chiral carbons

Meaning: A chiral carbon is an sp³ carbon with four different substituents. Swapping any two groups gives its enantiomer, a nonsuperimposable mirror image. Enantiomers share melting point and solubility but rotate plane-polarized light in opposite directions, and enzymes usually act on one form. In biochemistry, the α-carbon of an amino acid is chiral unless the side chain is H (glycine). Isoleucine and threonine have a second chiral carbon in the side chain. Open-chain glucose has four chiral carbons. Ring closure makes the anomeric carbon a fifth, giving α and β anomers.

Equation: Maximum stereoisomers = 2ⁿ, where n = the number of chiral carbons (a dimensionless count, so no SI unit).

On the MCAT: You may be asked to find the chiral carbons in a drawn structure, assign R or S from CIP priorities, or count stereoisomers. You may also be asked whether two sugars are enantiomers (every center inverted) or epimers/diastereomers (only some inverted). Protein amino acids are L, which is S for most of them. Cysteine is R because sulfur raises the priority of its side chain.

Common confusion: Chiral centers do not guarantee a chiral molecule. A meso compound has chiral carbons but an internal mirror plane, so 2ⁿ is a maximum. Also, D/L, R/S and (+)/(−) are separate labels. Only experiment gives the sign of rotation.