Question

Phenol samples that have been allowed to stand in air always possess a pink or red color because of oxidation. Write a mechanism for the oxidation of phenol by oxygen that could lead to one or more products that might be expected to be colored.

Short answer

The oxidation of phenol by oxygen involves a series of reactions including the formation of phenoxide ion (C6H5O-) after losing a proton in the presence of a base, generation of oxygen radicals (O•) through homolytic bond cleavage, and two radical addition reactions (Step 4 and 5) creating a peroxide radical species (C6H5O-O-O•-). This species gains a proton from another phenol molecule to form a hydroperoxide species (C6H5O-O-OH), which then undergoes rearrangement and cyclization reactions leading to the formation of colored products such as quinones or other complex, colored compounds. These products absorb light at specific wavelengths, giving the solution its pink or red color.

Step-by-step solution

Identify the reacting species

The reacting species in this exercise are phenol (C6H5OH) and oxygen (O2). Phenol is an aromatic compound with a hydroxyl (-OH) group attached to a benzene ring. Oxygen is a diatomic molecule that acts as an oxidizing agent.

Write the initial reaction between phenol and oxygen

In the presence of a base, phenol loses a proton (H+) to form a phenoxide ion (C6H5O-) that can react with oxygen more readily. The initial reaction is: C6H5OH + OH- ⇒ C6H5O- + H2O

Generate radicals by homolytic bond cleavage

Oxygen molecule (O2) undergoes homolytic bond cleavage, resulting in two oxygen radicals (O•). Each oxygen radical has an unpaired electron, making it more reactive: O2 → 2 O•

First radical addition

The phenoxide ion (C6H5O-) reacts with the oxygen radical (O•) in a radical addition reaction, generating a phenoxide radical: C6H5O- + O• → C6H5O-O•-

Second radical addition

The phenoxide radical (C6H5O-O•-) reacts with another oxygen radical (O•) through a second radical addition reaction, generating a peroxide radical species (C6H5O-O-O•-): C6H5O-O•- + O• → C6H5O-O-O•-

Proton transfer

The peroxide radical species (C6H5O-O-O•-) gains a proton from another phenol molecule, forming a hydroperoxide species (C6H5O-O-OH): C6H5O-O-O•- + C6H5OH → C6H5O-O-OH + C6H5O-

Rearrangement and formation of colored products

The hydroperoxide species (C6H5O-O-OH) undergoes rearrangement and cyclization reactions, leading to the formation of colored products such as quinones or more complex, colored compounds. These products absorb light at specific wavelengths, giving the solution its pink or red color. In conclusion, the oxidation of phenol by oxygen involves multiple radical addition steps, hydrogen transfer, and rearrangement reactions. This mechanism leads to the formation of colored products, which would be responsible for the observed pink or red color in phenol samples exposed to air.

Key concepts

Phenol Oxidation

Phenol oxidation is a process where phenol, an aromatic compound with a hydroxyl group, reacts with oxygen, often leading to the colored products. The mechanism of oxidation involves the interaction between the phenolic compound and atmospheric oxygen, which can occur in various ways. For students seeking to grasp the fundamentals, consider the initial formation of a phenoxide ion when phenol loses a proton in the presence of a base. This ion is more reactive towards oxygen, kicking off the series of radical-mediated reactions.

In the context of the exercise, the pink or red color development in phenol samples over time is attributed to the formation of complex organic structures during the oxidation process. These compounds can absorb light within the visible spectrum, giving the phenol solution its distinct coloration.

Radical Addition Reaction

A radical addition reaction is a type of chemical reaction that involves radicals, which are atoms, molecules, or ions with unpaired valence electrons, making them highly reactive. In our exercise, oxygen undergoes homolytic cleavage producing two oxygen radicals. These radicals then add to the phenoxide ion in a step-wise manner. The initial radical addition creates a new radical, which subsequently reacts with another oxygen radical to form a peroxide radical species.

This chain of events is crucial because each step sets the stage for the next, propagating a series of reactions that lead to significant structural changes in the phenolic compound. Understanding these radical mechanisms is fundamental in organic chemistry, as they are common in many types of reactions, including polymerizations and combustion.

Peroxide Formation

Peroxide formation plays a pivotal role in the oxidation of phenol. Peroxides are compounds containing an oxygen-oxygen single bond, and the process of their formation can be indicative of the radical nature of a reaction. In our example, the phenoxide radical reacts with an oxygen radical to form a peroxide radical. This structure is not stable and further reacts, ultimately resulting in the production of hydroperoxide.

In laboratory settings, peroxide formation can be both a useful reaction and a safety hazard, as some peroxides are explosively unstable. For educational purposes, it's critical to understand how these compounds are formed and the implications they bring to the overall chemical reaction taking place.

Organic Chemistry Reactions

Organic chemistry reactions are a diverse set of processes that involve carbon-containing compounds. In the context of our phenol oxidation exercise, multiple reaction types are showcased, including deprotonation, radical addition, and rearrangement. These reactions transform simple organic molecules into more complex structures, often with significant changes in properties, as evidenced by the formation of colored compounds from colorless phenol.

Focusing on the oxidation process allows students to appreciate how molecular complexity can arise from simpler precursors. It also exemplifies the interplay between various reaction types in building up the intricate molecules that define organic chemistry's vast landscape.