Why is this molecule named 1,1-chloroiodoethane, rather than 1-chloro-1-iodoethane?
The correct name is 1-chloro-1-iodoethane since the locants are placed immediately in front of the prefixes that represent the substituent groups.



Furthermore, since the release of the IUPAC recommendations of 1993, generally all locants are placed immediately in front of the part of the name to which they refer (e.g. cyclohex-2-en-1-one). The corresponding rules in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) read as follows.


  P-14.3.2 Position of locants
  
  Locants (numerals and/or letters) are placed immediately before that part of the name to which they relate, except in the case of the traditional contracted names when locants are placed at the front of their names.


 


  P-15.1.7.1.4 The longest chain is numbered from one end to the other by arabic numbers, the direction being chosen so as to give the lower locants to the substituent groups (side chains) [see P-14.4(f)]. The lower set of locants is defined as the set that, when compared term by term with other locant sets cited in order of increasing magnitude, has the lower term at the first point of difference (see P-14.3.5). The locants are placed immediately in front of the part of the name to which they refer. Identical simple substituent groups are indicated by multiplicative prefixes, such as ‘di’, ‘tri’, etc. [P-16.3.3(b)]. For compound or complex substituent groups (see P-29.4 and P-29.5), the multiplicative prefixes ‘his’, ‘tris’, ‘tetrakis-‘, etc. (P-14.2.2) are used as described in P-16.3.5(a).