Why is this S configuration ?

[Kobebucsfan]

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COOH
I
I
I
H3C-------C---------F
I
I
I
NH2


What is the configuration - i know its S but why ?

 

[Kobebucsfan]

COOH
I
I
I
H3C-------C---------F
I
I
I
NH2


What is the configuration - i know its S but why ?

dang it, spacing got messed up. pretend C is attached to COOH at 12'0clock and NH2 is attached to C at 6'0clock

 

[jhanago]

Is it in a fischer projection? If so you do it regular and flip it since CH3 is on one of the pointing up horizontal bonds

 

[bharat008]

COOH
I
I
I
H3C-------C---------F
I
I
I
NH2


What is the configuration - i know its S but why ?

FIrst of all look at the first connecting group to the Central Atom. Give them priority based on atomic mass so F is higher in mass compared to N. So, F is 1st priority, NH2 is second priority, COOH is 3rd and CH3 is 4th. Switch the CH3 and COOH so it would be clockwise (R) rotation. Since you switched 1 group initially you flip the R to S...
Since you switched CH3 with COOH you flip the configuration at end. So, R goes to S...

 

[FROGGBUSTER]

are priorities based on first point of difference or weight of the substituent for determining stereochemistry?

 

[bharat008]

are priorities based on first point of difference or weight of the substituent for determining stereochemistry?

Always the first point of difference. If they are same for example in the case of CH3 and COOH then look at second atom.
CH3 - H H H (arranged in decreasing atomic number)
COOH - OOO (arranged in decreasing atomic number)
Since O is higher in mass compared to H, COOH has higher priority compared to CH3

Another ex,
COOH - OOO (arranged in decreasing atomic number)
CHSH - SHH (arranged in decreasing atomic number)

S is higher in mass compared to O so CHSH is higher in priority compared to COOH..

HOpe this clears up the concept..

 

[FROGGBUSTER]

thanks for that.

when did we use mass of the substituent again? i coulda sworn we use it for something...

 

[bharat008]

thanks for that.

when did we use mass of the substituent again? i coulda sworn we use it for something...

mass of the substituent?? hmmm.. not sure ..
Can someone clarify if we ever use the mass of the substituents?/

As long as i remember we never use that.. but i could be wrong..

 

[Kobebucsfan]

FIrst of all look at the first connecting group to the Central Atom. Give them priority based on atomic mass so F is higher in mass compared to N. So, F is 1st priority, NH2 is second priority, COOH is 3rd and CH3 is 4th. Switch the CH3 and COOH so it would be clockwise (R) rotation. Since you switched 1 group initially you flip the R to S...
Since you switched CH3 with COOH you flip the configuration at end. So, R goes to S...

why would u switch CH3 and COOH ? is it b/c u want the prioirities clockwise order ?

 

[bharat008]

why would u switch CH3 and COOH ? is it b/c u want the prioirities clockwise order ?

Well, F is already 1st right and it is followed by NH2 then it should be COOH; however it shows CH3 so you switch them... This gives you R but since you made 1 switch; you switch R to S at the end..

Or you could switch COOH with NH2 and then switch COOH with CH3. This gives you S configuration (counter-clockwise) when you draw it. This time you made 2 switches so S goes to R which goes back to S..

1 switch = R goes to S
2 switch - R goes to S - goes to R...
3 switch and so on... but u won't encounter more than 2 switch..

So depends on how you want to do it..

 

[yappy]

http://en.wikipedia.org/wiki/Cahn%E2%80%93Ingold%E2%80%93Prelog_priority_rules


Assignment of priorities
R/S and E/Z descriptors are assigned by using a system for ranking priority of the groups attached to each stereocenter. This procedure, often known as the sequence rules, is the heart of the CIP system.
1. Compare the atomic number (Z) of the atoms directly attached to the stereocenter; the group having the atom of higher atomic number receives higher priority.

2. 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.

3. If there is still a tie, each atom in each of the two lists is replaced with a sub-list of the other atoms bonded to it (at distance 3 from the stereocenter), the sub-lists are arranged in decreasing order of atomic number, and the entire structure is again compared atom by atom. This process is repeated, each time with atoms one bond farther from the stereocenter, until the tie is broken.

 

[FROGGBUSTER]

i guess mass of substituents is never used then?

 

[jhanago]

Mass of Substituents is used by researchers sometimes when the R groups is HUGE. for single atoms and small groups its priority by number... but dont worry about the mass...at least until you start working on things like proteins

 

[AllahIsGreater]

I dont think it even makes a diffrence if you look at mass number, since as the atomic # goes up on the periodic table, so does the masses of the elements. so even though it may not be the "real" way.....i dont see how it wouldnt work. can anyone second this

 

[FROGGBUSTER]

I dont think it even makes a diffrence if you look at mass number, since as the atomic # goes up on the periodic table, so does the masses of the elements. so even though it may not be the "real" way.....i dont see how it wouldnt work. can anyone second this

no that doesn't work because what if you were comparing -CH2(SH) with -CH2(CH2CH2CH2CH2CH3)? -CH2(SH) would still be higher priority even though that substituent weighs less.

 

[bharat008]

no that doesn't work because what if you were comparing -CH2(SH) with -CH2(CH2CH2CH2CH2CH3)? -CH2(SH) would still be higher priority even though that substituent weighs less.

That protein is too small.. Sizes of proteins are in Kda.. Don't worry about it because you will never see that concept used on DAT.

Just focus on what i told u earlier and u will be fine..

 

[FROGGBUSTER]

That protein is too small.. Sizes of proteins are in Kda.. Don't worry about it because you will never see that concept used on DAT.

Just focus on what i told u earlier and u will be fine..

i think you misunderstood my post, i was just trying to explain something to the guy who posted above me, thanks though.

 

[bharat008]

i think you misunderstood my post, i was just trying to explain something to the guy who posted above me, thanks though.

😳

 

[rav4182]

ranking the priority groups goes clockwise. but since the 4-group is pointing towards you you have to flop your answer. so it goes from R to S.

 

[condog51]

-meso compounds

 

[rockclock]

Ok so I teach O-Chem at my University....... I am very familure with steriochemistry. R and S is assigned to a compound(s) that is usually OPTICALLY ACTIVE (racemic mixtures and meso compounds are exceptions)!!! Meaning it can rotate polarized light. R steriochemistry means that polarized light, when focused on the compound, is directed in the clockwise (this is argued still among chemists) direction off of the compound. S stereochemistry is just the opposite of R.

Furthermore steriochemistry is EXTREMELY DEPENDENT on molecular geometry!!!! It usually must always be Tetrahedral!!!! There is an alkene or two out there that can also be optically active, but you will not see them on the DAT...So before I go about making assumptions whether this is a fischer projection or assigning wedges and dashes where I think they should be...can you please state if this compound is oriented as a fischer projection; or please state which 2 of the 4 substituents covalently bonded to the central carbon is the wedge and dash respectively.

Here are terms you need to know for the DAT in regards to this topic:
-stereochemistry
-stereoisomers
-chiral
-achiral
-optically active
-cis/trans isomers
-chirality center (isomers)
-racemic mixture
-R/S
-D/L (bio chem)
-diastereomers
-meso compounds

WRONG! The R/S notation indicates configuration, while the +/- system indicates optical activity. There is no fixed connection between the two. You can't generalize that the R stereoisomer rotates light one way or the other. You could, however, say that the R enantiomer of a molecule with one chiral center rotates light the opposite direction of the S enantiomer.

http://en.wikipedia.org/wiki/Chirality_(chemistry)#By_configuration:_R-_and_S-

...great that you teach O.Chem at your university and are "familure" brah! 🙄

 

[jhanago]

WRONG! The R/S notation indicates configuration, while the +/- system indicates optical activity. There is no fixed connection between the two. A compound can be R-(+), while another may be R-(-), and the same goes for the S configuration.

http://en.wikipedia.org/wiki/Chirality_(chemistry)#By_configuration:_R-_and_S-

...great that you teach O.Chem at your university and are "familure" brah!🙄

Wow...

 

[FROGGBUSTER]

WRONG! The R/S notation indicates configuration, while the +/- system indicates optical activity. There is no fixed connection between the two. You can't generalize that the R stereoisomer rotates light one way or the other. You could, however, say that the R enantiomer of a molecule with one chiral center rotates light the opposite direction of the S enantiomer.

http://en.wikipedia.org/wiki/Chirality_(chemistry)#By_configuration:_R-_and_S-

...great that you teach O.Chem at your university and are "familure" brah! 🙄

you're a dick, and he said "usually."

 

[yappy]

He does have a point though... A compound may be named R or S but the only method of knowing what direction the R or the S form will rotate plane polorized light is through experiments. Therefore, he was incorrect in saying that you can tell that something is Dextrorotation or Levorotation based on Cahn-Ingold-Prelog R/S rules/naming.

But yeah... no tact lol.


EDIT: LOL I was reading some of Rockclocks older posts. They're awesome. Rockclock feel free to follow me around and blast me - I like your style man.

 

[condog51]

WRONG! The R/S notation indicates configuration, while the +/- system indicates optical activity. There is no fixed connection between the two. You can't generalize that the R stereoisomer rotates light one way or the other. You could, however, say that the R enantiomer of a molecule with one chiral center rotates light the opposite direction of the S enantiomer.

http://en.wikipedia.org/wiki/Chirality_(chemistry)#By_configuration:_R-_and_S-

...great that you teach O.Chem at your university and are "familure" brah! 🙄

The purpose of this forum is to help students. I do not want to get into a online argument/fight with you.

I was not wrong with my post. An R enantiomerically enriched species can direct plane polarized light to the right. Additionally I stated, "this is argued still among chemists" because it is not 100% true. Yes an R enantiomerically enriched species can direct plane polarized light to the left. But in OChem today it is being accepted more and more that R is associated with +, and S is associated with -. This is a practical application because R stereoselectivity is derived by ordering chiral center substituents in a clockwise fashion. Likewise + optically active species rotate plane polarized light to the right. Clockwise is synonymous with right, so an association can be made between the two. S and - is just the opposite of R and +. Yes it can ONLY TRULY be proven via an experiment, but for teaching purposes it is practical to relate the two, especially for students at an undergrad / DAT level.

As a practicing dentist this information will be irrelevant anyway. Unless you want to explain to someone how your cool polarized sunglasses work....


p.s. wikipedia isn't the law when it comes to figuring out fact from fiction.

 

[FROGGBUSTER]

EDIT: LOL I was reading some of Rockclocks older posts. They're awesome. Rockclock feel free to follow me around and blast me - I like your style man.

 

[rockclock]

The purpose of this forum is to help students. I do not want to get into a online argument/fight with you.

I was not wrong with my post. An R enantiomerically enriched species can direct plane polarized light to the right. Additionally I stated, "this is argued still among chemists" because it is not 100% true. Yes an R enantiomerically enriched species can direct plane polarized light to the left. But in OChem today it is being accepted more and more that R is associated with +, and S is associated with -. This is a practical application because R stereoselectivity is derived by ordering chiral center substituents in a clockwise fashion. Likewise + optically active species rotate plane polarized light to the right. Clockwise is synonymous with right, so an association can be made between the two. S and - is just the opposite of R and +. Yes it can ONLY TRULY be proven via an experiment, but for teaching purposes it is practical to relate the two, especially for students at an undergrad / DAT level.

As a practicing dentist this information will be irrelevant anyway. Unless you want to explain to someone how your cool polarized sunglasses work....


p.s. wikipedia isn't the law when it comes to figuring out fact from fiction.

if you want to tout your credentials (we're all impressed) and continue spreading misinformation by all means continue...

the fact of the matter is that you're wrong. this is a basic question that's asked at some point on an exam in every first semester organic class.

R/S absolute configuration is based on arbitrary CIP notation conventions. The rotation of plane polarized light is a quantum effect due to the chirality of polarized light and its interaction with chiral centers. Clockwise by atomic mass and clockwise rotation of light have no connection to each other. You could just as easily invent a replacement for the CIP system that names R/S centers by alphabetical order. That system still wouldn't have any relation to +/- optical activity. There's no "usually" or "for the DAT level" about it.

 

[rockclock]

you're a dick, and he said "usually."

true that. that "usually" was in the previous sentence, btw, and had no bearing on the point in question here.

i'm a dick for trying to set things straight on a basic point taught in ochem 1...yeah... i'll let you guys get back to circlejerking the guy who talks up his credentials and then spreads the wrong information...

please son, step aside.

 

[FROGGBUSTER]

ok phaggot.