Question

If a benzene ring contains several substituents, how are the relative locations of the substituents numbered in the systematic name given to the molecule?

Short answer

In the IUPAC numbering system for benzene rings with multiple substituents, start numbering from the carbon atom with the highest priority substituent or the one closest to the second substituent. Ensure the sum of the numbers assigned to the substituents is as low as possible. For simple disubstituted benzenes, use ortho- (o-, 1,2-disubstitution), meta- (m-, 1,3-disubstitution), and para- (p-, 1,4-disubstitution) prefixes to indicate their relative positions. Apply these rules to obtain the correct systematic name for any benzene molecule with multiple substituents.

Step-by-step solution

Understand the properties of benzene ring

A benzene ring is a hexagonal ring with alternating single and double bonds, with each carbon atom forming a bond with a hydrogen atom as well. It represents an aromatic compound and can have one or more substituents replacing one or more hydrogen atoms.

Identify the parent compound

Determine the parent compound by looking for the largest ring or chain with the highest number of substituents. When multiple substituents are present, the parent chain will be identified based on the substituent with the highest priority. The priority of the functional groups follows the IUPAC guidelines, with higher priority groups designated as the parent compound.

Assigning numbers to the substituents

Start numbering from the carbon atom with the highest priority substituent. If there is a tie in priority, start numbering from the one closest to the second substituent. When multiple substituents are present, number the carbon atoms in the benzene ring in such a way that the sum of the numbers is as low as possible.

Use ortho-, meta-, and para- for simple disubstituted benzenes

For simple disubstituted benzenes with two substituents, the prefixes ortho- (o-), meta- (m-), and para- (p-) can be used to indicate the positions relative to each other. Ortho- represents 1,2-disubstitution, meta- represents 1,3-disubstitution, and para- represents 1,4-disubstitution. Example: 1. If a benzene ring contains a nitro group and a methyl group, the nitro group has higher priority, so the numbering starts from the carbon attached to the nitro group. 2. Then, number the carbon atoms in the benzene ring clockwise or counterclockwise, ensuring that the methyl group gets the lowest possible number. 3. In this case, the systematic name would be 2-nitro-1-methylbenzene or 2-methyl-1-nitrobenzene, depending on the actual structure. Remember these rules and apply them on any benzene molecule with multiple substituents to obtain the correct IUPAC name.