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I have been trying to learn IUPAC nomenclature during the past few days.
I have been using the MarvinSketch software to play around with random structures and its generated names and comparing them to my own results. I stumbled across the following compound:
The nomenclature for the compound is shown as (1Z)‐1‐bromo‐2‐iodoethene
Is there any reason why (1Z)‐1‐iodo‐2‐bromoethene might be wrong? Is there a priority series among the halogens?
You have figured out the Z part of the nomenclature correctly. Now as to the question of why 1-bromo-2-iodo... and not 1-iodo-2-bromo..., we look at section P-14.4 (g) of the Nomenclature of Organic Chemistry. IUPAC Recommendations and Preferred Names 2013, which states the following:
P-14.4 NUMBERING
When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:
[...]
(g) locants for the substituent cited first as a prefix in the name;
According to this rule, the priority in numbering depends upon where each prefix is present within the name. You cannot have 2-bromo-1-iodo... or 2-iodo-1-bromo... as a possible name when the substituents are simple substituents.
Now, how do we decide which comes first? Is it the bromine or iodine group? For this we look into section P-14.5 which deals with alphanumeric order and states:
P-14.5 ALPHANUMERIC ORDER
[...]
P-14.5.1 Simple prefixes (i.e., those describing atoms and unsubstituted substituents) are arranged alphabetically; multiplicative prefixes, if necessary, are then inserted and do not alter the alphabetical order already established.
Therefore, since bromo- comes first alphabetically, we place it first before iodine and by P-14.4 (g), the bromine gets priority.
Therefore, the PIN of the given compound becomes (Z)-1-bromo-2-iodoethene
