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Chemical Isomers
by Erowid
This piece is heavily under construction. At this point it is mostly just notes on the topic. If you are already somewhat familiar with the subject this might help you out...if you're not it might just confuse you. Eventually I'll add in images and some examples to show what we're talking about.

TERMS
benzene ring 6 carbons in a ring
phenyl A benzene ring attached to something else
hydroxyl An oxygen with a single hydrogen attached
chiral asymmetrical
chiral center
A carbon with 4 different groups attached to it, generally one of these groups is a hydrogen. Also called a "stereogenic center" or "dissymetric site".
stereo-isomer The same molecule with different orientations of the legs
alpha-carbon The first carbon off the phenyl
racemic mixture A combination of + and - (D and L)


[D/L][+/-]
Each chiral center has 2 possible orientations dextro [D] or levo [L]. These refer to which direction a pure sample will polarize light which is shined through it in solution. Like a polarizing filter for a camera. The polymer coating is probably chiral. So ephedrine has 2 chiral centers and each has 2 orientations. If there are multiple chiral centers and they polarize light in differen directions, it's in between D/L. There is no way to tell D or L without shining a light on it to see which way it polarizes. D/L do not correlate exactly to positions in a 3d molecule model.

There are other ways for assigning the structure not based on how it polarizes light but rather on ranking the molecular weights of the groups attached to the chiral center.

[S/R]
Orient the view of the molecule so the alpha carbon (alpha to the phenyl or benzene ring)...the first chiral center... is in the center with the group with the least atomic mass attached so it's going away from you. [+/-] [S/R] depend only on this first chiral group. Now look at the 3 other groups (besides the hydrogen) attached to the chiral center. Look at the first atom of each group and pick the one with the highest molecular weight and call it 1. In the case of ephedrine it's the Oxygen.

Carbon=12
Nitrogen=14
Oxygen=16

Now look at the 2 remaining groups. Both have carbon as the first atom, so we need to look at a second atom in each group. Take the heaviest atom which is connected to each of the first atoms in each group. The benzene ring has 2 carbons attached to the first carbon. The other group has a nitrogen attached to the first carbon. So we number the group with the Nitrogen 2 and the benzene ring is 3. Now looking at the chiral center with the hydrogen pointed away from you, see which way the 1-->2-->3 goes around the circle. In this case it's counterclockwise (-).

clockwise = [-] or [R]
counterclockwise = [+] or [S]

(-)ephedrine = pseudoephedrine
a racemic mixture contains both + and - together. The natural extract from a plant is generally (though not always) a racemic mixture.

so D/L may correlate exactly to +/-, but not necessarily.

(L) LSD is inactive. the theory is that it may be the cause of some of the unwanted side effects
(D) LSD is the psychoactive isomer.