When naming branched alkenes/alkynes, the numbers of the carbons where the branches and multiple bonds are located are supposed to be the lowest possible. Do the branches take precedence over the multiple bonds, as for example, 2-methyl-3-pentyne? If the counting of the carbons for this molecule is flipped so that the triple bond is lower in number, it would be 3-methyl-2-pentyne. Is the first way correct?
With regard to numbering of locants within the same compound class, multiple bonds have seniority over simple substituent groups (e.g. methyl, ethyl, etc.) that are used for naming of branched alkanes.
On this matter, the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:
When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:
(i) low locants are given to hydro/dehydro prefixes (…) and ‘ene’ and ‘yne’ endings);
(ii) low locants are given first to multiple bonds as a set and then to double bonds (…);
(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;
Note that Rule e takes precedence over Rule f.
P-188.8.131.52 The presence of one or more double or triple bonds in an otherwise saturated parent hydride (…) is denoted by changing the ending ‘ane’ of the name of a saturated parent hydride to ‘ene’ or ‘yne’. Locants as low as possible are given to multiple bonds as a set, even though this may at times give ‘yne’ endings lower locants than ‘ene’ endings. If a choice remains, preference for low locants is given to the double bonds. In names, the ending ‘ene’ always precedes ‘yne’, with elision of the final letter ‘e’ in ‘ene’. Only the lower locant for a multiple bond is cited, except when the numerical difference between the two locants is greater than one, in which case the higher locant is enclosed in parentheses.
For example, a correct systematic name is 3-methylbut-1-ene (not 2-methylbut-3-ene).