Chemistry - Why do we insert a numeric multiplier after group positions when naming an isomer?

Solution 1:

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.

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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).

Solution 2:

Take 2,3 dimethylbutane for example. Why does one add the numerical multiplier di when one has already explicitly mentioned two locations? Why doesn't one just use 2,3 methylbutane?

Actually, it is the other way round. Multiplicative prefixes (di, tri, tetra, etc.) are always used in names to denote multiplicity of identical features in structures, whereas locants (1, 2, 3, etc.) may be omitted under certain conditions when there is no ambiguity, e.g.
‘trichloromethane’ (instead of ‘1,1,1-trichloromethane’) or
‘heptafluorobutanoic acid’ (instead of ‘2,2,3,3,4,4,4-heptafluorobutanoic acid’).