What is the configuration (R or S) of these molecules?

[sd34]

TL;DR

Hi. Can someone tell me which molecule is R or S?

For me, the first one is S and the second one is R.

 

[mjc123]

I think the first is R and the second is S.

Can you explain how you arrived at your answers?

 

[sd34]

I think the first is R and the second is S.

Can you explain how you arrived at your answers?

Yes, it’s correct.

Basically the mistake I made was that I gave the first priority to COOH instead of NH2 in the first molecule, whereas in the second one I forgot to switch from R to S.

 

[sd34]

Okay, I have another doubt.

Why the carbon on the right is S and not R?

Aren’t the priorities like this?

 

[mjc123]

You've forgotten the other C atom on the C 2 atoms from the stereocentre on the chain you labelled 2.

 

[sd34]

You've forgotten the other C atom on the C 2 atoms from the stereocentre on the chain you labelled 2.

Okay, but isn’t the priority still the same?

 

[mjc123]

No, that C trumps the H on C2 on the other chain.

 

[sd34]

No, that C trumps the H on C2 on the other chain.

If it’s not too much of a trouble, could you redraw the molecule?

 

[mjc123]

 

[sd34]

View attachment 245084

Thanks a lot! So basically I switched no.1 and no.2 in terms of priority.