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| Phenols Overview: Phenols Phenols is treated as a separate unit by the Chemistry Professor to allow inclusion at any point in the course. The topics addressed include nomenclature, physical properties, preparations, reactions, and analysis. In order to prevent confusion for the organic student, it is necessary to separate the study of phenols from the study of alcohols. Otherwise, there is a tendency among the novices to think of phenols as alcohols. Some textbook authors choose to place a study of phenols with a study of aromatics. Placing phenols in a separate unit permits their inclusion at whatever point the particular course designer feels is appropriate. The nomenclature of phenols addresses those unique names such as the cresols, catechol and friends, salicylic acid, vanillin, etc. Accompanying nomenclature is a study of physical properties, particularly the interesting differences in solubilities, as in the catechol group. Students see the impact of physical positioning of substituted rings when faced with the question of why the solubilities of catechol, resorcinol, and hydroquinone differ so markedly. Also addressed are the factors that impact the acidity of phenols. The question of why phenols are so much more acidic than alcohols is answered by showing the equilibrium components that stabilize the phenoxide ion. Problems addressing Ka of phenols help develop an understanding of the limited acidity of these compounds. That their acidity is sufficient for them to act as a good bactericide is explained. Two preparations are treated for phenols: diazzonium salts and benzenesulfonic acid. The reasons the first is a good lab preparation and the second is a good industrial preparation are addressed. The reactions of phenols addressed include neutralization, halogenation with chlorine, halogenation with bromine, nitration, sulfonation, Friedel-Crafts alkylation, oxidation, the Fries rearrangement, the Kolbe reaction. The ability of the OH group on phenols to act as a powerful activator changes the conditions required for electrophilic aromatic substitution. This is particularly noted in the manner in which halogenation and several other of the reactions proceeds. The mechanisms for each of the reactions is discussed and explained. The carbonation via the Kolbe reaction is presented as a method of producing salicylic acid.
Analysis of phenols involves test tube tests to suggest the presence
of this somewhat acidic group: pH, no effervescence of bicarbonate
solutions, and the formation of a yellow precipitate when phenols
are treated with bromine water. The major absorption bands are suggested
for IR identification, as well as the distinctive features found
in the HNMR and the UV-VIS. In mass spec, the prominent parent peak
which may be the base peak is discussed.
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