Problem 45 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Phenol is more readily soluble in: (a) NaOll solution (b) dil. \(\mathrm{HCl}\) (c) both \(\mathrm{NaOH}\) and \(\mathrm{HCl}\) (d) \(\mathrm{NaHCO}_{3}\) solution

Step-by-Step Solution

Answer

Phenol is more readily soluble in (a) NaOH solution.

Step 1: Understand the Chemical Nature of Phenol

Phenol is an organic compound that has a hydroxyl group (-OH) directly attached to an aromatic benzene ring, making it a weak acid. This acidity means it can react with strong bases to form phenoxide ions.

Step 2: Analyzing Phenol Solubility in Sodium Hydroxide (NaOH)

When phenol is reacted with a strong base like NaOH, it forms sodium phenoxide, which is soluble in water. This reaction occurs because the strong base deprotonates the weakly acidic phenol, increasing its solubility.

Step 3: Analyzing Phenol Solubility in Dilute Hydrochloric Acid (HCl)

Phenol is less soluble in dilute HCl than in NaOH because HCl is an acid. Since phenol is already a weak acid, it does not react significantly with another acid, leaving it in a less soluble state.

Step 4: Assess Joint Solubilities in NaOH and HCl

Comparing solubility in both NaOH and HCl, phenol will be more soluble in NaOH due to the formation of sodium phenoxide, whereas in HCl, it remains mostly unchanged and thus less soluble.

Step 5: Considering Solubility in Sodium Bicarbonate (NaHCO3)

Phenol does not react with bicarbonate ions in NaHCO3 as effectively as it does with NaOH because NaHCO3 is not a strong enough base to deprotonate phenol. Therefore, phenol's solubility in NaHCO3 is less than in NaOH.

Step 6: Conclusion: Compare and Determine the Best Soluble Medium

Based on the chemical reactions and interactions discussed, phenol dissolves best in NaOH solution among the given options.

Key Concepts

Weak Acid

Phenol is an excellent example of a weak acid. In chemistry, weak acids are substances that do not fully ionize in solution. This means that only a small fraction of the acid molecules break apart to release hydrogen ions (\( ext{H}^+ \)).
Phenol's weak acid nature is due to its structure. Its hydroxyl group (-OH) is attached to a benzene ring, which stabilizes the molecule. This makes phenol much less likely to donate its hydrogen ion compared to stronger acids like hydrochloric acid (HCl).
However, its partial ionization is still enough to engage in specific chemical reactions, especially with strong bases. Thus, when in the presence of a strong base such as sodium hydroxide (NaOH), phenol can donate its hydrogen ion, transforming into a more reactive ion, sodium phenoxide. Understanding this property is key in predicting the behavior of phenol in various solutions.

Sodium Phenoxide

When phenol reacts with sodium hydroxide (a strong base), it forms a compound known as sodium phenoxide. This reaction is critical because it increases phenol's solubility in water.
Formation Process:

• The hydroxide ion from NaOH deprotonates, or removes a hydrogen ion from the phenol.
• This results in the phenol converting into its anionic form, the phenoxide ion.
• The sodium cation (\( ext{Na}^+ \)) then pairs with this phenoxide ion to form sodium phenoxide.

The resulting sodium phenoxide is more ionic in nature and easily dissolves in water compared to molecular phenol. This transformation is why phenol dissolves better in solutions containing strong bases like NaOH than in acidic or neutral ones.

Acid-Base Reactions

Acid-base reactions are fundamental in predicting solubility trends and understanding chemical behavior of compounds like phenol. In an acid-base reaction, an acid donates a proton (hydrogen ion) to a base.
With phenol being a weak acid, it undergoes an acid-base reaction when introduced to a strong base like NaOH. This reaction results in the formation of sodium phenoxide, illustrating how acid-base interplay is crucial for solubility.
Key Reaction Insights:

• Strong bases like NaOH effectively deprotonate weak acids, increasing solubility by forming ions like the phenoxide ion.
• Acidic solutions or weak bases, such as HCl or NaHCO₃ respectively, do not significantly alter weak acids, thus impacting solubility less effectively.
• The balance between acidity and basicity in solutions largely dictates the state and solubility of compounds like phenol.

This knowledge is vital for predicting how substances will behave in different chemical environments and for conducting experiments.