Chemistry · General Principles Of Organic Chemistry · NEET
Just write the substituent name in front of the word benzene, joined as one word. For example, a -Br group gives bromobenzene, a -NO2 group gives nitrobenzene, and a -Cl group gives chlorobenzene. You do NOT need a number because with one group every ring position is the same by symmetry. Many of these also have common names used everywhere (for example C6H5-CH3 is methylbenzene but is almost always called toluene).
These are trivial-name prefixes used only when a benzene ring has exactly two substituents. Ortho (o-) means the two groups are on next-door carbons, positions 1,2. Meta (m-) means one carbon gap, positions 1,3. Para (p-) means opposite carbons, positions 1,4. So 1,3-dibromobenzene is the same as m-dibromobenzene. Remember they run 1,2 then 1,3 then 1,4 as you go o-, m-, p-.
Use ortho/meta/para ONLY for a disubstituted (two-group) ring. The moment the ring carries three or more substituents, these prefixes cannot be used and you must number the ring carbons and use locant numbers like 1,2,4-. So a two-group ring can go either way, but a three-group ring must use numbers.
Number the six ring carbons in whichever direction gives the substituents the smallest set of numbers. NCERT compares two dibromobenzenes: the correct name is 1,3-dibromobenzene (not 1,5-dibromobenzene), because 1,3 is lower than 1,5 for the same molecule. When more than one group is present, the substituents are then written in the name in alphabetical order.
Some benzene derivatives have a common name that is treated as the parent (base) compound, such as aniline (aminobenzene), phenol (hydroxybenzene), toluene (methylbenzene) and anisole (methoxybenzene). When you name a further-substituted version, numbering starts from the ring carbon carrying that base group (it becomes C-1), and you number in the direction that gives the next substituent the lowest number. Example: p-nitroaniline has the -NH2 as base (C-1) and -NO2 at C-4.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Ortho (o-) is just the trivial-name way of saying the 1,2 arrangement. Both names point to the exact same molecule with the two chlorine atoms on adjacent ring carbons.
No. Ortho/meta/para work only for two substituents. For three or more groups you must number the ring and use locants (like 1,2,4-trimethylbenzene), following the lowest locant rule and writing substituents in alphabetical order.
Numbering is chosen to give the lowest set of locants, and the substituents are listed alphabetically in the name (ethyl, fluoro, nitro). The number in front of each group tells its ring position, so fluoro sits on C-1, nitro on C-2, and ethyl on C-4, giving the lowest possible locant set.
Anisole is the common name for methoxybenzene (a -OCH3 group on benzene). When it is used as the base compound, the ring carbon bearing -OCH3 becomes C-1. So o-ethylanisole is the ortho (1,2) compound with an ethyl group next to the methoxy group.
Because a single naming slip (mixing meta and para, or missing the lowest locant rule) turns a giveaway mark into a wrong answer. Reactions of aromatic compounds in later chapters also assume you can read o-, m-, p- correctly, so getting the naming right early protects marks across organic chemistry.
