Kevin B. answered • 09/30/22
Enthusiastic Orgo Professor Can Help You Unlock Organic Chemistry
This is, uh, an interesting question.... First, I recommend heading to a standard organic chemistry textbook of your choice, and finding the section on naming of alkenes and alkynes (they are usually dealt with together), as this answer is geared to this specific question and not general naming. It's not that helpful to ask on this forum "How do I name organic compounds" (your other question) -- that would be a topic for an in-person tutoring session, more than something that can be answered in this format.
But this specific question can be answered! IUPAC names are always a series of little "bits" which specify different aspects of the structure. In this case, there are two numbers (or "locants") which specify the location on the parent chain of two different structural features, and three "word"s (for lack of a better term) which each indicate an aspect of the structure. The "methyl" means a one-carbon substituent (or -CH3 -- the three H's on the carbon are part of what "methyl" means). The "hex" means the parent carbon chain is six carbons long. And the suffix "-yne" means that there is a carbon-carbon triple bond. (The "-ene" suffix means a C=C double bond, and "-ane" means there are no double or triple bonds in the parent carbon chain.)
The number in front of each piece indicates its location on the parent 6-carbon chain. So "4-methyl" means the -CH3 substituent is on the 4th carbon of the chain. And the "3-yne" means the triple bond starts at the third carbon in the chain. But here's where you run into trouble! If the triple bond starts at C3, that means it is between the 3rd and 4th carbons. (If it were between the 2nd and 3rd carbons, it would have started at C2, so it would have been "2-yne".) But that's the problem! If you draw a 6-carbon chain with a triple bond between C3 and C4, you can't then put a methyl group on C4! The 4th carbon already has all 4 of its possible bonds "used up" in making the triple bond between C3 and C4, and then the single bond from C4 to C5 (to continue the six-carbon chain). As such, this molecule CANNOT EXIST. Try drawing the structure and you'll see -- in the structure described by the name above, the 4th carbon would violate the octet rule.
Again, without context, this is a weird question. I have on occasion asked questions like this, but always as one of several nomenclature questions with some sort of caveat like, "If the molecule cannot be drawn as described by the given name, briefly explain why." Then, most of the options are just fine, and a structure can be drawn for them, but maybe one is a "trick" like this, and the student has to be on their toes to make sure they realize the name cannot actually correspond to a structure.
