[pfnm]

Hi,

I was asked to name the compound in this image:



I came up with (2S,3R) 1-cyclopropyl-3-methyl-pentan-2-ol....however the right answer according to the lecturer is (2R,3R).

I'm wondering why the cyclopropyl substituent receives a lower priority according to the naming rules than the substituent on the other side which has the same number of carbons?

Is it because, starting at the methyl, we could trace a butyl chain to the left hand side, if that makes sense?



Thank you

[Smrt guy]

Priorities are based on atomic number.  When two or more atoms attached are the same, you have to move to the next group out.  So, in this case, you have O, H, and 2 C's.  Obviously O gets priority 1 and H 4.  For the two C's you look at what is attached to them.  In one case you have CHH (this is for the methylene attached to the cyclopropyl).  In the other you have CCH.  The first C is a draw so the next atom counts; in one case you have a C, in the other an H.  The C wins and the acyclic chain gets the 2nd priority.  In the case of double and triple bonds you count the number of bonds to the atoms (e.g. -CH=CH2 counts as CCH).

[pfnm]

Thank you very much, very clearly explained.

[pfnm]

In this instance (after SN1 reaction of (2R,3R)-1-cyclopropyl-3-methyl-pentan-2-ol, one of the products is the following:



(the other I guess would be the S isomer to form the optically impure racemic mixture).

My question is, why is this new product named as a cyclopropyl substituent? Don't we name based on the longest chain?

I would have called it (3R)-1-cyclopropyl-3-chloro-3-methyl-pentane. Is my answer incorrect?

[Smrt guy]

As for the second product I don't think they are referring to the other enantiomer.  Think about the initially formed product in an E1 mechanism.  The major product should be the one drawn, but there is a minor product as well.  As for the nomenclature, I cannot remember the relative priority of cyclic rings and straight chains.  That information should be in any undergraduate text (I lost mine years ago).