chiral center
Summary
A chiral center (stereocenter) is an atom, most often an sp³ carbon, bonded to four different groups, so the center has two possible configurations (R and S). A molecule with one such center is nonsuperimposable on its mirror image; n centers give at most 2ⁿ stereoisomers, fewer if meso.
Detail
Meaning: A chiral center is usually an sp³ carbon with four different substituents, compared branch by branch outward to the first difference. Swapping two groups inverts that center's configuration (R to S). With one chiral center this gives the enantiomer; with several, inverting only some gives a diastereomer, and the enantiomer needs every center inverted. Enantiomers share melting point and boiling point but rotate plane-polarized light in equal, opposite amounts and bind chiral enzymes differently.
Equation: Maximum number of stereoisomers = 2ⁿ, where n is the number of chiral centers (a unitless count).
On the MCAT: You may be asked to find chiral centers, assign R or S, or count stereoisomers. You may also classify a pair as enantiomers (every center inverted) or diastereomers (only some inverted). SN2 at a chiral center inverts the configuration, while SN1 tends to give a racemic mixture (no net rotation).
Common confusion: A chiral center does not guarantee a chiral molecule. A meso compound has two or more chiral centers and an internal mirror plane, so it is achiral and optically inactive, and it has fewer than 2ⁿ stereoisomers. Also, R or S does not predict (+) or (−) rotation, which is measured experimentally.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.