Absolute Configuration

[jdla]

How do you figure out the priorities?

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[tncekm]

By atomic number. If both are the same, go further back in the chain. If there are double or triple bonds, treat them as three separate connections. (E.g. R-(C=O)-R, the C is "essentially" bonded to two oxygens and takes priority over C-O-C, but not O-C-O).

 

[HCW212]

also, make sure the lowest priority group is facing away (usually hydrogen) or if it's coming out at you, then just reverse the configuration.

 

[BloodySurgeon]

I h8 2 b specific, but its not based on atomic weight but by the atomic number.

 

[tncekm]

I h8 2 b specific, but its not based on atomic weight but by the atomic number.

Yeah, my bad 😀 That's what I meant.

 

[jdla]

okay thanks.

 

[unsung]

Wait, I thought that was the way to figure out "relative configuration" ? What's the difference between absolute & relative configurations?

 

[tncekm]

I *believe* the D and L are "relative" configurations because their configurations are assigned relative to something else.

The absolute configuration is based on atomic # (priority).

 

[bluemonkey]

Wait, I thought that was the way to figure out "relative configuration" ? What's the difference between absolute & relative configurations?

Absolute config=+/-, the actual rotation of PPL

Relative=R/S, just tells us how the atoms are attached...

 

[tncekm]

Absolute config=+/-, the actual rotation of PPL

Relative=R/S, just tells us how the atoms are attached...

Actually, its definitely what I said earlier. I just looked it up (note: + and - are related to D and L):

An absolute configuration in stereochemistry is the spatial arrangement of the atoms of a chiral molecular entity (or group) and its stereochemical description e.g. R or S.[1]
http://en.wikipedia.org/wiki/Absolute_configuration

When the precise arrangement of substituents at a stereogenic center is known the absolute configuration of the molecule is known. This is usually accomplished by solving the x-ray crystal structure of a molecule, a method that is not always readily available.

The arrangement of atoms in an optically active molecule, based on chemical interconversion from or to a known compound, is a relative configuration. Relative, because there is no way of knowing just by looking at a structure whether the assignment of (+) or (-) is correlated to a particular isomer, R or S.
http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch07configurations.html

 

[bluemonkey]

oops. thanks.

 

[iA-MD2013]

tncekm, could you explain that? im kinda confused. thanks 🙂

 

[tncekm]

Okay, sure.

Absolute configuration My best guess is they call it absolute, because it can be absolutely repeated and absolutely determined by assigning R and S configurations to any chiral molecule based on priority of constituents (via molecular number).

Relative configuration is literally relative to some other feature on a molecule. This example may not be exact, but the point remains the same: There are D and L carbohydrates, right? We use D carbohydrates, like D-Glucose.

D (dextrorotary, on the right) is the "relative" configuration of D-Glucose because of how the hydroxyl group is aligned (to the right), relative to the most oxidized carbon, in a fischer projection.

Note: often you'll see something like: D-(+)-moleculename, and I don't believe that the (D/L) or (+/-) are directly correlated; i.e. I believe you can have a D-(+)-molecule and a D-(-)-molecule.

The same thing applies to AAs. The amine group next to the most oxidized carbon is used to distinguish AA's as D or L, with D (dextrorotary) being the amine group is to the right on a fischer projection and with L (levorotary) being the amine group nearest the most oxidized carbon is located to the left on the fischer projection.

L-Glutamine

D-Glutamine

 

[iA-MD2013]

But doesn't D=R and L=S? So aren't you saying the same thing when you are determining the absolute and the relative configurations.
So, would 5R-glucose be absolute? It's the same as saying D-glucose...

 

[tncekm]

No, D doesn't = R and L doesn't = S. They "can", but that's just a coincidence. Technically, all of the absolute configurations on D and L glucose are opposite because D and L glucose are enantiomers.

Without determining any absolute configurations, you can put glucose in a fischer projection with its most oxidized carbon at the top and determine if its D or L glucose, just based off of the position (left, L, or right, D) of the nearest hydroxyl group.

R/S, D/L, +/- describe different things.

R/S describes how a chiral center is organized with regards to the priority of the constituents (which is based off of atomic #).

D/L describe the left or right position of the constituent attached to the carbon immediately below the most oxidized carbon at the top of a fischer projection.

+/- describe which way plane polarized light is rotated.

Maybe this wiki link I found will help: http://en.wikipedia.org/wiki/Optical_isomerism#By_optical_activity:_.28.2B.29-_and_.28.E2.88.92.29-

 

[iA-MD2013]

No, D doesn't = R and L doesn't = S. They "can", but that's just a coincidence. Technically, all of the absolute configurations on D and L glucose are opposite because D and L glucose are enantiomers.

Without determining any absolute configurations, you can put glucose in a fischer projection with its most oxidized carbon at the top and determine if its D or L glucose, just based off of the position (left, L, or right, D) of the nearest hydroxyl group.

R/S, D/L, +/- describe different things.

R/S describes how a chiral center is organized with regards to the priority of the constituents (which is based off of atomic #).

D/L describe the left or right position of the constituent attached to the carbon immediately below the most oxidized carbon at the top of a fischer projection.

+/- describe which way plane polarized light is rotated.

Maybe this wiki link I found will help: http://en.wikipedia.org/wiki/Optical_isomerism#By_optical_activity:_.28.2B.29-_and_.28.E2.88.92.29-

Wow, I hate stereochem. THANK YOU! that clears everything up wonderfully.
One more question, If a compound is dextratory, it is (+) but not necessarily D, right?

 

[tncekm]

Yeah, I believe that's the case. Although, that was something i was unclear on as well. But, I think that wiki article cleared that one for me. Either way, I think these details are probably more than we need to know.

 

[iA-MD2013]

Yeah, I believe that's the case. Although, that was something i was unclear on as well. But, I think that wiki article cleared that one for me. Either way, I think these details are probably more than we need to know.

Thanks for your help!

 

[tncekm]

NP--helping others helps me, too 🙂