Problem 157 - A Complete Resource Book in Chemistry for JEE Main

Question

Under different conditions, nitration of phenol yields 1\. o-nitrophenol 2\. p-nitrophenol 3\. \(2,4,6\)-trinitrophenol The correct sequence of decreasing order of acidic nature of these phenols is (a) \(3,2,1\) (b) \(1,2,3\) (c) \(2,1,3\) (d) \(3,1,2\)

Step-by-Step Solution

Answer

The correct sequence is (a): 3,2,1.

Step 1: Understand the Structures

Identify the structures of the compounds mentioned in the question: 1. **o-nitrophenol** has a nitro group at the ortho position. 2. **p-nitrophenol** has a nitro group at the para position. 3. **2,4,6-trinitrophenol** (also known as picric acid) has nitro groups at the 2, 4, and 6 positions.

Step 2: Analyze the Effect of the Nitro Group

Remember that the nitro group is an electron-withdrawing group (EWG), which increases the acidity of phenol by stabilizing the negative charge on the phenoxide ion through resonance and inductive effects.

Step 3: Evaluate the Number of Nitro Groups

Compare the number of nitro groups present in each compound: - **2,4,6-trinitrophenol** has three nitro groups, providing significant acid stabilization. - **p-nitrophenol** has one nitro group, effectively stabilizing the phenoxide ion. - **o-nitrophenol** also has one nitro group but the proximity to the hydroxyl group decreases resonance stabilization compared to the para position.

Step 4: Assess the Acidity Order

Based on the electron-withdrawing power of the nitro groups: - **2,4,6-trinitrophenol** is the most acidic because it has three electron-withdrawing nitro groups. - **p-nitrophenol** is more acidic than **o-nitrophenol** due to better resonance stabilization when the nitro group is at the para position.

Step 5: Determine the Correct Sequence

From the above evaluations, the order of decreasing acidity is **2,4,6-trinitrophenol**, **p-nitrophenol**, and then **o-nitrophenol**, which corresponds to sequence (a): **3,2,1**.

Key Concepts

Nitration of Phenol

Nitration of phenol is a chemical reaction where a nitro group (\(\text{NO}_2\)) is added to the phenol compound. The process involves treating phenol with a nitrating agent such as nitric acid. This reaction can occur under controlled conditions and usually results in the formation of nitrophenols. Depending on the conditions, nitration can produce various isomers, including:

• o-nitrophenol, with the nitro group attached to the ortho position.
• p-nitrophenol, with the nitro group attached to the para position.
• 2,4,6-trinitrophenol, known as picric acid, with three nitro groups attached to the phenol ring.

This process is significant because it modifies the properties of phenol, particularly its acidity. By introducing electron-withdrawing groups like nitro, the compound becomes more acidic.

Electron-withdrawing Groups

Electron-withdrawing groups (EWGs) play a crucial role in chemistry, especially when they are attached to an aromatic system like phenol. The nitro group (\(\text{NO}_2\)) is a prime example of such a group. EWGs increase acidity by stabilizing the conjugate base. When phenol loses a hydrogen ion (H\(^+\)), the remaining phenoxide ion can be destabilized by the negative charge. However, when an EWG is present, it can help to stabilize this charge through:

• Resonance effects: EWGs allow the charge to be delocalized across the aromatic ring.
• Inductive effects: EWGs pull electron density away from the charge, thereby stabilizing it.

This stabilization is more pronounced when more EWGs are present, as seen with 2,4,6-trinitrophenol, which has three nitro groups, making it much more acidic than mono-substituted phenols.

Resonance and Inductive Effects

To understand the role of resonance and inductive effects in phenols, it's important to know how these effects affect the acid-base properties. When phenol loses a proton, forming the phenoxide ion, the negative charge created can be stabilized by delocalization across the aromatic ring, a process known as resonance stabilization. Nitro groups enhance this effect by providing resonance structures where the negative charge can be further spread out. This reduces the energy of the phenoxide ion:

• Resonance: Allows the charge to move throughout the benzene ring, sharing the load and decreasing overall energy.
• Inductive: Affects the electron density across bonds, pulling electron density away from the negative charge and stabilizing it.

These combined effects make compounds like p-nitrophenol more acidic than o-nitrophenol, given better resonance stabilization when the nitro group is at the para position.

Ortho, Meta, Para Positions

These terms describe the positions of substituents on an aromatic ring relative to a reference group, often the hydroxyl group in phenols.

• Ortho position (o-): Adjacent to the reference group, for example, the nitro group directly next to the hydroxyl in o-nitrophenol.
• Meta position (m-): One position removed from the reference group.
• Para position (p-): Located opposite the reference group, as seen in p-nitrophenol, where the nitro group is directly across from the hydroxyl group.

These positions influence the electronic and physical properties of the molecule due to steric and electronic effects. In the case of phenols, the para position allows for more efficient resonance stabilization of the phenoxide ion compared to the ortho position. This is why p-nitrophenol tends to be more acidic than o-nitrophenol, as it maximizes stabilization through better delocalization."}]} рустайте a coding-style format. For example: NOT: Headline: The first concept NOT: The first concept has to be...vement advice. Be detailed and only choose the most important topics from the list. Remove any **headlines** with a '*' instead define headlines through context in paragraphs. Remove any **title** with a '*' as the title shall be irrelevant to your text. - Make sure the definition and main points are clearly presented as always the sub-headings could be its own mini-article. - Regularly use bullet points to clear up text and show examples. Use bullet points to clearly list elements or comparisons, where
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