Which of the following reagents cannot be used to distinguish between benzyl alcohol and phenol? (a) \(\mathrm{FeCl}_{3}\) (b) \(\mathrm{NaOH}\) (aqueous) (c) \(\mathrm{Br}_{2} / \mathrm{CCl}_{4}\) (d) \(\mathrm{NaHCO}_{3}\) solution

Organic chemistry often requires us to identify and differentiate between compounds. One specific test, the FeCl3 test for phenols, is a classic and straightforward way to distinguish phenols from other types of organic compounds. When phenols are treated with ferric chloride (FeCl3) in a test, a significant color change due to complex formation is observed.

For instance, phenol itself reacts to form a ferric phenoxide complex that exhibits a violet or purple coloration. This color change is a strong indication of the phenolic group in the compound. Other alcohols, like benzyl alcohol, do not undergo this reaction, allowing us to distinguish them from phenols easily. This test is particularly useful because of its simplicity and the immediate visual feedback it provides.

Another distinguishing reaction involves the reaction of phenols with sodium hydroxide (NaOH). When aqueous NaOH is added to phenol, sodium phenoxide is produced, which is highly soluble in water, leading to a clear solution.

This occurs because the phenolic -OH group has acidic properties, allowing it to react with bases like NaOH. On the other hand, benzyl alcohol does not exhibit this behavior due to its non-acidic nature, resulting in no reaction with NaOH. This difference provides a convenient method to differentiate phenols from alcohols that do not contain an acidic -OH group.

The bromination of phenol is a reaction that demonstrates the chemical reactivity and the distinct behavior of phenols under electrophilic aromatic substitution conditions. When bromine (Br2) dissolved in a non-polar solvent like carbon tetrachloride (CCl4) is introduced to phenol, a rapid reaction occurs without the need for a catalyst.

The result is 2,4,6-tribromophenol, showing that the phenolic OH group activates the benzene ring towards electrophilic substitution. Alcohols like benzyl alcohol, lacking such activating groups, do not react under the same conditions, hence bromine in CCl4 serves as an excellent reagent to discriminate between phenol and other types of alcohols.

The NaHCO3 test is another analytical tool used to discern between carboxylic acids, phenols, and less acidic compounds. Sodium bicarbonate (NaHCO3), a weak base, will react with carboxylic acids and sufficiently acidic phenols to form their corresponding sodium salts and release carbon dioxide gas.

This effervescence of CO2 can easily be observed and serves as an indicator of the presence of an acidic proton within the compound. For example, phenol’s acidity allows it to react with NaHCO3 to form sodium phenoxide and generate bubbling from CO2 evolution. On the flip side, benzyl alcohol, with a much less acidic hydrogen, does not react, making the NaHCO3 test another convenient method for distinguishing phenolic compounds from less acidic alcohols.