Stereochemistry and Chirality: every key term you need (+ practice quiz)
25 flashcard terms for Organic Chemistry I Topic 4, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 15-question quiz — free, no account needed.
Compounds with the same connectivity but a different spatial arrangement of atoms. They cannot be interconverted by rotation about single bonds alone.
Chirality
The property of a structure that is not superimposable on its mirror image. In practice it usually arises from a carbon bearing four different groups.
Stereocenter
An atom where exchanging any two attached groups produces a different stereoisomer. A carbon with four distinct substituents is the standard example.
Enantiomers
Nonsuperimposable mirror images. They share every physical property except the direction they rotate plane-polarized light and how they behave toward other chiral things.
Diastereomers
Stereoisomers that are not mirror images of one another. Unlike enantiomers they have genuinely different melting points, solubilities, and reaction rates.
Meso compound
A molecule containing stereocenters yet achiral overall because an internal mirror plane makes one half the reflection of the other. It is optically inactive.
Cahn-Ingold-Prelog priority
A ranking scheme that assigns priority by atomic number at the first point of difference, exploring outward atom by atom until a tiebreak appears.
R configuration
With the lowest priority group pointed away from the viewer, priorities one through three proceed clockwise. The label describes geometry, not the direction of optical rotation.
S configuration
With the lowest priority group pointed away, priorities one through three proceed counterclockwise. Reversing any two groups at that center flips the descriptor.
Optical rotation
The angle by which a chiral sample turns plane-polarized light, measured in a polarimeter. Its sign must be determined experimentally rather than deduced from a drawing.
Specific rotation
Observed rotation normalized for concentration and path length so it becomes a reproducible physical constant for a given compound, solvent, and temperature.
Racemic mixture
An equal blend of both enantiomers. It shows no net optical rotation because the two contributions cancel exactly, though each component remains chiral.
Enantiomeric excess
The percentage by which one enantiomer outweighs the other in a sample. A value of zero describes a racemate and one hundred describes a single pure enantiomer.
Fischer projection
A flat drawing where horizontal lines point toward the viewer and vertical lines point away. Rotating one by ninety degrees inverts the configuration it represents.
Wedge and dash notation
A drawing convention where a solid wedge projects toward the viewer and a hashed line recedes. Two swaps of substituents return the original configuration.
Maximum stereoisomer count
For a compound with n independent stereocenters the ceiling is two raised to the n power. Internal symmetry lowers that count by producing meso forms.
Alkene geometric isomerism
Restricted rotation about a pi bond fixes substituents on the same or opposite sides. The resulting isomers are diastereomers with distinct physical properties.
E and Z descriptors
Priority is assigned on each alkene carbon separately; higher priorities on opposite sides gives E and on the same side gives Z. This system supersedes ambiguous cis and trans labels.
Optical activity requirement
A sample rotates polarized light only if it is chiral and not racemic. Meso compounds and racemates both read zero on a polarimeter.
Resolution of enantiomers
Separating a racemate by converting it into diastereomeric salts with a single enantiomer of a chiral acid or base, which can then be separated by crystallization.
Chiral drug activity
Because biological receptors are themselves chiral, two enantiomers of the same drug can differ enormously in potency, metabolism, and toxicity.
Epimers
Diastereomers differing in configuration at exactly one stereocenter while all others match. The term appears constantly in carbohydrate chemistry.
Conformers versus configurations
Conformers interconvert by rotation about single bonds at room temperature; configurations require breaking bonds. Only the latter counts as separate stereoisomers.
Internal mirror plane
A symmetry element that reflects one half of a molecule onto the other. Its presence in any accessible arrangement guarantees the molecule is achiral.
Stereochemical inversion at carbon
A configuration change in which three bonds flip through like an umbrella in wind. It converts one descriptor into the other at that center.