Problem 89 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Which one of the following statements is correct? (a) phenol is more acidic than carbonic acid (b) phenol is less acidic than ethyl alcohol (c) phenol is more acidic than ethanol (d) phenol is more acidic than acetic acid

Step-by-Step Solution

Answer

The correct statement is (c) phenol is more acidic than ethanol.

Step 1: Identify Acidic Strength Order

To solve the problem, we have to determine the acidic strength of each compound. Typically, acidic strength can be evaluated by looking at the stability of the conjugate base formed after donating a proton. In general, carbonic acid \((H_2CO_3)\), acetic acid \((CH_3COOH)\), phenol \((C_6H_5OH)\), and ethyl alcohol \((C_2H_5OH)\) exhibit different acidic strengths in order: \text{carbonic acid} > \text{acetic acid} > \text{phenol} > \text{ethyl alcohol}. Therefore, phenol is more acidic than ethanol (ethyl alcohol) but less acidic than acetic acid and carbonic acid.

Step 2: Analyze Each Statement

Now, let's evaluate each provided statement according to the acidic strength order: (a) Phenol is more acidic than carbonic acid - Incorrect. (b) Phenol is less acidic than ethyl alcohol - Incorrect. (c) Phenol is more acidic than ethanol - Correct. (d) Phenol is more acidic than acetic acid - Incorrect.

Step 3: Conclusion

After analyzing each statement, the correct answer is (c), stating that phenol is more acidic than ethanol (ethyl alcohol). This is consistent with the known order of acidity.

Key Concepts

Phenol

Phenol is a complex organic compound with the molecular formula \(C_6H_5OH\). It belongs to a class of compounds known as aromatic alcohols. Unlike typical alcohols, phenol has unique acidic properties due to the aromatic ring connected to the hydroxyl group.
When phenol loses a proton (\(H^+\)), it forms a phenoxide ion, \(C_6H_5O^-\). The stability of this ion is enhanced by resonance within the aromatic ring. This resonant stabilization increases the acidity of phenol compared to typical alcohols like ethanol.

• Phenol's acidity: medium.
• Primary reason for acidity: resonance stabilization.

Although more acidic than ethanol, phenol is still less acidic than acetic acid and carbonic acid. Its moderate acidity is evident by its ability to react with strong bases to form phenoxide salts.

Ethanol

Ethanol is a widely used type of alcohol with the chemical formula \(C_2H_5OH\). It is a non-aromatic alcohol and is commonly known as ethyl alcohol.
In terms of acidity, ethanol is weaker than phenol. When it releases a proton, it forms an ethoxide ion, \(C_2H_5O^-\). The ethoxide ion lacks the resonance stabilization found in phenol, making it less stable. This instability contributes to ethanol's weaker acidic properties.

• Ethanol's acidity: weak.
• Stability: ethoxide ion is less stable due to lack of resonance.

Ethanol is generally not considered acidic compared to most compounds. Instead, it behaves more as a neutral compound, often used as a solvent and in consumables due to its low reactivity.

Acetic Acid

Acetic acid, with the formula \(CH_3COOH\), is an essential carboxylic acid. It is best known as the main component of vinegar apart from water. Its acidity is relatively high for an organic compound, making it a frequent subject in discussions of acid strength.
The acidity of acetic acid comes from its ability to form acetate ions, \(CH_3COO^-\), which are resonance stabilized. The resonance involves delocalization of electrons between the oxygen atoms, stabilizing the acetate ion.

• Acetic acid's acidity: stronger than phenol.
• Primary reason for acidity: resonance within the acetate ion.

This strengthens acetic acid, making it more reactive with bases compared to phenol. Its acidic properties allow it to act in various chemical reactions, especially in organic synthesis and food preservation.

Carbonic Acid

Carbonic acid \(H_2CO_3\) is a weak acid that forms in solution when carbon dioxide dissolves in water. Its formula indicates the presence of two hydrogen atoms that can be released as protons.When carbonic acid loses a proton, it forms the bicarbonate ion \(HCO_3^-\). If it loses another, it forms the carbonate ion \(CO_3^{2-}\). These ionic forms provide buffering capacities to maintain pH stability in natural water systems and physiological processes.

• Carbonic acid's acidity: highest among common organic acids like phenol and ethanol.
• Primary reason for acidity: easy reversibility and ion stability.

Carbonic acid plays significant roles in maintaining equilibrium within the environment and biological organisms. Due to its unique properties, it is more acidic than phenol and acetic acid, although it often exists transiently in aqueous systems.