Chirality: Chiral Atoms VS Chiral Molecules
A molecule is termed chiral if it cannot be superimposed onto its mirror image through any combination of rotations, translations, or certain conformational changes. Chiral molecules exist in two non-identical mirror-image forms, known as enantiomers. While they share the same chemical properties, except when interacting with other chiral entities, their physical properties are identical except for their differing optical activities. A racemic mixture, which contains equal proportions of both enantiomers, exhibits distinct chemical and physical properties compared to the individual pure enantiomers.
The origin of chirality in molecules is usually due to the presence of a stereogenic element. The most common type is a stereogenic center (stereocenter), often represented by a carbon atom bonded to four different groups in a tetrahedral arrangement. Other elements, such as nitrogen, can also act as stereogenic centers, provided they are connected to four different substituents.
Each stereocenter can adopt one of two configurations, R or S, leading to the formation of stereoisomers, including diastereomers and enantiomers. In chiral molecules with multiple stereocenters, the enantiomer is the stereoisomer where each center has the opposite configuration. Molecules with a single stereogenic carbon are inherently chiral, while those with several such carbons are typically chiral but not always. Specifically, if the arrangement of stereocenters allows the molecule to adopt a symmetrical conformation (internal plane of symmetry), it becomes achiral and is recognized as a meso compound. Molecules with central chirality are those with one or more stereocenters.
Summary:
-
Chiral atoms are usually carbon atoms, sometimes nitrogen and sulfur atoms, which have 4 different substituents
-
Chiral molecules are molecules which contain at least one chiral atom and no internal plane of symmetry
-
Chiral molecules have enantiomers
-
Enantiomers
-
Must contain at least one stereocenter
-
Are molecules with more than one stereocenter that must not have an internal plane of symmetry
-
If an internal plane of symmetry is present, the molecule is a meso compound and is not optically-active
-
-
Are non-superimposable mirror images
-
Rotate plane-polarized light by equal magnitude, but in opposite directions
-
The rotation of plane-polarized light is an experimental observation and not related to R/S designations
-
-
To draw the enantiomer of a given molecule, invert all stereocenters (change wedges to dashes and dashes to wedges, R to S, S to R)
Diastereomers
Diastereomers are a unique class of stereoisomers that are not mirror images of each other and have non-matching configurations. They emerge when two or more stereoisomers of a compound exhibit varying arrangements at one or more (but not all) corresponding stereocenters. If the difference between two diastereoisomers is at a single stereocenter, they are classified as epimers. Each stereocenter can lead to two distinct configurations, often doubling the number of potential stereoisomers.
Contrary to enantiomers, which are pairs of stereoisomers that are mirror images and differ at all stereocenters, diastereomers do not share this property. Enantiomers of a compound with more than one stereocenter can also be considered diastereomers relative to other stereoisomers of the same compound that are not their mirror image. Notably, diastereomers exhibit distinct physical properties and typically have differing chemical reactivities.
In essence, diastereomers are unique in both their physical attributes and their chemical interactions with other substances. They not only differ in physical properties but also in their chemical reactivity, which is how a compound reacts with others.
-
To draw the diastereomer of a given molecule, invert one or more stereocenters, but not all (change one or more wedges to dashes and dashes to wedges, R to S, S to R, but not all of them)
Meso Compounds
A meso compound, or a meso isomer, is a type of stereoisomer that, despite having two or more stereocenters, is not chiral and is optically inactive. This is because it can be superposed on its mirror image. It’s important to note that superposition is different from superimposition, where any two objects can be placed over one another, irrespective of their similarity. In the case of a meso compound, the superposition is possible due to the identical nature of the compound and its mirror image.
Summary:
-
Have stereocenters but are achiral
-
All meso compounds are achiral but not all achiral molecules are meso compounds (such as molecules that don’t have stereocenters)
-
If we rotate the molecule on the left around its vertical axis we get the molecule on the right, meaning they’re the same molecule
Alkene Stereochemistry
Determining E/Z for Alkenes
-
Divide the double bond in half, across the bond
-
Look at the pairs of substituents on each carbon separately to determine priorities
-
Use the Cahn-Ingold-Prelog rules to determine priorities
-
If the higher-priority groups on each carbon are on the same side, the alkene is “Z”
-
If the higher-priority groups on each carbon are on opposite sides, the alkene is “E”
Keywords
chirality | enantiomer | diastereomer | meso | alkene stereochemistry | E alkene | Z alkene