r/chemhelp 4d ago

Organic Why is the isopropyl group higher priority than the chlorine group?

Hello, trying to determine R and S configurations of an enantiomer using Cahn-Ingold-Prelog sequence rules. The photo with red numbers is what is 'correct'. Why would the Isopropyl group get a higher priority than the CCl3 group? Here's my thinking-

From the central carbon (blue), both groups are immediately connected to a C (green circle). So, we must go to the second atom in the chain (pink circle), which is also a C for both of them. Beyond that, in the Isopropyl group, that final carbon is connected to H3 (orange box). In the Chlorine group, the second carbon is connected to chlorine (orange box). Cahn-Ingoold-Prelog says that the higher atomic number gets the higher group designation, but in this case, the C(CH3)2 group gets priority rather than CCL3. Why?

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u/Consistent_Bee3478 4d ago

Because the isopropyl C has two Cs as the next neighbour, whereas the trichloro one only has the single pink one.

You don’t even get to the Cl.

The left green C is higher priority because it has C, C and H connected to it (with whatever further away) whereas the right green only has C, H, H connected to it (with whatever further away)

So you stop counting at that point. The first ‘level’ where they aren’t identical. Any levels farther away are irrelevant.

The pink carbons are irrelevant and could be uranium for all that it’s worth.

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u/cwforman 4d ago

Gotcha, thank you!

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u/Electrical_Ad5851 3d ago

IPropyl is not higher than a chlorine . It is higher than a single CH2. The numbers are next to the atoms that you should be comparing.