Problem 70 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Arrange the following in increasing acidic character (i) Phenol (ii) m-nitrophenol (iii) \(\mathrm{m}\) -chlorophenol (iv) \(\mathrm{m}\) -cresol (a) iv \(<\mathrm{i}<\mathrm{iii}<\mathrm{ii}\) (b) iv \(<\mathrm{i}<\mathrm{ii}<\mathrm{iii}\) (c) \(i<\) iv \(<\) iii \(<\) ii (d) iii \(<\mathrm{ii}<\mathrm{iv}<\mathrm{i}\)

Step-by-Step Solution

Answer

Correct answer is (a) iv < i < iii < ii.

Step 1: Understand Acidic Character

The acidic character of phenolic compounds depends on the ability of the compound to donate a proton. The electron-withdrawing groups increase the acidic strength by stabilizing the negative charge on the oxygen atom after losing a proton. Conversely, electron-donating groups decrease acidic strength.

Step 2: Analyze Phenol (i)

Phenol is a weak acid with the phenolic hydroxyl group capable of donating a proton. There are no additional substituents directly affecting its acidic character in the standard phenol molecule.

Step 3: Analyze m-Nitrophenol (ii)

m-Nitrophenol has a nitro group (NO_2) at the meta position, which is a strong electron-withdrawing group. This increases the acidic character by stabilizing the phenoxide ion (phenol after losing a proton).

Step 4: Analyze m-Chlorophenol (iii)

m-Chlorophenol has a chlorine atom at the meta position. Chlorine is an electron-withdrawing group, albeit weaker than the nitro group (NO_2), thus increasing the acidic character, but less so than m-nitrophenol.

Step 5: Analyze m-Cresol (iv)

m-Cresol has a methyl group (CH_3) at the meta position, which is an electron-donating group. This decreases the acidic character relative to phenol.

Step 6: Arrange Compounds

Based on the analysis: m-Nitrophenol (ii) is the most acidic due to the strong electron-withdrawing nitro group. m-Chlorophenol (iii) follows due to its electron-withdrawing chlorine. Phenol (i) is next as a baseline, and m-Cresol (iv) is the least acidic due to the electron-donating methyl group. Thus, the order is: iv < i < iii < ii.

Key Concepts

Electron-Withdrawing Groups

When we talk about electron-withdrawing groups in phenolic compounds, we're referring to groups that attract electrons towards themselves away from the rest of the molecule. This behavior impacts the acidic nature of phenols significantly.

• The presence of electron-withdrawing groups, such as the nitro group ( O_2 ), helps stabilize the negative charge that forms on the oxygen atom after a proton is donated.
• By stabilizing this negative charge, these groups increase the tendency of the compound to release a proton, thus enhancing its acidic character.
• Common electron-withdrawing groups include nitro ( O_2 ), cyano ( CN ), and halogens like chlorine ( Cl ).

Understanding this can explain why m-nitrophenol is more acidic than phenol, as the nitro group at the meta position is a strong electron-withdrawing entity that significantly stabilizes the ion formed after proton donation.

Electron-Donating Groups

In contrast to electron-withdrawing groups, electron-donating groups push electrons away from themselves and towards the rest of the molecule. This action decreases the overall acidity of the phenolic compound.

• Electron-donating groups destabilize the negative charge on the oxygen atom after proton donation, reducing the compound's ability to act as an acid.
• Common examples include alkyl groups like methyl ( CH_3 ) and ethyl ( C_2H_5 ).
• In the case of m-cresol, the methyl group is an electron-donating group that decreases the acidity by destabilizing the resulting phenoxide ion.

This effect explains why m-cresol is less acidic than phenol, as the presence of a methyl group reduces the compound's ability to release a proton.

Phenolic Compounds

Phenolic compounds are a class of organic compounds characterized by a hydroxyl group attached to an aromatic hydrocarbon. These compounds are known for their unique chemical properties, particularly their ability to act as weak acids.

• The acidic nature of phenolic compounds is due to the presence of the hydroxyl group, which can donate a proton, forming a phenoxide ion.
• The strength of acidity in phenolic compounds varies depending on the substituents attached to the aromatic ring.
• Compounds such as m-nitrophenol, m-chlorophenol, and m-cresol demonstrate different acidic strengths due to the nature of their substituent groups.

Understanding these variations helps predict and explain the behavior of different phenolic compounds in chemical reactions.

Proton Donation

Proton donation is the process by which an acidic substance, like a phenolic compound, donates a hydrogen ion ( H^+ ) to a base, creating its conjugate base.

• In phenolic compounds, the hydroxyl group is the site of proton donation, leading to the formation of a phenoxide ion.
• The ability of a phenolic compound to donate a proton is a key factor in determining its acidity; stronger acids more readily give up their protons.
• The presence of different substituents on the aromatic ring affects how easily this donation occurs, influencing acidic strength.

By studying proton donation, chemists can better understand the reactivity and properties of phenolic compounds, like the distinct behavior observed in m-nitrophenol compared to m-cresol.