[CHEMISTRY 11] ARENE (AROMATIC HYDROCARBON) - Kite Books, Connecting Knowledge, Creative Horizons

Nomenclature and General Properties of Aromatic Hydrocarbons
📌 The term "mì tôm" (instant noodles with shrimp flavor) is used generically in Vietnamese, even if actual shrimp is absent, due to historical branding where early packaging featured shrimp illustrations.
📌 Aromatic hydrocarbons, or "hợp chất thơm," are defined by the presence of one or more benzene rings (C₆H₆), despite many compounds in this class lacking a pleasant aroma.
📌 Benzene (C₆H₆) is the simplest aromatic hydrocarbon, featuring a six-membered ring with three alternating double bonds (conjugated $\pi$ system) and internal bond angles of $120^\circ$.
📌 Homologues of benzene follow the general formula $\text{C}_n\text{H}_{2n-6}$ where $n \ge 6$; for $n=7$, the compound is $\text{C}_7\text{H}_8$ (Toluene), and for $n=8$, the molecular formula is $\text{C}_8\text{H}_{10}$, yielding four structural isomers with benzene rings.

Nomenclature of Benzene Derivatives
📌 Toluene ($\text{C}_6\text{H}_5\text{CH}_3$) is systematically named Methylbenzene.
📌 Di-substituted benzene derivatives are named based on the relative positions of the two substituents: ortho (o or 1,2), meta (m or 1,3), and para (p or 1,4).
📌 Xylenes are common names for dimethylbenzenes: o-Xylene (1,2-dimethylbenzene), m-Xylene (1,3-dimethylbenzene), and p-Xylene (1,4-dimethylbenzene).
📌 The IUPAC nomenclature for di-substituted benzenes requires numbering the ring carbons to give the substituent positions the lowest possible sum (e.g., 1,2-dimethylbenzene is preferred over 1,6-dimethylbenzene).

Physical and Chemical Properties
📌 Aromatic hydrocarbons are typically liquids or solids at room temperature, have a characteristic odor, are nearly insoluble or slightly soluble in water, and are less dense than water.
📌 Due to the stable, conjugated $\pi$ system in the benzene ring, aromatic compounds primarily undergo substitution reactions and are resistant to addition reactions under mild conditions.
📌 Characteristic substitution reactions include halogenation (e.g., with $\text{Br}_2$ catalyzed by $\text{FeBr}_3$) and nitration (e.g., with $\text{HNO}_3$ catalyzed by concentrated $\text{H}_2\text{SO}_4$).
📌 Substitution on alkyl-substituted aromatics (like Toluene) preferentially occurs at the ortho and para positions when using electrophilic substitution.
📌 Addition reactions (e.g., hydrogenation with $\text{H}_2$/Ni or chlorination with $\text{Cl}_2$/UV light) require harsh conditions (high temperature/pressure or UV radiation) due to the stability of the aromatic ring.

Oxidation Reactions and Industrial Relevance
📌 In oxidation reactions using $\text{KMnO}_4$, benzene does not react because it lacks an oxidizable alkyl side chain, whereas Toluene reacts, oxidizing the $\text{CH}_3$ group to a carboxyl group ($\text{COOH}$).
📌 The complete combustion of benzene yields $\text{CO}_2$ and $\text{H}_2\text{O}$, releasing significant heat.
📌 Aromatic hydrocarbons are sourced from petroleum refining (reforming process) and coal tar distillation, and they serve as important raw materials for synthesizing polymers, dyes, pharmaceuticals, and solvents.

Key Points & Insights
➡️ Aromatic compounds are named based on the stability of the benzene ring's delocalized $\pi$ system, which dictates a preference for electrophilic substitution over addition reactions.
➡️ When naming di-substituted benzenes, ensure the numbering minimizes the sum of the substituent positions (e.g., 1,3-dimethylbenzene is preferred).
➡️ Toluene's side chain ($\text{CH}_3$) is oxidized by hot, acidic $\text{KMnO}_4$ to form Benzoic Acid ($\text{C}_6\text{H}_5\text{COOH}$), which is a standard method for identifying alkyl side chains.
➡️ To slow down the drying time of paint (keeping the solvent longer), choose the aromatic solvent with the higher boiling point, as it evaporates slower (e.g., Xylene over Toluene due to higher molecular mass).

📸 Video summarized with SummaryTube.com on Feb 21, 2026, 05:54 UTC