[Doodsrsly]

I'm currently in my first semester of organic chemistry, and I've been studying prior to the beginning of the semester. I thought I had a pretty thorough understanding of stereochemistry. But the other day, my instructor was telling us that Alkenes always get priority over alkanes. I thought that sounded right.. But the method I learned was to see what the carbons are attached to. What if I had a substituent that had three t-butyl groups attached to it's central carbon? C(C(CH3)3)3, sort of thing.. The priority on that would be listed as C,C,C on that central atom, while the alkene would simply be C,C,H. This is assuming we jump directly onto the alkene and this other substituent right off the parent chain. I brought this to my instructors attention and he simply told me that alkenes always get priority over alkanes. Can someone please tell me why this is?

[Dan]

The priority on that would be listed as C,C,C on that central atom, while the alkene would simply be C,C,H. This is assuming we jump directly onto the alkene and this other substituent right off the parent chain. I brought this to my instructors attention and he simply told me that alkenes always get priority over alkanes. Can someone please tell me why this is?

Your instructor is either wrong or misunderstood your question, you are absolutely right. There are occasions when alkanes will take priority over alkenes.

Here's an example - the structure below has been named by the ChemDraw software, which is strictly adherent to the CIP rules. The stereogenic centre is defined as R, which shows that the ^tBu substituent has taken priority over the vinyl substituent, as you would logically expect.