How do you figure out the priorities?

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).

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.

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

I h8 2 b specific, but its not based on atomic weight but by the atomic number.
Yeah, my bad :D That's what I meant.

okay thanks.

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

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).

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...

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

oops. thanks.

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

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.

http://mooni.fccj.org/~ethall/stereo/dglucose.gif

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.

http://www.biocheminfo.org/klotho/fischer/L-glutamine_fischer.gif
L-Glutamine
http://www.biocheminfo.org/klotho/fischer/D-glutamine_fischer.gif
D-Glutamine

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...

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-

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?

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.

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!

NP--helping others helps me, too :)