Question

Give two reactions that show the acidic nature of phenol. Compare acidity of phenol with that of ethanol.

Short answer

Phenol is acidic; it reacts with \(NaOH\) and \(Na\). Phenol is more acidic than ethanol.

Step-by-step solution

Understanding Phenol's Acidity

Phenol (\(C_6H_5OH\)) is an aromatic compound with a hydroxyl group directly bonded to the benzene ring. The acidic nature is due to the ability of phenol to donate its hydrogen ion (proton) from the hydroxyl group, forming a phenoxide ion (\(C_6H_5O^-\)). This ion is resonance-stabilized within the phenyl ring.

Reaction with a Strong Base

When phenol reacts with a strong base like sodium hydroxide (\(NaOH\)), it forms sodium phenoxide and water. The reaction is\[ C_6H_5OH + NaOH \rightarrow C_6H_5ONa + H_2O \]This reaction demonstrates phenol's ability to donate a proton, showing its acidic nature.

Reaction with Metal

Phenol can also react with active metals like sodium or potassium. For example, the reaction with sodium (\(Na\)) produces sodium phenoxide and hydrogen gas:\[ 2C_6H_5OH + 2Na \rightarrow 2C_6H_5ONa + H_2 \uparrow \]The evolution of hydrogen gas here illustrates phenol's acidic nature as it displaces hydrogen.

Comparing Acidity with Ethanol

Ethanol (\(C_2H_5OH\)) is a simple aliphatic alcohol. When comparing the acidity, phenol is more acidic than ethanol. This is because the phenoxide ion is resonance-stabilized, making it easier to lose the hydrogen ion, whereas the ethoxide ion (\(C_2H_5O^-\)) lacks such resonance stabilization.

Conclusion on Acidity

The reactions with \(NaOH\) and \(Na\) highlight phenol's stronger acidic nature. Phenol is more acidic than ethanol due to the resonance stabilization of its conjugate base. Ethanol, lacking this, is less acidic.

Key concepts

Reactions of Phenol

Phenol, known chemically as \( C_6H_5OH \), is an aromatic compound that exhibits acidic properties due to its ability to donate a hydrogen ion (proton) from its hydroxyl group. To highlight its acidic nature, we can look at two specific reactions:

• Reaction with a Strong Base: When phenol reacts with a strong base like sodium hydroxide (\( NaOH \)), it forms sodium phenoxide (\( C_6H_5ONa \)) and water. The balanced reaction is: \[ C_6H_5OH + NaOH \rightarrow C_6H_5ONa + H_2O \] This reaction indicates that phenol can donate a proton, showing its acidic nature.
• Reaction with Active Metals: When phenol encounters active metals such as sodium (\( Na \)), it reacts to form sodium phenoxide and produces hydrogen gas. The reaction is: \[ 2C_6H_5OH + 2Na \rightarrow 2C_6H_5ONa + H_2 \uparrow \] The evolution of hydrogen gas signifies phenol's acidic property because it indicates the displacement of hydrogen.

These reactions demonstrate how phenol can act like an acid by donating a proton and forming products that include phenoxide ions.

Phenol vs Ethanol Acidity

Phenol and ethanol, though both contain a hydroxyl group, differ significantly in their acidic strength. Phenol \( C_6H_5OH \) is more acidic than ethanol \( C_2H_5OH \). Let's explore why:

• Resonance Stabilization in Phenol: Phenol’s acidic strength is primarily due to the stabilization provided by the aromatic benzene ring. When phenol loses a proton, it forms a phenoxide ion \( C_6H_5O^- \), which is extensively resonance-stabilized. This stabilization helps the phenoxide ion to manage the negative charge more effectively, thus favoring deprotonation.
• Ethanol’s Lack of Resonance: Ethanol, on the other hand, forms the ethoxide ion \( C_2H_5O^- \) upon losing a proton. However, this ion lacks resonance stabilization because ethanol’s alkyl group does not provide any such structure to stabilize the negative charge. Consequently, ethanol is less willing to lose its hydrogen ion compared to phenol.

In essence, when we compare the acidity of these two compounds, phenol's unique ability to distribute and stabilize the negative charge makes it more acidic than ethanol.

Resonance Stabilization of Phenoxide

The resonance stabilization of the phenoxide ion \( C_6H_5O^- \) is a central concept in understanding why phenol is more acidic than many other alcohols like ethanol. This stabilization occurs due to the delocalization of electrons in the phenoxide ion.

• Delocalization of Electrons: When phenol loses a hydrogen ion, it becomes a phenoxide ion, in which the negative charge on the oxygen can be delocalized over the benzene ring. This delocalization is possible because of the overlapping of \( p \)-orbitals, allowing the electrons to be spread out over multiple atoms.
• Resonance Structures: Several resonance structures can be drawn for the phenoxide ion, each depicting the spreading of the negative charge over the aromatic ring. These structures show that the negative charge is not localized on one atom but is shared among the entire structure, making the phenoxide ion more stable.
• Impact on Acidity: As a result of this stability, the phenol molecule can more easily donate a hydrogen ion compared to alcohols like ethanol, where no resonance is involved. This increased ease of deprotonation is why phenol is a stronger acid.

Resonance stabilization is a compelling concept that helps explain not only the acidity of phenol but also its reactivity and interactions in various chemical settings.