Chapter 5: Stereochemistry: Chiral Molecules – Key Terms
Terms for Chapter 5: Stereochemistry: Chiral Molecules
C
Chirality (Sec 5.1, 5.3, 5.6): The property of nonidentity of an object with its mirror image.
Chirality center (Sec 5.3, 5.12, 5.17): A tetrahedral atom with four different groups attached to it, so that interchanging any two groups produces a stereoisomer.
Chiral molecule (Sec 5.3, 5.10, 5.12): A molecule that is not superposable on its mirror image, that has handedness, and an enantiomer of the molecule exists.
cis-trans Isomers (Sec 1.13B, 4.5, 4.13, 5.2, 7.2): Diastereomers that differ in their stereochemistry at adjacent atoms of a double bond or on different atoms of a ring; the cis-isomer has two substituents on the same side of a double bond or ring; the trans-isomer has two substituents on opposite sides of a double bond or ring. Also called geometric isomers.
Configuration (Sec 5.7, 5.15): The particular arrangement of atoms (or groups) in space that is characteristic of a given stereoisomer.
Conformational stereoisomers (Sec 4.9A, 5.14A): Two stereoisomers whose three-dimensional structures differ only due to rotations about single bonds, and they are interconvertible by bond rotations.
Constitutional isomers (Sec 1.3, 4.2, 5.2): Compounds that have the same molecular formula, but that differ in their connectivity, i.e., their atoms are connected in different ways. Also called structural isomers.
D
Dextrorotatory (Sec 5.8B): An optically active compound that rotates plane-polarized light clockwise; designated d or (+).
Diastereomers (Sec 5.2): Stereoisomers that are not mirror images of each other. Also called geometric isomers.
E
Enantiomeric purity (Sec 5.9B): A measure of the enantiomeric excess; calculated for a mixture of enantiomers by dividing the difference of moles of the two enantiomers by the total moles of both enantiomers, then multiplying by 100%.
Enantiomers (Sec 5.2, 5.3, 5.7, 5.8): Stereoisomers that are nonsuperposable mirror images of each other.
F
Fischer projection formula (Sec 5.13): A two-dimensional formula for representing the configuration of a chiral molecule; written with the main carbon chain extending from top to bottom with all groups eclipsed; vertical lines represent bonds that project behind the plane of the page (or that lie in it); horizontal lines represent bonds that project out of the plane of the page.
K
Kinetic resolution (Sec 5.10B): Production of a preponderance of one stereoisomer over the other by a reaction in which the rates of the stereoisomers are different.
L
Levorotatory (Sec 5.8B): An optically active compound that rotates plane-polarized light counterclockwise; designated l or (–).
M
Meso compound (Sec 5.12A): An optically inactive compound that is achiral, even though it contains chirality centers (tetrahedral atoms with four different groups attached), which is usually a result of having a plane of symmetry in the molecule.
O
Optical purity (Sec 5.9B): A measure of the enantiomeric excess; calculated for a mixture of enantiomers by dividing the observed specific rotation for the mixture by the specific rotation of the pure enantiomer, then multiplying by 100%.
Optically active compound (Sec 5.8, 5.9): A compound that rotates the plane of polarization of plane-polarized light.
P
Plane of symmetry (Sec 5.6, 5.12A): An imaginary plane that bisects an achiral molecule in a way such that the two halves of the molecule are mirror images of each other.
Plane-polarized light (Sec 5.8A): Ordinary light in which the oscillations of the electrical field occur only in one plane.
R
R,S system (Sec 5.7): A method for designating the configuration of chirality centers, which was devised by Cahn, Ingold, and Prelog.
Racemic mixture (Sec 5.9A, 5.10A): An equimolar mixture of enantiomers that is optically inactive; designated (±). Also called racemate, or racemic form.
Racemization (Sec 5.10A, 6.12A): A reaction that transforms an optically active compound into a racemic form, which occurs when a reaction causes chiral molecules to be converted into an achiral intermediate.
Relative configuration (Sec 5.15A): Two different chiral molecules in which similar or identical groups in each occupy the same position in space.
Resolution (Sec 5.16B): The process by which the enantiomers of a racemic form are separated.
S
Specific rotation, [α] (Sec 5.8C): A standardized measurement of the observed rotation of plane-polarized light by an optically active compound; [α] = α / ( c ∙ l ), where α is the observed rotation, c is the concentration in g/mL, and l is the pathlength in dm.
Stereogenic center (Sec 5.3): An atom at which an interchange of groups produces a stereoisomer. Also called stereocenter.
Stereoisomers (Sec 1.13B, 4.9A, 4.13, 5.1 – 5.16): Compounds with the same molecular formula and the same connectivity, but are not superposable because they differ in the arrangement of their atoms in three-dimensional space; includes enantiomers and diastereomers, but excludes constitutional isomers.
Stereoselective reaction (Sec 5.10B, 8.21, 12.3C): A reaction that produces a preponderance of one stereoisomer, where a stereogenic center is altered or created; an enantioselective reaction produces a preponderance of one enantiomer over the other; a diastereoselective reaction produces a preponderance of one diastereomer over the other.
Superposable (Sec 1.13B, 5.1): Two objects that can be placed on top of each other so that all parts of each coincide. Also called superimposable.
A
Achiral molecule (Sec 5.3): A molecule that is superposable on its mirror image, that lacks handedness, and no enantiomer of the molecule can exist.
Absolute configuration (Sec 5.l5A): The three-dimensional arrangement of atoms or groups at a chirality center.
Atropisomers (Sec 5.18): Conformational isomers that are stable compounds which can be separated.