Absolute Configuration
Started by jdla
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).
I h8 2 b specific, but its not based on atomic weight but by the atomic number.
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
Advertisement - Members don't see this ad
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
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
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...
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-
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?
Thanks for your help!
