Naming an Alkyne

[Dianna Grigorian 1C]

In the textbook on page 19, an example is given of naming an alkyne with the formula (CH3)2CHC --- CH. How do we know that the longest carbon-carbon chain has four bonds and should be butyne rather than a chain of 5 carbons?

[Jamie_Lin_3I]

Hello!
I would suggest drawing the structure out with a chain of 4 carbons and a chain of 5 carbons. You will notice that for the chain of 5 carbons, the structure doesn't quite work out. There are 2 CH3. If you use the 5 carbon chain, you can only place the 2 groups of CH3 at the ends of the structure. If this is the case, you cannot place the triple bond anywhere because the rest of the carbons have 2 bonds already.
If you try with the 4 carbon chain however, you could use one CH3 group as a substituent and you can place the triple bond on the end.