[dun13203171]

Hi everyone, I missed a lecture at the start of the year on assigning r and s sterochemistry and I am a bit confused as to this example.

The imageis not very clear, that is a wedged line in the left example and a dashed on the right example.

I was given the answer of r for the the one on the left and S for the one on the right.

Could someone please go through the process with me for assigning the particular configuration in these examples.

thanks.

Also any tips or tricks to help get better at assigning these as we have been told tricky ones will be on the exam. Anyone know of any interactive learning tooks, websites, etc?

thanks

[Babcock_Hall]

Welcome to the forum.  It is a forum rule that you must show your attempt before we can help you.  Perhaps you could start by identifying any stereogenic carbon atoms.  The next step would be to assign priorities around this center.

[earthnation112]

I was stuck also, the following website helped alot:

http://chemwiki.ucdavis.edu/Core/Organic_Chemistry/Chirality/Absolute_Configuration,_R-S_Sequence_Rules

Hope it benefits you also.

[Dan]

dun13203171, on a side note, the descriptors r and s (used to define the configuration of pseudoasymmetric centres) do not mean the same thing as the descriptors R and S (used to define the configuration of asymmetric centres).

[dun13203171]

dun13203171, on a side note, the descriptors r and s (used to define the configuration of pseudoasymmetric centres) do not mean the same thing as the descriptors R and S (used to define the configuration of asymmetric centres).

Hi dan,
           Thanks for your reply, is there anyway you could explain that with examples? In other words I do not understand.

[dun13203171]

Welcome to the forum.  It is a forum rule that you must show your attempt before we can help you.  Perhaps you could start by identifying any stereogenic carbon atoms.  The next step would be to assign priorities around this center.

I will use the left as an example. I have identified the chiral carbon as one on the cyclic ring where the wedged line is attached. The hydrogen attached to this, I give the lowest priority #4. There is a carbonyl group attached the chiral carbon, with another two carbons attached to the chiral carbon simply bonded to 2 hydrogens each, thus I give this carbonyl carbon the highest priority #1 as it is bonded to an oxygen (higher atomic number than hydrogen. to decide between the other two carbons, I work my way down the chain each side and I find a double bond, which counts as a carbon bonded to two carbons and thus I give it priority #2  and the next carbon I give #3 priority.

Since there is a wedged line present, i've determined the hydrogen, the lowest priority group, to be point behind the plane of the page, i then ignore it, and look at the direction from the highest priority group (lowest number) to the #3 group, it goes in a clockwise direction and thus is determined as R?


I do find it difficult to determine r and s, when a Hydrogen isn't the lowest priority and the carbon skeleton is laid out in such a way that makes it very difficult to place the lowest priority group to the back of the page if you know what I mean, is there any tricks to do this?

thanks

[Dan]

There is a carbonyl group attached the chiral carbon

You posted the structures of limonene - no carbonyls present

I do find it difficult to determine r and s

Here is a link for pseudoasymmetry (I will try to find a more detailed one): http://goldbook.iupac.org/P04921.html

The descriptors r/s are not appropriate for limonene (there are no pseudoasymmetric centres), the correct descriptors are R/S.

[dun13203171]

Hi dan,
          a mistake on my part in regards to the carbonyls.

Guys could you give me an indication if I am using a correct method.



This is an example from my notes.

It gives H as lowest priority.

"When we look at the atoms attached to the Carbons we can see
   1. 3*Hydrogen atoms (methyl chain)
        2. 2*hydrogen atoms and 1 oxygen atom (OH chain)
   3. 3*oxygen atoms" (COOH chain)

so it gives the acid group priority, followed by the alcohol and then the methyl.

Now I thought you looked at what was directly attached to each carbon.

I agree with the methyl carbon - attached to HHH
and the OH carbon                  attched to  HHO
but the next carbon, has 2 hydrogens and another carbon attached, but in the notes they have quoted the atoms attached to the carbonyl carbon. Is this correct, as if I had of done it my way, I would have given priority to the alcohol, then acid chain, then methyl...I am very confused now.

[Dan]

I agree with the methyl carbon - attached to HHH
and the OH carbon                  attched to  HHO
but the next carbon, has 2 hydrogens and another carbon attached, but in the notes they have quoted the atoms attached to the carbonyl carbon. Is this correct, as if I had of done it my way, I would have given priority to the alcohol, then acid chain, then methyl...I am very confused now.

You are correct, the notes are wrong.

Priority 4: H
Priority 3: CH₂ H
Priority 2: CH₂ C HHC   [CH₂CH₂CO₂H]
Priority 1: CH₂ C HHO   [CH₂CH₂OH]