Phenols
Phenols are a class of organic compounds characterized by a hydroxyl group (-OH) directly bonded to an aromatic hydrocarbon ring. This structural feature distinguishes phenols from alcohols, where the hydroxyl group is attached to a saturated carbon atom. The aromatic ring's influence on the hydroxyl group and vice versa gives phenols distinct chemical and physical properties.
Phenols are weakly acidic, more so than aliphatic alcohols due to the stabilization of the phenoxide anion (formed upon deprotonation) by resonance within the aromatic ring. This resonance delocalization distributes the negative charge, making the anion more stable and thus favoring the deprotonation.
Properties:
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Acidity: As mentioned, phenols exhibit weak acidity, reacting with strong bases to form salts called phenoxides or phenolates. The pKa values of phenols typically range from 9 to 11, making them stronger acids than alcohols but weaker than carboxylic acids.
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Reactivity: The aromatic ring in phenols is highly reactive towards electrophilic aromatic substitution reactions. The hydroxyl group is an activating and ortho-/para- directing group. This means electrophiles will preferentially attack the aromatic ring at the positions ortho and para to the hydroxyl group.
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Hydrogen Bonding: Phenols can form hydrogen bonds with other molecules, including water. This hydrogen bonding influences their solubility and boiling points. Smaller phenols are more soluble in water than larger ones due to the dominance of hydrogen bonding over hydrophobic effects.
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Oxidation: Phenols are easily oxidized, often resulting in colored products. This oxidation can be exploited in various applications, such as in antioxidants.
Occurrence and Uses:
Phenols are found in various natural sources, including plants, where they contribute to the flavors, colors, and antioxidant properties. Synthetic phenols are widely used in the production of polymers (such as phenolic resins), disinfectants, antiseptics, pharmaceuticals (e.g., aspirin), dyes, herbicides, and fungicides. Examples of common phenols include phenol itself (carbolic acid), cresols (methylphenols), and catechol (1,2-dihydroxybenzene).