Oxidation of Phenols to Quinones: Videos & Practice Problems

Phenol can be oxidized to quinone using dichromate (Cr₂O₇²⁻), resulting in a conjugated six-membered ring with carbonyls positioned either 1,2 or 1,4. The 1,4-quinone, known as 1,4-benzoquinone, can be reduced to hydroquinone or catechol using reducing agents like tin(II) chloride or catalytic hydrogenation. Both phenol and hydroquinone can revert to 1,4-quinone, illustrating the interconversion between these compounds through oxidation-reduction reactions.

Oxidation of Phenols to Quinones Concept 1

The oxidation of phenol to quinone is an important chemical transformation that occurs in the presence of strong oxidizing agents, such as dichromate ions (\( \text{Cr}_2\text{O}_7^{2-} \)). This reaction highlights the unique properties of phenol compared to other alcohols, particularly tertiary alcohols, which do not undergo oxidation due to the absence of alpha hydrogens on the carbon bearing the hydroxyl group.

Quinone, specifically 1,4-benzoquinone, is characterized by its conjugated six-membered ring structure, which contains two carbonyl groups positioned 1,4 to each other. The nomenclature of quinone can be broken down as follows: "cyclo" indicates a ring structure, "hexa" refers to the six carbon atoms in the ring, "diene" signifies the presence of two double bonds, and "dione" denotes the two carbonyl groups. Thus, 1,4-benzoquinone can also be referred to as cyclohexadienedione.

In contrast to tertiary alcohols, which do not react due to the lack of reactive hydrogens, phenol can be oxidized effectively to form quinone. The resulting quinone features a conjugated system with alternating double bonds and carbonyls, contributing to its stability and reactivity. This transformation is significant in organic chemistry, showcasing the distinct reactivity of phenolic compounds.

Oxidation of Phenols to Quinones Concept 2

Phenol can be oxidized to form quinone, a compound that can further undergo reduction to yield either catechol or hydroquinone. The reduction process can be achieved using Tin(II) chloride in the presence of hydrochloric acid or through catalytic hydrogenation, which involves hydrogen gas (H₂) and a metal catalyst such as nickel, palladium, or platinum.

When considering the structure of quinone, it can exist in two forms based on the orientation of the carbonyl groups: 1,2-quinone and 1,4-quinone. The reduction of 1,2-quinone results in catechol, characterized by a benzene ring with two hydroxyl (–OH) groups positioned adjacent to each other. Conversely, the reduction of 1,4-quinone yields hydroquinone, where the hydroxyl groups are positioned opposite each other on the benzene ring.

Both catechol and hydroquinone can be oxidized back to their respective quinone forms, demonstrating the reversible nature of these oxidation-reduction reactions. Notably, both phenol and hydroquinone can be oxidized to 1,4-quinone, highlighting a commonality in their chemical behavior. Understanding these transformations is crucial for grasping the dynamics of oxidation and reduction in organic chemistry.

Oxidation of Phenols to Quinones Example 1

To synthesize a diether starting from phenol, we can utilize a series of chemical reactions that involve oxidation, reduction, and the Williamson ether synthesis. The process begins with phenol, which can be transformed into a quinone through oxidation using a strong oxidizing agent such as dichromate. This reaction introduces a carbonyl group, resulting in a 1,4-quinone structure.

Next, the quinone can be reduced to hydroquinone using tin(II) chloride in the presence of hydrochloric acid. This reduction yields a compound with two hydroxyl (–OH) groups positioned para to each other on the benzene ring, which is essential for the subsequent steps.

With hydroquinone in hand, we can now perform the Williamson ether synthesis twice to convert each hydroxyl group into an ether. The first step involves deprotonating one of the –OH groups using a strong base, such as potassium hydroxide (KOH), to form a phenoxide ion. This ion can then undergo an SN2 reaction with an appropriate alkyl halide, which should have a two-carbon chain to ensure the desired structure of the ether. The reaction results in the formation of the first ether.

Repeating this process for the second –OH group involves the same deprotonation and SN2 reaction with another alkyl halide, leading to the formation of the second ether. It is important to note that both reactions can be summarized as using KOH in equivalent amounts followed by the respective alkyl halides to achieve the final diether product.

In summary, the key steps to synthesize the diether from phenol include:

1. Oxidation of phenol to quinone using dichromate.
2. Reduction of quinone to hydroquinone using tin(II) chloride and hydrochloric acid.
3. Deprotonation of each –OH group with KOH followed by SN2 reactions with alkyl halides to form the ethers.

This method effectively demonstrates the transformation of phenol into a diether through a combination of oxidation, reduction, and ether synthesis techniques.

Determine the products made during each of the following steps. 

• A.
• B.
• C.
• D.

Answer: (D)

Beginning from phenol, determine the chemical steps needed to prepare the following compound. 

To synthesize a complex compound starting from phenol, we can follow a systematic approach that involves oxidation and a Diels-Alder reaction. The initial compound, phenol, can be transformed into quinone through oxidation. This step is crucial as quinone serves as a key intermediate in the synthesis process.

To achieve this oxidation, we utilize a strong oxidizing agent, specifically sodium dichromate in the presence of sulfuric acid (H₂SO₄). This reaction converts phenol into quinone, which is characterized by its two carbonyl groups.

Once we have quinone, we can proceed to form the desired compound by employing the Diels-Alder reaction. This reaction is a [4+2] cycloaddition that allows for the formation of six-membered rings. In this case, we will use two equivalents of conjugated dienes to react with quinone. The Diels-Alder reaction is particularly suitable here because it retains a pi bond in the final product, indicating that the reaction has successfully formed the two six-membered rings.

In summary, the overall synthetic pathway involves first oxidizing phenol to quinone and then performing the Diels-Alder reaction twice to construct the final compound. This method highlights the utility of quinone as a dienophile in the Diels-Alder reaction, making it a strategic choice for synthesizing complex organic structures.

Here’s what students ask on this topic:

What is the mechanism of oxidation of phenol to quinone?

The oxidation of phenol to quinone involves the use of a strong oxidizing agent like dichromate (Cr₂O₇²⁻). The mechanism starts with the formation of a phenoxyl radical through the loss of a hydrogen atom from the hydroxyl group of phenol. This radical then undergoes further oxidation to form a quinone. The quinone structure is a conjugated six-membered ring with carbonyl groups positioned either 1,2 or 1,4 to each other. The overall reaction can be summarized as:

$C_6{H}_5\left(\mathrm{OH}\right)+{\mathrm{Cr}}_2{O}_7^-\to C_6{H}_4\left(O\right)+{\mathrm{Cr}}_3^+$

What are the common reducing agents used to convert quinone to hydroquinone?

Quinone can be reduced to hydroquinone using several common reducing agents. Two primary methods include:

• Tin(II) chloride (SnCl₂) over hydrochloric acid (HCl): This method involves the use of SnCl₂ in an acidic medium to reduce quinone to hydroquinone.
• Catalytic hydrogenation: This method uses hydrogen gas (H₂) in the presence of a metal catalyst such as nickel (Ni), palladium (Pd), or platinum (Pt) to achieve the reduction.

The reduction reaction can be summarized as:

$C_6{H}_4\left(O\right)+H\_2\to C_6{H}_4\left(\mathrm{OH}\right)$

What is the difference between 1,2-quinone and 1,4-quinone?

The difference between 1,2-quinone and 1,4-quinone lies in the positions of the carbonyl groups on the benzene ring:

• 1,2-Quinone: In this structure, the carbonyl groups are adjacent to each other, positioned at the 1 and 2 positions on the benzene ring. This is also known as ortho-quinone.
• 1,4-Quinone: In this structure, the carbonyl groups are positioned at the 1 and 4 positions on the benzene ring, making them para to each other. This is also known as para-quinone or 1,4-benzoquinone.

Both structures are conjugated systems, but their chemical properties and reactivity can differ due to the different positions of the carbonyl groups.

Can phenol be oxidized to quinone using mild oxidizing agents?

Phenol typically requires strong oxidizing agents to be converted to quinone. Commonly used strong oxidizing agents include dichromate (Cr₂O₇²⁻) and permanganate (MnO₄⁻). Mild oxidizing agents generally do not have sufficient oxidizing power to convert phenol to quinone. The reaction involves the formation of a phenoxyl radical and subsequent oxidation steps, which necessitate a strong oxidizing environment. Therefore, mild oxidizing agents are not effective for this transformation.

What are the applications of quinones in organic chemistry?

Quinones have several important applications in organic chemistry and related fields:

• Electron Transfer Agents: Quinones are involved in electron transfer processes in biological systems, such as in the electron transport chain in cellular respiration.
• Organic Synthesis: Quinones are used as intermediates in the synthesis of various organic compounds, including dyes, pharmaceuticals, and agrochemicals.
• Redox Reactions: Quinones participate in redox reactions, making them useful in studying oxidation-reduction mechanisms.
• Antioxidants: Some quinones have antioxidant properties and are used in the formulation of stabilizers for polymers and other materials.

These applications highlight the versatility and importance of quinones in both industrial and biological contexts.