isomer
Summary
Isomers are different compounds with the same molecular formula but different arrangements of atoms. The MCAT sorts them into constitutional isomers, which differ in connectivity, and stereoisomers, which share connectivity but differ in 3D arrangement (enantiomers and diastereomers).
Detail
Meaning: Isomers are distinct compounds that share a molecular formula but differ in how the atoms are arranged, so their properties differ. Constitutional (structural) isomers differ in connectivity, such as butane and isobutane, or ethanol and dimethyl ether. Stereoisomers have the same connectivity but differ in spatial arrangement. They divide into enantiomers (nonsuperimposable mirror images) and diastereomers (stereoisomers that are not mirror images, including cis-trans isomers).
Equation: Maximum number of stereoisomers = 2ⁿ, where n is the number of stereocenters (a dimensionless count). Meso compounds make the actual number smaller.
On the MCAT: You are shown two structures and asked for their relationship. Check the formula first, then connectivity, then configuration at each stereocenter: all centers inverted means enantiomers, some but not all inverted means diastereomers. Enantiomers share melting point, boiling point and solubility in achiral solvents but rotate plane-polarized light in opposite directions. Diastereomers and constitutional isomers differ in physical properties.
Common confusion: Having stereocenters does not guarantee chirality: a meso compound has an internal mirror plane and is achiral. Also, conformers (interconverting by rotation about single bonds) are not separate compounds at room temperature, whereas stereoisomers generally need bonds broken to interconvert.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
- Organic Chemistry (Brown, Iverson, Anslyn, Foote)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.