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?


Unsaturated bonds take precedence over "branches". Consider these bonds as functional groups and name the compound, it will make you number carbons correctly.

Also the compound you mentioned above can't possibly exist as it has 5 bonds on $\ce{C_3}$.

  • $\begingroup$ Thanks, the above structural formula of the molecule was to be drawn on an assignment I was given. Also, is 2, 3-dimethyl 5-heptyne a possible compound, or do you see something wrong there too? $\endgroup$ – ChemStudent May 10 '15 at 2:44
  • $\begingroup$ The naming you did is wrong once again @ChemStudent it should be- 5,6-dimethyl 2-heptyne. $\endgroup$ – Rajat Jain May 10 '15 at 2:55
  • $\begingroup$ Thank you - this worksheet packet has troubles. These were the names that were on the sheet with instructions to draw the structural formulas. I appreciate your assistance. $\endgroup$ – ChemStudent May 10 '15 at 3:02

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:


(e) saturation/unsaturation:

  (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- 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).



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