Final answer:
To determine the number of substituents on a compound, we examine the groups that are attached to the main carbon chain or ring. Substituents are numbered starting from the end closest to them and listed alphabetically in the compound name, with prefixes like 'di-' and 'tri-' indicating multiple identical groups.
Step-by-step explanation:
To establish the number of substituents on a given compound, especially when dealing with compounds such as benzenes or alkanes, one must determine the number and positions of the groups that are not part of the main carbon chain or ring structure. These groups are known as substituents and can include alkyl groups, halogens, and other functional groups. When naming a molecule with multiple substituents, start numbering the longest carbon chain at the end closest to a substituent to get the lowest possible numbers for the locants of the substituents. Substituents are listed in alphabetical order, and prefixes such as di-, tri-, and tetra- are used to indicate the number of identical substituents.
For example, a molecule with two methyl groups on adjacent carbons would be named as a dimethyl compound with the numbers indicating the carbons they are attached to, such as 2,3-dimethylbutane. Additionally, cyclic compounds like benzene that have substituents will have the positions relative to each other indicated by numbering the ring, and the substituents then named accordingly.