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Need help with assigning priority


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#1 Really Lost

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Posted 1 April 2012 - 06:26 AM

I have a compound that looks like this

COOH
{

H [[[[[[[[[[[[[[C1[[[[[[[[[[[[[ NH2
}
NH2[[[[[[[[[[[C2[[[[[[[[[[[[[ COOH
[
H

I know that NH2>COOH so NH2 is 1st priority. But when I am assigning second priority to I give it the COOH at the top or do I look below and use the NH2 attached to the second stereocenter C2?

I am unsure with a group such as NH2 which normally has higher priority than COOH will still still have higher priority when it is attached to another carbon and not attached to the stereocenter.

Any ideas?
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#2 hypervalent_iodine

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Posted 1 April 2012 - 08:22 AM

I'm having trouble making sense of your structure. Could you possibly attach an actual drawing with conventional notation instead of brackets?
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#3 Really Lost

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Posted 1 April 2012 - 09:07 AM

Oh the brackets arent actually brackets I just needed to use something to draw lines. Look at it as if it were a fischer projection. Unfortuanetly when I posted the space got all messed up. picture it as one horizontal line with to parallel horizontal lines.The C1 and the C2 represent the two carbon stereocenters at the crossing points

Edited by Really Lost, 1 April 2012 - 09:09 AM.

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#4 hypervalent_iodine

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Posted 1 April 2012 - 10:10 AM

I figured they weren't brackets, but you didn't go to any effort to explain what they represented. It's much easier to just draw something on paper or in ChemDraw - heck, even in paint.

Anyway, the next priority would go to the COOH. When assigning priority, you have to take it one atom at a time. The carbon on the carboxylic acid moiety is sp2 hybridized, which means it has a higher priority than the stereogenic carbon, which is sp3. If you were to replace the COOH with, say, an isopropyl group or an isobutyl group (see below), then you would look at the next atom along the chain. In this case, the stereogenic carbon would have a higher priority since it is bound to a nitrogen and the other carbon is bound only to more carbons.

Posted Image
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#5 Really Lost

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Posted 1 April 2012 - 05:04 PM

Thanks for your response.

Perhaps I do not understand proper Priority Rules that well

As I understand it goes like this.

1. We first look at the atomic number of the attached group. but we tend not to include the mass of the carbons in terms of priorty. CH3 and CH2CH3 for example tend to get the lowest priority except for -H

2. The more carbons between Stereocenter and the substiuent generally mean it gets a lower priority. However if they are of equal distance from the stereocenter, the substituant with the double bond (sp2) will get priority as is the case with the fischer projection you showed.

The thing I didn't understand was that when you look at the first sterocenter. the COOH and the NH2 attached to the second stereocenter are the same number of carbons away from the stereocenter. Is it only because of the sp2 on the carboxyl that makes it a higher priority over an NH2 when it is attached another carbon? When an NH2 is attached directly to the stereo center it is considered a higher priority than COOH?

If we replaced the NH2 attached to the second stereocenter with Iodine, would the COOH above the first stereocenter still have a higher priority?

I have attached Pic of the fischer projection of this compound

Attached Thumbnails

  • Fischer projection.jpg

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#6 hypervalent_iodine

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Posted 1 April 2012 - 05:39 PM

When you assign priority, you take it one atom at a time; you don't consider what exists outside of that unless you have two of the same atoms with identical hybridisation.

On your first stereocentre we have four atoms to consider (image on the far left of the picture below): a nitrogen, a hydrogen, one sp3 carbon and one sp2 carbon. The rest is ignored for now.


Posted Image

The nitrogen and the hydrogen are easy to order, but the carbons are (at first glance) a little trickier. To distinguish which one has higher priority you need to look at the hybridisation. Since the carbon on the carbonyl group is sp2, it has a higher priority than the second carbon, which is sp3. The fact that that sp3 carbon also has a nitrogen attached to it is irrelevant, since we don't need to look that far to be able to order them.

However, if we take the other example I drew (see the two images on the right), which has an isopropyl group attached to it, the story becomes a little different. Now attached to the first stereocentre we have one hydrogen, one nitrogen and two sp3 carbons (middle structure). Again, the nitrogen and hydrogen are simple to order. The carbons, however, are identical, so in order to distinguish between the two groups and assign priority, we need to look at the next atoms along (far right structure). The isopropyl carbon attached to the first stereocentre is bonded to two sp3 carbons and a hydrogen, whereas the other carbon (the second stereocentre) is bound to a nitrogen, a hydrogen and an sp2 carbon centre. The isopropyl group is therefore a lower priority than the second stereocentre.

I hope I'm making sense. It's rather late here and my track record for making posts / doing anything at this time isn't crash hot.
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#7 Really Lost

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Posted 1 April 2012 - 06:01 PM

Thanks for your reply I think I understand it better. In the drawing I provided the COOH would still be higher priority than the attached to the second stereocenter Iodine because the Sp2 carbon still takes precedence.

You look one carbon at a time after the (Oxygens, Bromines, Fluorines etc) until you find priority.


I have one final question

If we have two OH that are of equal distance (in carbons) from the stereocenter. See drawing, how do we prioritizes the two? Is the CH2CH2OH counted as being the same priority as the CH2OH which is another Carbon away from the stereocenter. I have not drawn the other substituents.

Attached Thumbnails

  • Fischer Projection 2.jpg

Edited by Really Lost, 1 April 2012 - 06:30 PM.

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#8 hypervalent_iodine

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Posted 2 April 2012 - 03:49 AM

Well, that's not stereogenic, so you don't have to prioritise it :P

Also, here's a better (read: more correct) explanation for your problem:

If there is a tie, we must consider the atoms at distance 2 from the stereocenter—as a list is made for each group of the atoms bonded to the one directly attached to the stereocenter. Each list is arranged in order of decreasing atomic number. Then the lists are compared atom by atom; at the earliest difference, the group containing the atom of higher atomic number receives higher priority.


(From wikipedia)


Oxygen has a higher priority than nitrogen. When we look distance 2 from the stereocentre of your original drawing and we list the atoms joined to each carbon, they differ in their highest priority and the COOH gourp is a higher priority than your second stereocentre.

The structure with the iodine in it (I didn't really look at that last night) goes the other way. Iodine is higher priority than oxygen, so when we look distance two from stereocentre 1, the COOH is now a lower priority than the second stereocentre.
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