The first important simplified criteria for the choice of a principal chain in such compounds are:
- greater number of suffixes
- longest chain
- greater number of multiple bonds
- lower locants for suffixes
- lower locants for multiple bonds
The corresponding wording of the nomenclature rules (taken from Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book)) reads as follows:
P-44.1 SENIORITY ORDER FOR PARENT STRUCTURES
When there is a choice, the senior parent structure is chosen by applying the following criteria, in order, until a decision is reached. These criteria must always be applied before those applicable to rings and ring systems (see P-44.2) and to chains (see P-44.3). Then criteria applicable to both chains and rings or ring systems given in P-44.4 are considered.
P-44.1.1 The senior parent structure has the maximum number of substituents corresponding to the principal characteristic group (suffix) or senior parent hydride in accord with the seniority of classes (P-41) and the seniority of suffixes (P-43).
P-44.3.2 The principal chain has the greater number of skeletal atoms [criterion (b) in P-44.3].
P-44.4.1 If the criteria of P-44.1 through P-44.3, where applicable, do not effect a choice of a senior parent structure, the following criteria are applied successively until there are no alternatives remaining. These criteria are illustrated in P-188.8.131.52 through P-184.108.40.206.
The senior ring, ring system, or principal chain:
(a) has the greater number of multiple bonds (P-220.127.116.11);
(b) has the greater number of double bonds (P-18.104.22.168);
(h) has the lower locant for an attached group expressed as a suffix (P-22.214.171.124);
(j) has the lower locant(s) for endings or prefixes that express changes in the level of hydrogenation, i.e., for ‘ene’ and ‘yne’ endings and ‘hydro/dehydro’ prefixes (P-126.96.36.199);
Since the compound that is given in the question does not have any principal characteristic group (a characteristic group chosen for citation at the end of a name by means of a suffix or a class name, or implied by a trivial name), the first criterion (greater number of suffixes) given in P-44.1.1 is not relevant in this case.
The second criterion given in P-44.3.2 is the longest chain. However, both possible names, (3Z)-3-ethyl-2-methylhex-3-ene and (3Z)-3-(propan-2-yl)hex-3-ene, indicate a principal chain with six skeletal atoms.
Also, both possible principal chains include the double bond. Therefore, the criteria given in P-44.4.1 (a) (greater number of multiple bonds) and (b) (greater number of double bonds) cannot be used to reach a decision.
The criterion given in P-44.4.1 (h) (lower locant for the suffix) is not relevant since the compound that is given in the question does not have any suffix.
Finally, the criterion given in P-44.4.1 (j) stipulates that the principal chain shall have the lower locant for the double bond, i.e., for the ‘ene’ ending. However, both names, (3Z)-3-ethyl-2-methylhex-3-ene and (3Z)-3-(propan-2-yl)hex-3-ene, have the locant 3 for the ‘ene’ ending.
Further criteria can be found in Section P-45:
P-45 SELECTION OF PREFERRED IUPAC NAMES
Two or more names may result based on the same senior parent structure selected according to P-44, because of different substitution patterns or multiple occurrences of the same senior parent structure. (…) Application of
the criteria in this section will generate the preferred IUPAC name.
P-45.2.1 The preferred IUPAC name is based on the senior parent structure that has the maximum number of substituents cited as prefixes (other than ‘hydro/dehydro’) to the parent structure.
In accordance with P-45.2.1, the preferred principal chain corresponds to (3Z)-3-ethyl-2-methylhex-3-ene, which has a principal chain with two substituents (3-ethyl, and 2-methyl), whereas the principal chain of (3Z)-3-(propan-2-yl)hex-3-ene has only one substituent (3-(propan-2-yl)).