# Deciding higher priority order between a triple bond and tertiary carbon

I want to know which of the groups will be given higher priority $$\ce{-C#CH}$$ or $$\ce{-C(CH3)3}$$ and why. I tried resolving the triple bond to single bonds such that they are phantom atoms. But that gave me a structure in which there are no atoms after the first point of difference. How is this (see image) wrong and if this is correct then isn't it a contradiction? After first point of difference, there are two difference one is nothing v/s $$\ce H$$ and other is $$\ce C$$ vs $$\ce H$$ and both situation seem to contradict in giving their respective group higher priority order.

• I'll point out that there are different schemes for naming compounds. So the answer probably depends on exactly which one that you are using... – MaxW Jul 17 '18 at 15:27
• @marxw for E and Z or R and S we need to first give them priority order. And i want to know which priority is correct and why. – Jasmine Jul 17 '18 at 16:12
• At 2nd point of difference, the phantom carbon is attached to the original carbon and hence its a C-C v/s C-H and its obvious that the former will be given higher priority. Its not right to say that the phantom C is not linked to any atom if its really linked to the original C – Jasmine Jul 18 '18 at 18:27

## 1 Answer

From the Wikipedia article on CIP rules.

If an atom A is double-bonded to an atom B, A is treated as being singly bonded to two atoms: B and a "ghost atom" that is a duplicate of B (has the same atomic number) but is not attached to anything except A.

Ghost atoms are attached to resolve the triple/double bond.

So according to CIP rules the two compounds can be resolved as
The pink atoms are ghost carbons

From here we can see that ethyne has a greater preference over isobutyl as after the first point of difference we move over to the next highest order carbon.In ethyne the second carbon is connected to $$\ce{C2H}$$ whereas the next highest order carbon in isobutyl is connected to $$\ce{H3}$$.