Why do we insert a numeric multiplier after group positions when naming an isomer?
take 2,3-dimethylbutane


In the case of different substituents on different or the same atom, we use a combination of the position and the name of the substituent, such as in


2-chloro-3-methoxy...
3-cyano-4-bromo..., etc.


Following this rule, 1,2-dimethoxyethane would be 1-methoxy-2-methoxyethane. Using the multiplier di, together with the positions, just shortens and simplifies the name and still is unambiguous.

The multiplier di explicitly denotes that there are two identical substituents at the given positions and not just one that bridges two centres.



An example for such a situation would be 1,4-methanonaphthalene (1). Note that even in this context, di can be used, as in 1,4:5,8-dimethanonaphthalene (2).