Conditional Absolute configuration changes in SN2 rxs

[SaintJude]

In the section on inversions typical of SN2 reaction, Kaplan says:

One important thing to be aware of is that the known inversion in SN2 reactions will lead to an inversion of the absolute configuration (R changes to S, vice versa) ONLY if the leaving group and the nucleophile have the same priority.
IF the nucleophile and the leaving group have DIFFERENT priorities, even though the molecule will still flip [relative configuration will still occur] the designation will not be changed.

Does anyone have an example of this?

So if bromide "displaces" Iodide in an SN2 reactions, we all know that an inversion of relative configuration will definitely occur. But will inversion of absolute configuration occur (change from R to S) ?

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[MedPR]

In the section on inversions typical of SN2 reaction, Kaplan says:

One important thing to be aware of is that the known inversion in SN2 reactions will lead to an inversion of the absolute configuration (R changes to S, vice versa) ONLY if the leaving group and the nucleophile have the same priority.
IF the nucleophile and the leaving group have DIFFERENT priorities, even though the molecule will still flip [relative configuration will still occur] the designation will not be changed.

Does anyone have an example of this?

So if bromide "displaces" Iodide in an SN2 reactions, we all know that an inversion of relative configuration will definitely occur. But will inversion of absolute configuration occur (change from R to S) ?

It depends on the other substituents. If you have 2-iodo-2-methylbutane, for example, the absolute configuration will change since the leaving group (I-) is first priority, and when you add the Bromine (nucleophile) it will also be the first priority. Edit: Bad example, that might not even undergo SN2 since it is secondary.

(R)-FluoroIodomethanol + Br- ---------> (S)-Bromofluoromethanol

However, say you had something like (R)-1-bromo-1-fluorobutane. If you add Methoxide, you will not get absolute inversion to the (S) isomer because Bromine is 1, but Methoxy group will be 2, since the first priority would now be the Fluorine.

Thanks for making this thread. I wasn't aware that there was a shortcut for absolute inversion. I've just been checking by figuring out what the new configuration was (what a waste of time).

 

[MT Headed]

You might need to draw this one out...

(R)-bromofluoroiodomethane gets hit with Cl-. The iodide ion leaves. Result (R)-bromochlorofluoromethane.

It's a somewhat contrived example, but the general case of my example is to have your substituents ordered 1,2,3,4, attack it with a nucleophile that sits between 2 and 3, and have the high priority 1 group leave.

 

[MT Headed]

(R)-Iodomethanol

(Psst... keep trying... iodomethanol is not chiral... it has two H substituents 🙂 )

 

[MedPR]

(Psst... keep trying... iodomethanol is not chiral... it has two H substituents 🙂 )

Oh damn 😀

 

[dranet]

Hell! That is making life difficult but what Kaplan said can be true. I have never seen that in an MCAT q but it is a good curveball to think about. i found this in Sparknotes
The phrase "inversion of configuration" may lead you to believe that the absolute configuration must switch after SN2 attack. This is not always true. Recall that absolute stereochemical configuration is determined via the Cahn-Ingold-Prelog (CIP) system, and categorized with the labels "R" and "S." The absolute configuration inverts (from "R" to "S" or vice-versa) only when the nucleophile and leaving group have the same CIP priority relative to the other substituents. Since good nucleophiles and leaving gro ups tend towards equally high CIP priorities, most SN2 reactions do result in a switch of absolute configuration. If the nucleophile and leaving group have different relative CIP priorities, however, the absolute configuration does not necessarily change even though inversion occurs. Keep on your toes.
A favorite test question presents a SN2 reaction that results in 100% retention of both absolute and general configuration. In this case, it's likely that two SN2 reactions take place. Two inversions yield a net retention of configuration
" I also found this example of SN2 inversion versus retention by googling images of such at http://www.absoluteastronomy.com/topics/SNi and that its the solvent that can change the inversion to retention of abs configuaration

 

[SaintJude]

Ah yes, it makes sense now ! Brilliant, thank you! Definitely recommend drawing it like MT Headed said.