Nomenclature II Aromatic Hydrocarbons II lec # 08 II Dr. Rizwana

Aromatic hydrocarbons get far more complicated in nomenclature once more than one substituent shows up on the benzene ring—and the lecture lays out the rules for naming mono-, di-, tri-, and poly-substituted benzenes, including common “special names” that override the basic prefix/suffix pattern.

For mono-substituted benzene, the naming is straightforward: treat the substituent as a prefix and attach it to “benzene.” Examples given include ethylbenzene (from an ethyl substituent), nitrobenzene (nitro substituent), bromo-benzene (bromine substituent), isopropylbenzene (isopropyl substituent), acetophenone (an acetyl/ketone-type substituent), benzoic acid (carboxylic acid substituent), benzaldehyde (aldehyde substituent), and aniline (amino substituent). The key idea is consistent: the substituent’s functional identity determines the name, either through a direct prefix approach or through a suffix/pattern tweak for functional groups like aldehydes and acids.

When two substituents appear, the lecture distinguishes three naming layers: (1) positional descriptors (ortho, meta, para) when both groups are of the same “type,” (2) alphabetic ordering when the substituents differ, and (3) special parent names when one group gives benzene a recognized common name. For positional naming, the ring is numbered so that adjacent positions become ortho, separated by one carbon become meta, and opposite positions become para. Thus, a dichlorobenzene example is named using the positions (e.g., 1,2- or 1,4- patterns) or equivalently as ortho-/meta-/para-dichlorobenzene.

For two different substituents, the lecture applies alphabetic rules: the substituent that comes first alphabetically is written first, and its position is specified using ring numbering. A worked example pairs chlorine and nitro, producing a name of the form “(position) chloro (position) nitrobenzene,” with the order determined alphabetically.

The “special names” become crucial when certain functional groups attach to benzene. Hydroxy gives phenol; methoxy gives anisole; amino gives aniline; carboxylic acid gives benzoic acid; aldehyde gives benzaldehyde; and sulfonic acid gives benzenesulfonic acid. In cases where two functional groups compete to define a parent name, the lecture emphasizes preference rules: the group with higher priority becomes the parent, while the other group is treated as a substituent with its position.

The same logic scales to tri- and higher substitution. For tri-substituted benzenes, the lecture uses carbon numbering to assign positions (e.g., 1,2,4 patterns) and then builds the name using substituent positions and the appropriate parent chain name (such as tolyl as the parent when methyl is part of the core naming). It also shows how amino/acid combinations map to names like aminobenzoic acid derivatives, and how multiple halogens plus phenol lead to names like tribromophenol.

Finally, the lecture highlights a set of widely used “recognition names” for disubstituted benzenes: o-xylene, m-xylene, and p-xylene for two methyl groups in ortho/meta/para positions; catechol for two hydroxyls in ortho positions; resorcinol for two hydroxyls in meta positions; and hydroquinone for two hydroxyls in para positions. It closes with mixed functional examples such as salicylaldehyde (ortho-hydroxy + aldehyde), o-toluidine (ortho-amino + methyl), and cresol (methyl + hydroxyl in the specified position). These names are presented as exam-critical because they often appear as direct formula-identification or functional-group-composition questions.