO}_3 \), since its acidity is not sufficient to liberate carbon dioxide from bicarbonate.

Benzyl Alcohol

Benzyl alcohol distinguishes itself from phenol by the position of its hydroxyl group. Here, the hydroxyl is attached to a benzyl group, not directly to the aromatic ring. This structure imparts benzyl alcohol with a neutral character, unlike the weak acidity of phenol.
Benzyl alcohol has several important features:

• It is a colorless liquid with a mild pleasant aromatic odor.
• It finds usage as a solvent in inks, paints, and coatings.
• Lacks acidity, showing no reaction with bases like \( \mathrm{NaOH} \) or \( \mathrm{NaHCO}_3 \).

Due to its neutral nature, benzyl alcohol does not undergo the same chemical reactions as phenol, which affects how we distinguish it using chemical reagents.

Distinguishing Reagents

Distinguishing reagents are substances used to tell two similar compounds apart based on their chemical properties. In organic chemistry, these reagents help identify whether a particular reaction will occur with specific compounds.
Here are some common reactions to distinguish phenol from benzyl alcohol:

• **NaOH test:** Phenol reacts due to its weak acidity, forming phenoxide ion, whereas benzyl alcohol does not react.
• **Bromine/CCl4 test:** Phenol undergoes electrophilic substitution, easily brominating due to its aromatic nature. In contrast, benzyl alcohol remains unreactive under these conditions.
• **NaHCO3 test:** Neither phenol nor benzyl alcohol reacts, as both lack the necessary acidity.

Effective distinguishing relies on observing chemical reactions or the lack thereof. This highlights their intrinsic properties.

Chemical Reactions

Chemical reactions involve rearranging atoms to form new substances. They often involve breaking and forming bonds, leading to a transformation in the substance's properties.
In organic chemistry, understanding these transformations is crucial.

• Phenol's acidity allows it to react with \( \mathrm{NaOH} \), forming a new ionic compound.
• Benzyl alcohol, lacking this acidic property, remains unchanged.
• Bromine (\( \mathrm{Br}_2 \)) reacts with phenol to form a brominated product via electrophilic aromatic substitution.

These transformations help identify and understand compound behavior in reactions. By applying specific reagents, chemists can predict and utilize these reactions to analyze and distinguish organic compounds.