Hydroxyl Group Detection (Phenolic $$-\text{OH}$$)

Formula / Core Reaction:

$$3\text{Ar-OH} + \text{FeCl}_3 \rightarrow \text{Fe}(\text{OAr})_3 + 3\text{HCl}$$

$$\text{Phenol} + \text{Phthalic Anhydride} \xrightarrow[\Delta]{\text{conc. } \text{H}_2\text{SO}_4} \text{Phenolphthalein} \xrightarrow{\text{NaOH}} \text{Pink Color}$$

Concept: Phenols react with neutral ferric chloride ($$\text{FeCl}_3$$) to form deeply colored coordination complexes (violet, blue, or green). In the phthalein dye test, condensation with phthalic anhydride produces phenolphthalein, which turns pink in alkaline media.

Notation / Key Observation:

• $$\text{Fe}(\text{OAr})_3$$: Iron-aryloxy complex (Violet/Blue/Green coloration);
• Phenolphthalein anion: Pink color in dilute $$\text{NaOH}$$ ($$\text{pH} > 8.3$$).