Newman Projection Priority (Ek OChem #130)

[DrknoSDN]

I am having a hard time finding justification for the answer provided by Examkrackers 1001 Ochem for #130.
The problem is with prioritizing groups on Newman Projections.

The questions shows the distal carbon having an up pointing Aldehyde (CHO), and an bottom right Alcohol (-OH).
The front carbon has the Alcohol (-OH) top left. (60 deg from CHO, and 180 deg from -OH)

I answered that it should be considered Anti-staggered because the the front alcohol is 180 degrees from the back -OH.

The answer says "Gauche" because the two largest groups are 60 degrees apart.

Is it priority based on atomic number or group Size? Online answers are giving me varied responses. Very confused, please help.

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[premed1001]

-

 

[DrknoSDN]

That was my assumption for the priority, and what I found online. But examkrackers did not post anything about errors for that problem on their site so I assumed I was mistaken.

For clarification, An Alcohol gets a higher priority than an Aldehyde when both groups are attached to the same Carbon...?
Therefore Alcohols that are 180 degrees apart are anti-staggered regardless of the aldehyde location.

ty for reply.

 

[premed1001]

actually now i am confused, i thought it was based on atomic number of the alpha atom but apparently not

based on their logic they are using the total atomic weights of the attached molecules, not functional group dependent

so
CHO weighs more than OH weighs more than H


here is the problem for anyone wondering

http://gyazo.com/77916287289534a3503f37edc9ad4a57

 

[DrknoSDN]

ty for replying premed1001... If you land on a definitive answer let me know.

I don't see how EK could want you to use the group weight over atomic number, because things like attached Esters (-COOCH3) would have very high priority, unless that's how it works.

 

[premed1001]

ty for replying premed1001... If you land on a definitive answer let me know.

I don't see how EK could want you to use the group weight over atomic number, because things like attached Esters (-COOCH3) would have very high priority, unless that's how it works.

i think it has to do with the electron clouds of the bonds repelling against the ones on the adjacent molecule across the newman projetion

 

[DrknoSDN]

That would cause increased steric hindrance. But the questions is saying when an (-OH) group is 60 degrees from an aldehyde on an adjacent carbon, and 180 degrees from the adjacent (-OH) group, the book is saying that is a gauche conformation.

Ignoring any steric factors it should still be called "anti" based on atomic number priority.

 

[premed1001]

That would cause increased steric hindrance. But the questions is saying when an (-OH) group is 60 degrees from an aldehyde on an adjacent carbon, and 180 degrees from the adjacent (-OH) group, the book is saying that is a gauche conformation.

Ignoring any steric factors it should still be called "anti" based on atomic number priority.

on one molecule OH is the largest group right?
but on the other molecule even though there is an OH, there is a "Heavier" molecule with more electron clouds and IT (CHO) is the one they are using to deterine configuration not the OH
since the OH on one groups is 60 degrees to the larger CHO group on the other molecule it is considered gauche

 

[premed1001]

im suprised at how lacking information is available to the process of defining priorities. this is the best i could find where they say "often the largest group"

http://gyazo.com/ba59fa56eefc72c20a6ea5e6ba31123e


once again i am surprised as i always thought it was the largest atom, and only if the atoms are identicle you would look at the next attached atom

 

[DrknoSDN]

Going to go with UCLA and almost every other college website out there... http://www.chem.ucla.edu/harding/tutorials/stereochem/rsez.pdf
-OH Gets priority over Aldehyde and even -COOH groups, and it's completely based on atomic number (weight), in the event of a tie you go to the next attached atom.

Problem 137 also asks:
"In the assignment of absolute configuration, what priority does the CHO group have in the structure shown below"
The groups attached to the chiral carbon are:
-Br
-OH
-CHO
-CH3

Their answer is (2),
[edit]
This is the correct answer if you assume the highest priority is 4. So (2) would place it below -Br and below -OH.

 

[Czarcasm]

You base it off the closest group interactions. Had the carbonyl group of the aldehyde been replaced with a hydrogen, then it would of been valid to say that both -OH groups were anti to one another. But in this case, you have two groups bumping into each other that are much closer, so this takes predominance. That's why the answer was gauche.

You didn't have to consider priorities to answer this question. However, just for the sake of knowing, if a question asks you to select the most stable conformation (newman projection), that's when priorities should be considered. Before considering any rules, just ask yourself, what will most stabilize the structure. A lot of people who jump right into priorities can bump into some mistakes. An example of this is when you're considering two -OH groups that are gauche vs. anti. Some might assume wrongly that having those groups anti to one another is more stable. But in this case, both OH groups are capable of hydrogen bonding, and this significantly increases the stability of the structure. Now on the other hand, if they instead replaced one of the -OH groups with a methyl, there are no stabilizing factors at play here. Instead, the most stable structure is one that reduces both torsional and steric strain. In this situation, anti would be the best choice.

 

[Czarcasm]

Going to go with UCLA and almost every other college website out there... http://www.chem.ucla.edu/harding/tutorials/stereochem/rsez.pdf
-OH Gets priority over Aldehyde and even -COOH groups, and it's completely based on atomic number (weight), in the event of a tie you go to the next attached atom.

EK has a ton of questions wrong in regards to Newman Projections. Beware.

Another exemplified error in problem 137 asking:

"In the assignment of absolute configuration, what priority does the CHO group have in the structure shown below"
The groups attached to the chiral carbon are:
-Br
-OH
-CHO
-CH3

Their answer is (2), when clearly the priority is in the order it's listed above...
If you learned this from EK, unlearn it and go elsewhere.

These rules are only considered when you have to choose between two groups, one of which MUST be in a gauche position relative to one another, while the other more distant to avoid sterics. For example, let's say one carbon has 2 H's attached and 1 Methyl group -- and the carbon it's bonded to has 1 Methyl Group, 1 T-Butyl group, and 1 H.

You would likely be given a number of choices to choose from, with either staggered or eclipsed interactions and they'd likely ask you to choose the most stable structure. In this scenario, you want the methyl group as far apart to either group, so staggered is more favorable to eclipsed (less stable). But regardless of which staggered position you choose, there will be gauche interactions no matter what. You have to consider which gauche interaction is least sterically hindered: methyl or t-butyl. In this case, methyl, being less branched would be the better choice.

In this scenario it was pretty straight forward but sometimes it can be difficult to compare two groups. For example, a halide vs. a methyl group. I honestly can't remember which of these two are given priority (they would likely tell you, or provide a table), but what I do know is that the priority would go to the one with the smaller electron cloud to avoid both torsional and steric strain. Now which of the two has a smaller electron cloud ...who really knows, but this is essentially why they give you priority rules.

 

[DrknoSDN]

Ok, so I can accept that Gauche interactions occur when one functional group is 60 degrees from any functional group on the other carbon.
Thats like saying if one of the carbons has 2 functional groups anti conformation is not possible and priorities do not matter.

[edit]
Clarified. Ty

 

[DrknoSDN]

Actually your explanation clears that up Czarcasm. Thank You.

Apparently the highest priority is (4),,, and not (1).

So their answer of giving it a priority of (2) means that it's behind 4 (Br) and 3 (-OH) ... instead of being 2nd highest in priority.

 

[Czarcasm]

Ok, so I can accept that Gauche interactions occur when one functional group is 60 degrees from any functional group on the other carbon.
Thats like saying if one of the carbons has 2 functional groups anti conformation is not possible.

However, I don't see how the second example can have them give a higher priority to an aldehyde than to an alcohol on a single chiral carbon fischer projection.

Yep, I just glanced at that question and it's definitely wrong. -OH has 2nd priority, not the carbonyl group. I agree, there is a bunch of errata in EK 1001's. I would recommend you visit their website. I don't know if it's still there but there use to be a published list of errata corrections.

 

[Czarcasm]

Actually your explanation clears that up Czarcasm. Thank You.

Apparently the highest priority is (4),,, and not (1).

So their answer of giving it a priority of (2) means that it's behind 4 (Br) and 3 (-OH) ... instead of being 2nd highest in priority.

Sorry, I think you're confusing two different concepts.

Question 130 is regarding Newman Projections. The priority rules considered here are in regards to electron cloud size and is based on reducing both torsional and steric constraints. Generally, you only consider these rules when you're trying to determine which substituent to put in the gauche position, given a choice. Typically, you won't need to refer to them since most newman projections are relatively straight forward to answer.

Question 137 is asking you about absolute configuration. This has to do with Cahn-Ingold-Prelog Rules and has nothing to do with sterics but is just an arbitrary set of rules (based on atomic number) and is defined to either determine whether a chiral center is in the R or S configuration.

 

[premed1001]

Sorry, I think you're confusing two different concepts.

Question 130 is regarding Newman Projections. The priority rules considered here are in regards to electron cloud size and is based on reducing both torsional and steric constraints. Generally, you only consider these rules when you're trying to determine which substituent to put in the gauche position, given a choice. Typically, you won't need to refer to them since most newman projections are relatively straight forward to answer.

Question 137 is asking you about absolute configuration. This has to do with Cahn-Ingold-Prelog Rules and has nothing to do with sterics but is just an arbitrary set of rules (based on atomic number) and is defined to either determine whether a chiral center is in the R or S configuration.

so in R S priority it is purely based on atomic number of the alpha atoms?

and in newman projections priority is based on the molecular weights of the substituents?

is this correct?

 

[DrknoSDN]

Im going with the understanding that R/S priority is based on atomic number only.

Newman projection "assignment" of anti-staggered, gauche, eclipsed, fully-eclipsed is completely based on how "close" one functional group is to another and almost ignores priority rules.

Oddly enough online almost all newman projections have single R groups, and 2 hydrogens on each of the 2 chiral carbons. Probably to avoid the multi-substituted priority confusion entirely.

 

[Czarcasm]

so in R S priority it is purely based on atomic number of the alpha atoms?

and in newman projections priority is based on the molecular weights of the substituents?

is this correct?

R and S is determined by atomic number, but there's more to it than that (for example when directly bonded to the same atom). The rules go into more detail as to what they do when there's a tie, but essentially, you keep comparing atomic numbers until there's a point of difference.

Newman Projections are just representations of structure and is used to represent c-c bonds due to their free rotation. You can have all sorts of configurations for a given molecule and in fact at any given instant, there are staggered or eclipsed configurations. If a question is specifically asking you to choose an answer choice based on the most stable newman configuration, then you have to understand two things: that eclipse are less stable than staggered conformations -- and that for staggered conformations, you want bulky groups to be as far appart from each other as possible. But as I mentioned earlier, this isn't always possible, particularly when you have more than 1 bulky group attached to a carbon and in this scenario, you don't compare the molecular weights but instead the relative electron cloud of each substituent. It's really tough to establish this comparison so generally, they'll show you a table and basically tell you for example: bromine's electron cloud is smaller than a methyl group, even though bromine has a larger atomic number.

Sorry guys, I'm not always the most articulate or best at explanations but I hope this makes sense. You might find it more helpful to refer to your review book for further insight.

 

[DrknoSDN]

Yep, I just glanced at that question and it's definitely wrong. -OH has 2nd priority, not the carbonyl group. I agree, there is a bunch of errata in EK 1001's. I would recommend you visit their website. I don't know if it's still there but there use to be a published list of errata corrections.

Hilariously I now believe that EK 137 is correct... I looked around online and found that the most common notation for priority assignment is to assign the highest priority with the number 4 instead of 1... This would mean that when they said it has a priority of (2), thats the 2nd lowest priority... not the 2nd highest.

The ucla link that i posted earlier also does it this way. They assign a priority 1 to hydrogen and go up from there.

 

[Czarcasm]

Hilariously I now believe that EK 137 is correct... I looked around online and found that the most common notation for priority assignment is to assign the highest priority with the number 4 instead of 1... This would mean that when they said it has a priority of (2), thats the 2nd lowest priority... not the 2nd highest.

*sigh* That's completely wrong. The only instance I can think of where things go outta whack is when the lowest priority group points in the plane of the page. (Remember for R/S configuration, you have to draw an arrow between the 1-2-3 priority groups, with the 4th priority pointing away). Some people have trouble picturing this, but there are tricks to help with this, and one of these is taking the lowest priority group (in the plane of the page) and swaping it's number with the priority pointing away from you. Find the configuration. Then realizing that swapping these two results in an entirely different configuration (anytime you exchange two groups), you take the opposite of what you got as the configuration of the represented structure. So if after swapping you determined it was S configuration, the real structure is instead R.

Okay, I'm gonna stop here because I suck at explanations, but I would highly recommend you review this in your review book. I think pictures help a ton!

 

[DrknoSDN]

Pictures do help: http://www.chem.ucla.edu/harding/tutorials/stereochem/rsez.pdf
page three shows priorities.
(4) -OH, given highest priority
(3) -COH
(2) -CH3
(1) -H, lowest priority...

Basically the lowest priority gets the lowest number of one. Highest priority gets highest number of four. That makes the solutions provided correct.

 

[Czarcasm]

Pictures do help: http://www.chem.ucla.edu/harding/tutorials/stereochem/rsez.pdf
page three shows priorities.
(4) -OH, given highest priority
(3) -COH
(2) -CH3
(1) -H, lowest priority...

Basically the lowest priority gets the lowest number of one. Highest priority gets highest number of four. That makes the solutions provided correct.

Okay, I suppose either convention can work -- but just about everywhere I've seen this presented, including multiple organic books, the norm is to give the highest priority #1 and the lowest priority #4. If you're gonna stick to numbering things that way though, the important thing is to stay consistent with it, otherwise it'll result in a bunch of mistakes.