Absolute (R/S) Configuration question

[studentdoctor08]

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I'm a little confused as to how to determine absolute configuration in Fischer projections. For example, in the figure attached, why is C-3 on D-ribose R? I understand that since the H is sticking out, that we must invert the original configuration. I know that OH has the highest priority because of the O, but how do you know which atom is the next highest priority since they are both C? I know you are supposed to go in order of the molecules until you determine a "tie-breaker" but in this case how do you know what order the subsequent atoms are?

http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/Images/pentoses.gif

Thanks for the help.

 

[487806]

I'm a little confused as to how to determine absolute configuration in Fischer projections. For example, in the figure attached, why is C-3 on D-ribose R? I understand that since the H is sticking out, that we must invert the original configuration. I know that OH has the highest priority because of the O, but how do you know which atom is the next highest priority since they are both C? I know you are supposed to go in order of the molecules until you determine a "tie-breaker" but in this case how do you know what order the subsequent atoms are?

http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/Images/pentoses.gif

Thanks for the help.

You're right. You have to find the next carbon until they break the tie-breaker. At C-3, OH has the highest priority (1). Now you have two carbons bonded to another carbon (C-2 and C-4). C-2 is bonded to C (C-1), which is double bonded to O (so 2 C-O bonds). C-4 is bonded to C (C-5), which is bonded to one O. But two O's have a higher priority than one O. So C-2 has a higher priority than C-4. Numbering the priorities would give a counterclockwise rotation, but because H is located horizontally (and outward), the configuration is R.

Does it make sense?

 

[RSConfiguration]

I'm a little confused as to how to determine absolute configuration in Fischer projections. For example, in the figure attached, why is C-3 on D-ribose R? I understand that since the H is sticking out, that we must invert the original configuration. I know that OH has the highest priority because of the O, but how do you know which atom is the next highest priority since they are both C? I know you are supposed to go in order of the molecules until you determine a "tie-breaker" but in this case how do you know what order the subsequent atoms are?

http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/Images/pentoses.gif

Thanks for the help.

Using the following method will HELP you Tremendously. It has several examples, including, Fischer Projections. The website may be viewed better with either Firefox or Internet Explorer, yet it does work in both browsers. I need to fix it, so it will work in multiple browsers. In any case, at the bottom of the homepage are a list of molecular representation example hyperlinks, to look at those examples, just click on the hyperlink. All of the examples use the McLaurin Method. 🙂

The question you posed to the forum is quite simple using the McLaurin Method. Just click on the hyperlink that demonstrates how to determine the configuration in Fischer Projections. 🙂

The website is FREE to everyone and is on the Internet 365 days a year, decade after decade!

The examples A, C, D, E and F (of Fischer Projections), all have similar configurations to that of the 1st example you provided, that is, D-Ribose with an R configuration.

Please pass this method and website on to your fellow students, in need of help with configurations/RS Configuration/Absolute Configuration/Stereochemistry. It does NOT use substitution or inversion. It is quite straight forward. You can use either your right hand or your left hand, yet you do not use both hands. I used my right hand for all the examples on the website, yet, you can use your left hand and it will work just as good. The McLaurin Method makes it as simple, as fast and less confusing than any method ever developed, either for students and/or professors. Think of the method like using a model set, that is, the ball and stick model set. I hope you like it and find it Very useful !

The web addresses are:

http://www.AbsoluteConfiguration.com
and
http://www.RSconfiguration.com

Great Question and Good Luck with your future exams! 🙂

Tim

PS: Using your example of of D-Ribose with an R configuration at C3:

C1 has 2 bonds to separate O (oxygen) elements and 1 bond to a H (hydrogen) element, whereas C5 has 1 bond to an O (oxygen) element and 2 bonds to separate H (hydrogen) elements, therefore C1 will be assigned the priority #2 and C5 will be assigned the priority #3, therefore the R configuration.

 

[gettheleadout]

For Fischer projections this is simple: if the hydroxyl is on the right, the carbon is R, if the hydroxyl is on the left, the carbon is S.

 

[RSConfiguration]

For Fischer projections this is simple: if the hydroxyl is on the right, the carbon is R, if the hydroxyl is on the left, the carbon is S.

I am sure 'gettheleadout' was referring to the four (4) structures that are mentioned at the beginning of this thread, that is, of those 4 aldopentoses. If that is the case, 'gettheleadout' would be correct, YET, to state "For Fischer projections this is simple . . .", reads as though, for any and all Fischer projections, of which, 'getheleadout' would be wrong. Specificity is extremely important when posting, in order to help students.

If the Fischer Projections, differed in the functional groups and/or elements bonded (attached) to C1 and C5, the priority of C1 and C5 would have to be determined, for each structure (molecule), please understand that students look to this forum for help, of which, at times, requires specificity.

Therefore, before making such observations about what the configuration is, according to the location of the hydroxyl group(s), that is, the -OH group is on the left or the right side of the Fischer Projection (at the C2, C3 and C4 carbons), it would have a specific configuration of S and R, respectively, please make 'note' to the student reading this thread, which Fischer Projections, one is referring too.

It would have been better if 'gettheleadout' added the specificity that is needed in his/her response.

For example, 'For the Fischer Projections of the four (4) aldopentoses, in question, it can be observed that when the hydroxyl (-OH) groups on C2, C3 and C4, are on the left, than the configuration is S. And when the hydroxyl (-OH) groups are located on the right at C2, C3 and C4, than the configuration is R.

Without such specificity, a student could misinterpret 'gettheleadout' response, to be true for any/all Fischer Projections when hyroxyl (-OH) groups are bonded to Stereogenic Carbons, located between the terminal carbons of Fischer Projections, of which, 'gettheleadout' response, would be False (wrong).

Have a Great Week 🙂

Tim 🙂

PS: I think, 'studentdoctor08' was looking for an answer to how to prioritize between C1 and C5, when the configuration at C3 (the middle carbon element of the of the Fischer Projection of D-Ribose R), is in question. I am sure every major Organic Chemistry textbook has that information, including books authored by Solomons & Fryhle, Wade, Carey, McMurry and so-on.

There are no real 'Tricks of the Trade' regarding assigning priority to the 4 elements/groups bonded to the Stereogenic Carbon. An example of a 'Trick of the Trade', is that of 'Opposite Configuration', as opposed to that, of a method that mimics (depicts) the model set, as much as humanly possible.

 

[gettheleadout]

Sorry, I should have been more specific.

For Fischer projections of typical monosaccharides, a OH on the right means the carbon has R configuration. If, in a hexose sugar, for example, the C6 hydroxyl was replaced by a different functional group, priority between it and the aldehyde or ketone group would have to be determined first before assigning configuration to the internal chiral carbons.