Question

Hexachlorophene, a substance used in the manufacture of germicidal soaps, is prepared by reaction of \(2,4,5\) -trichlorophenol with formaldehyde in the presence of concentrated sulfuric acid. Propose a mechanism for the reaction.

Short answer

Hexachlorophene forms through a mechanism involving protonation, nucleophilic attack, formation of an oxocarbenium ion, and Friedel-Crafts alkylation reactions.

Step-by-step solution

Protonation of Formaldehyde

In the presence of concentrated sulfuric acid, the carbonyl oxygen of formaldehyde is protonated. This increases the electrophilicity of the carbon atom in formaldehyde, making it more susceptible to nucleophilic attack. The structure of protonated formaldehyde can be depicted as \( \text{CH}_2\text{OH}^+ \).

Nucleophilic Attack by 2,4,5-Trichlorophenol

The hydroxyl group of 2,4,5-trichlorophenol performs a nucleophilic attack on the activated formaldehyde. This forms a new carbon-oxygen bond and leads to the creation of a hemiacetal intermediate, linking the two molecules together.

Formation of Oxocarbenium Intermediate

The hemiacetal intermediate undergoes a dehydration reaction, losing a molecule of water. This results in the formation of an oxocarbenium ion, which is a highly reactive intermediate characterized by a positive charge on the carbon atom linked to the oxygen.

Friedel-Crafts Alkylation Reaction

The oxocarbenium ion acts as an electrophile and is susceptible to attack by the aromatic ring of 2,4,5-trichlorophenol. The aromatic ring's electron-rich nature facilitates the Friedel-Crafts alkylation, leading to the attachment of the trichlorophenol moiety to formaldehyde.

Rearrangement and Completion of Product Formation

Finally, there might be rearrangement or further nucleophilic attacks leading to the formation of hexachlorophene. This compound forms when a second molecule of 2,4,5-trichlorophenol reacts in a similar mechanism, completing the bisphenolic structure of hexachlorophene.

Key concepts

Friedel-Crafts Alkylation

In organic chemistry, Friedel-Crafts alkylation is a fundamental reaction used to introduce alkyl groups into aromatic rings. This reaction involves the electrophilic attack on the aromatic ring by an alkylated cation. In our exercise, an oxocarbenium ion serves as this electrophile. The reaction is facilitated by the electron-rich nature of aromatic compounds like 2,4,5-trichlorophenol.

• Alkyl groups are added to the aromatic ring, enhancing its functionality.
• The reaction requires a catalyst, often a strong acid like sulfuric acid, to proceed.
• In our case, the catalyst helps form the reactive oxocarbenium ion.

Understanding this concept helps us see how complex aromatic systems can be constructed by selectively adding new functional groups.

Nucleophilic Attack

Nucleophilic attack is a key step in many organic reactions. This is where a nucleophile, a species rich in electrons, attacks an electrophile, a species deficient in electrons. In the reaction of 2,4,5-trichlorophenol with formaldehyde, the hydroxyl group acts as the nucleophile.

• Nucleophiles seek out positive or partially positive centers in molecules.
• This attack forms a new bond, here between the oxygen atom of the hydroxyl group and the carbon atom in formaldehyde.
• The efficient formation of new bonds is crucial for building complex molecules from simpler starting materials.

These interactions are foundational in forming various organic compounds.

Protonation

Protonation is a process where a proton (H⁺) is added to an atom or a molecule, usually increasing its reactivity. In the mechanism, formaldehyde undergoes protonation, making it more electrophilic due to the increased positive charge on the carbon.

• This step is catalyzed by strong acids, such as sulfuric acid in this case.
• Protonation transforms the normally stable formaldehyde into a reactive species capable of participating in further chemical reactions.
• Formally, the protonated formaldehyde can be represented as CH₂OH⁺.

Protonation enhances reactivity, enabling subsequent steps in the reaction mechanism, such as nucleophilic attack.

Oxocarbenium Ion

The oxocarbenium ion is a pivotal intermediate in many organic reactions. Formed from the dehydration of a hemiacetal intermediate, it features a carbon atom double-bonded to an oxygen atom and bearing a positive charge.

• This ion is highly electrophilic, making it very reactive and suitable for further chemical transformations.
• Its presence in the reaction pathway is crucial, as it facilitates the electrophilic addition to the aromatic ring.
• In this particular mechanism, the oxocarbenium ion allows for the subsequent Friedel-Crafts alkylation.

Mastering the concept of oxocarbenium ions is essential for understanding how complex organic molecules can be constructed step by step.

Electrophilic Addition

Electrophilic addition is a reaction where a chemical species with a positive charge (the electrophile) adds to a molecule, commonly an alkene or aromatic compound. It involves the addition of an electrophile to the electron-rich areas of a molecule.

• In our discussed mechanism, the oxocarbenium ion acts as the electrophile.
• It interacts with the aromatic ring system of trichlorophenol, forming a more complex structure.
• Such additions are vital in reactions where new carbon-carbon bonds are formed.

Electrophilic additions enable chemists to synthesize more complex organic molecules from simpler precursors, opening pathways to innovative compounds.