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:

enter image description here

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?

  • $\begingroup$ @orthocresol (1Z)‐1‐bromo‐2‐iodoethene is different from (1Z)‐1‐iodo‐2‐bromoethene and not just a rearrangement of prefixes since 1-bromo prefix is used in the first name and 2-bromo prefix is used in the second one. $\endgroup$
    – Prajval K
    Aug 7, 2021 at 11:26
  • $\begingroup$ Related: Order of citation of alkyl and halide prefixes in IUPAC names $\endgroup$
    – Loong
    Aug 7, 2021 at 12:09

1 Answer 1


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:


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-14.5.1 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

enter image description here

  • $\begingroup$ For a PIN you need 1Z not just Z $\endgroup$ Aug 7, 2021 at 12:47
  • $\begingroup$ Is it necessary. since the there can only be one location for the double bond to exist? $\endgroup$ Aug 7, 2021 at 13:14
  • $\begingroup$ @SafdarFaisal Check P-14.3.3 of IUPAC Blue Book 2013, for a name to be a PIN, dropping of a locant like '1' is generally permissible if without ambiguity, it is not allowed in PIN. $\endgroup$
    – Prajval K
    Aug 7, 2021 at 13:24

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