Phenol is the starting material for the synthesis of \(2,3,4,5,6\)-pentachlorophenol, known alternatively as pentachlorophenol or more simply as penta. At one time, penta was widely used as a wood preservative for decks, siding, and outdoor wood furniture. Draw the structural formula for pentachlorophenol, and describe its synthesis from phenol.

The chemical synthesis of pentachlorophenol begins with phenol, a simple aromatic compound with a hydroxyl group (-OH) attached to a benzene ring. In the context of its transformation to pentachlorophenol, the hydroxyl group on phenol acts as an activator for the aromatic ring, making it more reactive towards electrophilic substitution reactions such as chlorination.

The synthesis process proceeds through three key steps:

1. Activation of phenol by treatment with a Lewis acid catalyst like aluminum chloride (AlCl₃) or ferric chloride (FeCl₃). This step increases the susceptibility of the ring towards electrophilic attack.
2. Chlorination, where chlorine (Cl₂) gas is introduced. The activated ring undergoes a series of electrophilic aromatic substitution reactions, where chlorine atoms replace hydrogen atoms at the ortho and para positions relative to the hydroxyl group (positions 2, 3, 4, 5, and 6 on the ring).
3. Hydrolysis of the chlorinated complex, which serves to regenerate the hydroxyl group and yield pentachlorophenol.

It's important to control the reaction conditions carefully to prevent over-chlorination or side reactions. The final product, pentachlorophenol, has been used as a potent biocide, particularly as a wood preservative, although its usage is now restricted due to environmental concerns.

Phenol is an aromatic compound with the molecular formula C₆H₅OH, consisting of a benzene ring bonded to a hydroxyl group. The presence of the electron-donating hydroxyl group increases the electron density on the benzene ring, particularly at the ortho and para positions, which influences its reactivity.

The increased electron density makes these positions more susceptible to electrophilic substitution reactions—a category of reactions where an electrophile replaces an atom (typically hydrogen) in an aromatic system. This reactivity is pivotal in the synthesis of more complex molecules like pentachlorophenol. Additionally, the acidity of the hydroxyl hydrogen is higher compared to aliphatic alcohols, making phenol an aromatic alcohol with unique properties.

Through the interaction of the lone pair of electrons on the oxygen atom with the pi-electron system of the benzene ring, the hydroxyl group influences the entire structure of phenol, setting the stage for its chemical transformations.

Chlorination reactions are characterized by the substitution of hydrogen atoms with chlorine atoms. In the context of transforming phenol into pentachlorophenol, a specific chlorination mechanism called electrophilic aromatic substitution (EAS) takes place.

The mechanism involves the following steps:

1. Formation of a reactive chlorine species: Chlorine gas, in the presence of a catalyst, forms a chloronium ion or complexes with the catalyst, increasing its reactivity toward the aromatic ring.
2. Formation of an arenium ion intermediate: The reactive chlorine species attacks the aromatic ring of the activated phenol, temporarily disrupting its pi-electron cloud and forming a resonance-stabilized intermediate.
3. Restoration of aromaticity: A base (often the chloride anion formed in the initial step) removes a proton from the arenium ion, restoring the aromatic system and yielding the chlorinated product.

Each chlorination step increases the electron-withdrawing character of the ring, making subsequent chlorinations less reactive. As a result, careful control of reaction conditions is crucial to achieving the desired level of chlorination. In the case of pentachlorophenol synthesis, five such substitution events are orchestrated to obtain the target molecule.