Stereochemistry — the three-dimensional arrangement of atoms — is where organic chemistry becomes genuinely three-dimensional, and it matters enormously in biology and medicine because living systems are exquisitely sensitive to a molecule's "handedness." The central concept is chirality: a molecule is chiral if it is non-superimposable on its mirror image, exactly like your left and right hands (the word comes from the Greek for "hand"). No matter how you rotate your left hand, it never perfectly overlays your right — same parts, opposite arrangement.

The most common cause of chirality is a stereocenter (chiral center): a carbon bonded to four different groups. Swap any two groups on such a carbon and you get a different molecule that is its mirror image of the original — the relationship explored fully in the next track.

To name which spatial arrangement you have at a stereocenter, use the Cahn–Ingold–Prelog (CIP) system to assign R or S. The procedure:

1. Assign priorities (1–4) to the four groups on the stereocenter by atomic number — the atom directly attached with the highest atomic number gets priority 1, lowest gets 4. If there is a tie at the first atom, move outward to the next atoms and compare until the tie breaks. (A double bond counts as if that atom were duplicated.)

2. Orient the molecule so the lowest priority group (4) points away from you, into the page.

3. Trace the path from priority 1 → 2 → 3. If it goes clockwise, the center is R (Latin rectus, right); if counterclockwise, it is S (sinister, left).

4. A vital shortcut: if the lowest-priority group is pointing toward you instead of away, determine the rotation anyway and then reverse your answer (what looked clockwise is really S, and vice versa).

R and S are a naming convention for a single stereocenter's configuration — they say nothing on their own about how a molecule interacts with other chiral things, or how it behaves in a polarimeter. Those questions, and what happens once a molecule has more than one stereocenter, are exactly where the next two tracks pick up.

"Left Hand, Right Hand"