Kevin B. answered • 04/30/23
Enthusiastic Orgo Professor Can Help You Unlock Organic Chemistry
Sure! Before reading ahead, I suggest that you make your best effort to construct a name yourself, by going to the section in your textbook on alkyne/alkene nomenclature, reading it, checking out some practice problems for examples, and then giving it a stab yourself. Getting an answer online is ALWAYS worse that figuring out how to generate one yourself....
Nomenclature is about a lot of things, but two big themes are functional group priority and number of carbon atoms in the chain of your molecule. Typically, you first need to identify the longest carbon chain which contains the highest priority functional group in the molecule as the first order of business. In this molecule, it is cyclic, and all the carbons (11 of them) are included in the same ring. (If some carbons were branches off the ring, the naming would be different). There are three FG's in the molecule -- an alkyne (triple bond), an alkene (double bond) and an alkyl halide (fluoride). All three are "included" in the longest chain (the 11-carbon ring) so there's no funny business in choosing the longest chain. As for "priority", you definitely need to go to a reference to go deeper into relative priority, as that is a longer discussion than can be had here. But the highest priority functional group here is the alkene (double bond) -- the alkyne is lower priority, and the fluoride (or any alkyl halide) has no "priority" at all, when considering priority.
Constructing the name of the molecule is basically like constructing a single (very long) word which specifies all the pieces of the molecule, and indicates their relative positioning in the molecule with numbers ("locants"). In this case, there are 5 "pieces" of this molecule you need to communicate about -- # of carbons in the ring, the two FG's (alkene and alkyne), the fluorine, and the fact that it is cyclic. Each of those facts will be communicated by a portion of the name.
The "parent" chain of the molecule is kinda the "root" of the word -- the center, around which the other pieces are built. The parent chain is indicated by the number of carbons in the longest chain -- 11 here. Also in this case, the fact that it is cyclic is also part of the "parent" or "root". Eleven carbons is indicated by the root "undec", where "dec" would be 10 carbons, "non" would be 9, "oct" would be 8, with "hept", "hex", and "pent" making sense (4, 3, 2, and 1 carbons are unique -- go look those roots up). In order to indicate the ring, it is given the prefix "cyclo". So "cycloundec" means a 11-carbon ring.
Once the parent/root is determined, the primary functional group is given a suffix -- there are lots of different suffixes which indicate different functional groups (and you will need to learn them all), but here the suffixes you need to know are "ene" for alkene (double bond), and "yne" for alkyne (triple bond). There are also rules about when you can -- and when you cannot -- "combine" suffixes, but for "ene" and "yne" you can combine them both at the end of the root. So this is now "cycloundecenyne" (-ene comes before -yne, don't ask me why, and you drop the trailing "e" from -en, so -enyne indicates BOTH a double bond AND a triple bond in the parent chain).
Since the fluorine does NOT have any "priority" it gets named as a "substituent" of the parent, with a PREfix, rather than a suffix. Any portion of the molecule which can't be included in the parent/root and suffix will be named with a prefix, as a substituent. (And here, that's only the -F). The name for a fluorine substituent is "fluoro", so the molecule is now "fluoroundecenyne". (Told you the word would be long -- and it's going to get longer).
So far so good! But we have not specified the relative location of any of these "pieces" of the molecule! The double/triple/fluoro could be anywhere in the 11-carbon ring -- we haven't specified (yet). The name of the molecule must FULLY describe the structure, so we need to give a "locant" which tells "where" each piece exists.
That involves numbering the carbons in the chain! Because this is a cyclic molecule, the numbering is both simpler and more complex than a linear chain. In a linear chain, you MUST start counting from the END of the chain (and you pick the end closest to the highest priority FG). In a cyclic molecule, there IS no "end" of the chain.... So you can pick any carbon to start your counting! But which carbon should you pick? There are a couple of guidelines in that choice which you need to follow (not all of which are mentioned here, since some don't apply to this molecule).
First, the choice is made to give the highest "priority" FG the lowest number. In this molecule, that is the alkene, so I have to start counting with the double bond. But! You could start counting with either end of the C=C! Another guideline -- when you start counting a double (or triple) bond, the other half of the multiple bond MUST be the next carbon in the chain. In the molecule above, you CANNOT start counting at the top-right carbon of the double bond, and then count counterclockwise towards the -F to give the -F the number 3. That does not follow this rule -- if you start with the top-right double bonded carbon, you would need to count clockwise, since the "other half" of the double bond MUST be C2. In order to count in the direction of the fluorine, you would need to start with the bottom-left C of the double bond, so the second carbon of the double bond is C2, and F would be on C4. That same rule applies to triple bonds.
Lastly, as a general rule, when counting, you want to provide the lowest possible numbers at the first point of difference (though functional group priority takes precedence over this rule). In this case, there are two ways to number the carbons, starting with the alkene. Starting at the bottom-left double-bonded carbon and counting counterclockwise, the F is on C4, and the alkyne starts on C6 (and is thus assumed to continue to C7, which it does). The other possible way is to start counting at the top-right double bonded C, and count clockwise -- this would give the alkyne starting at C7 (continuing to C8), and the fluorine at C10.
Interestingly, it turns out that the position of the -F actually doesn't matter in this case, because the alkyne (triple bond) is a FG which has some "priority" in naming/numbering, while the -F has no priority. The first case is the correct answer, but only because the alkyne has a lower number going counterclockwise. If the triple bond were moved one atom to the left, it would start at C6 going clockwise, but C7 going counterclockwise. In that case, you'd need to number clockwise, in which the F would be given the locant 10.
So now the molecule is "4-fluorocycloundec-1-en-6-yne". Actually, the "-1-" is redundant -- in the cyclic molecule, the highest priority FG (in this case, the alkene) is always C1, so numbering the highest priority FG is unnecessary. You could actually write "4-fluorocycloundecen-6-yne" and unambiguously describe the molecule.
But there is one bit left, which is stereochemistry! Here, I am NOT going to go into how to do R/S nomenclature, but the stereochemistry at the -F is "S", and the double bond is "trans". (Head to your o-chem textbook for a description of how to determine those labels). So the FINAL, COMPLETE name for this molecule (the stereochemical labels are put in front) is: "trans-(S)-4-fluorocycloundecen-6-yne". (An alternative way to designate the stereochemistry of the double bond is with "E".) All the pieces of the molecule are now fully described, with their relative positioning (both regiochemistry and stereochemistry).
