chirality
Summary
Chirality is the property of a molecule that cannot be superimposed on its mirror image. It usually arises from a stereocenter, such as an sp³ carbon with four different groups. Testable point: a chiral molecule has an enantiomer, but a compound with stereocenters and an internal mirror plane (meso) is achiral.
Detail
Meaning: A molecule is chiral when it is not superimposable on its mirror image. The two mirror-image forms are enantiomers. The usual cause is a stereocenter, typically an sp³ carbon bonded to four different groups. Enantiomers have identical melting points, boiling points and solubilities in achiral solvents. They rotate plane-polarized light by equal amounts in opposite directions, and they interact differently with other chiral molecules such as enzymes. A molecule with an internal mirror plane is achiral.
Equation: maximum stereoisomers = 2ⁿ, where n is the number of stereocenters (a dimensionless count). Meso compounds make the actual number smaller.
On the MCAT: Expect questions that ask you to find stereocenters, assign R or S, and classify a pair as identical, enantiomers, diastereomers or meso. A racemic (50:50) mixture shows no net optical rotation. In biochemistry the same idea appears because protein amino acids are mostly L and glycine is achiral. Enzymes usually act on one enantiomer.
Common confusion: Having stereocenters does not make a molecule chiral. Meso-tartaric acid has two stereocenters but an internal mirror plane, so it is achiral and optically inactive. Also, R or S does not tell you the direction of rotation (+ or −), which must be measured. Inverting only some stereocenters gives a diastereomer, not an enantiomer.
Sources
- Organic Chemistry
- Organic Chemistry: Structure and Function
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.