Man-made or natural phenols are planar organic compounds based on one or more six-carbon aromatic (benzene) rings to which one or more hydroxy groups (-O—H) are bonded (Figure 1).
The H atom of the —O—H group can dissociate from the O atom when the phenol molecule is dissolved in water, so phenols are weakly acidic. The term phenol now is used for any of a group of related acidic compounds that are hydroxyl derivatives of aromatic hydrocarbons. Included in this definition are a huge number of compounds of commercial importance such as cresols, catechols, quinols, xylenols, guaiacol, and resor-cinol. These groups can be identified as follows:
• cresol — a phenol with one methyl group in the ortho-(o-), meta- (m-), or para- (p-) position;
• guaiacol — a phenol with one methoxy group (-O— CH3) in the o-position;
• xylenol — phenol with two methyl groups (e.g., 2,4-dimethyl phenol);
• catechol — a phenol with a second hydroxy group in the o-position;
• resorcinol — a phenol with a second hydroxy group in the m-position;
• quinol (or hydroquinone) — a phenol with a second hydroxy group in the p-position;
• naphthol — two joined aromatic rings, with a hydroxy group on at least one ring.
Functional groups, such as hydroxy, methyl (—CH3), nitro
(—NO2), chloro (—Cl), fluoro (—F), or bromo (—Br) can substitute for one or more hydrogen atoms on a basic phenolic structure. A vast range of more complex substi-tuents is also possible. These can include (but are not limited to) alkylated (H-saturated) carbon chains, branched carbon-based structures, or other simple or complex ring-based structures, with or without their own substituents. Two aromatic six-carbon rings, each with or without substituents, also can bond covalently to each other, creating various biphenols. Three-ring structures, such as flavones, are common naturally occurring phenolic compounds.
Substituted simple phenols are named according to the type of substituting groups and their positions relative to the initial hydroxy group. In this nomenclature, positions 2, 3, and 4 (the carbon to which the initial -OH is bonded is counted as the first position) are sometimes referred to as the ortho- (o-), meta- (m-), and para- (p-) positions, respectively. Substitutions can include sulfur (S) for the O of the defining -OH group, leading to thiophenols, which also can support substituents. This permits (for example) compounds such as 4-bromothiophenol, which contains an S substitution for O in the phenol-defining -OH group, and a bromine substitution at the 4 (or p-) position.
Phenolics are sparingly to moderately soluble in water, but the acidity and aqueous solubility of the more highly substituted one-ring phenols are strongly affected by the types and positions of the substituent groups. The effects of substituents on acidity and solubility are especially large if the substituents are electron-withdrawing groups located at the o- or p-positions. The pKa for phenol, for example, is 10.0, whereas the pKa for p-nitrophenol is 7.2. Additional nitro groups radically lower the pKa relative to the nonsubstituted parent compound (Table 1).
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