Recognizing Stereoisomers: Constitutional, Diastereomers, And Enantiomers

Constitutional isomers, diastereomers, and enantiomers are types of stereoisomers, which are sets of molecules with identical chemical formulas and connectivity but distinct spatial arrangements of atoms. Constitutional isomers have the same formula but differ in the arrangement of atoms and connections between them, resulting in different molecules with unique properties. To identify constitutional isomers, one must first determine the molecular formula and then calculate the HDI to predict structural features. Diastereomers are stereoisomers that are not mirror images of each other and have different physical properties. They occur when a compound has two or more stereocenters, and while they are not superimposable, they are not mirror images. Enantiomers, on the other hand, are stereoisomers that are mirror images of each other and are non-superimposable. They have the same makeup but differ in stereochemistry, causing them to bend polarized light differently.

Explore related products

Organic Chemistry Isomerism and Reactions Essential Workbook Volume 2: Organic Reactions, Geometric, Optical, All Type Of Isomers 270+ Exercises with Solutions

$13.99

Enantiomer Separation: Fundamentals and Practical Methods (Subcellular Biochemistry)

$56.7 $169.99

The Separation of Enantiomers by Capillary Electrophoresis (CHROMATOGRAPHIA CE-Series, 6)

$43.19 $54.99

Enantiomers: Webster's Facts and Phrases

$28.95

Enantiomer: Webster's Timeline History, 1959 - 2007

$28.95

Separation of Enantiomers: Synthetic Methods

$151

Constitutional isomers have the same formula, different connectivity

Constitutional isomers have the same molecular formula but different connectivity. They are also known as structural isomers. For example, propionic acid and 1-hydroxy-2-propanone share the same molecular formula, C3H6O2, but they are connected differently and are therefore constitutional isomers. Another example is ethanol (C2H6O) and dimethyl ether (C2H6O), which have the same molecular formula and mass but different atomic connectivity, and so are also constitutional isomers.

Constitutional isomers can have the same or different functional groups. For instance, ethyl alcohol and dimethyl ether have the same molecular formula, but their functional groups differ. The atomic connectivity is C-C-O in ethyl alcohol, and the oxygen atom is part of an alcohol. Conversely, the C-O-C connectivity in the isomer forms an ether.

Constitutional isomers can also have the same functional groups, but these groups are located at different points on the carbon skeleton. An example of this is 1-propanol and 2-propanol, which both have a hydroxyl group, but this group is on different carbon atoms.

Constitutional isomers must have the same chemical formula, or else there is no possibility of isomerism. However, it is important to remember that molecules are free to move around and can be drawn in different ways, so it is not always easy to identify whether two molecules are constitutional isomers. For example, butane and isobutane are constitutional isomers, but they have different carbon backbones. Butane has an uninterrupted chain of carbon atoms, whereas isobutane has only three carbon atoms connected in sequence.

Constitutional isomers are distinct from stereoisomers, which have the same connectivity but differ in their arrangement in space. Stereoisomers can be further divided into enantiomers and diastereomers. Enantiomers are non-superimposable mirror images of each other, whereas diastereomers are not.

Stereoisomers have the same connectivity, different arrangement

Stereoisomers are isomeric molecules that have the same molecular constitution but differ in their three-dimensional spatial arrangement of atoms. They can be further classified into enantiomers and diastereomers.

Constitutional Isomers vs Stereoisomers

Constitutional isomers have the same molecular formula but differ in their connectivity. In other words, they have the same parts but are arranged differently. For example, 2-methylpropane and butane, or 1-pentanol and ethyl propyl ether.

On the other hand, stereoisomers have the same connectivity but differ in their arrangement in space. For instance, 2-hexene and 3-methyl-1-pentene have the same connectivity but differ in the arrangement of their groups in space about the double bond.

Enantiomers

Enantiomers are stereoisomers that are non-superimposable mirror images. They have identical physical properties except for optical rotation. Enantiomers always have the same connectivity but opposite R, S designations. For example, (2R, 3R)-tartaric acid and (2S, 3S)-tartaric acid are enantiomers.

Diastereomers

Diastereomers are stereoisomers that are not non-superimposable mirror images. They have the same connectivity but differ in the configuration of at least one but not all chiral centers. For instance, (2R, 3R)-tartaric acid and (2R, 3S)-tartaric acid are diastereomers.

Identifying Stereoisomers

To identify stereoisomers, one can use techniques such as liquid chromatography and supercritical fluid chromatography, which are valuable tools in various fields, including pharmaceuticals, food, and agriculture. Another method is to build models of the molecules and compare their structures.

Enantiomers are non-superimposable mirror images

Enantiomers are defined as a pair of stereoisomers that are non-superimposable mirror images of one another. This means they are molecules made up of identical atoms, bonded together in the same way, i.e., they have the same connectivity. However, the 3D arrangement of the atoms in enantiomers is different, as these molecules are mirror images of each other. You cannot superimpose one enantiomer onto the other without breaking and remaking bonds.

The mirror image method is used to determine whether optical isomers exist. If the mirror image of a molecule can be rotated to look identical to the original molecule, it is superimposable, and no optical isomers exist. On the other hand, if the mirror image cannot be rotated to match the original molecule, it is non-superimposable, and the molecule has optical isomers. Non-superimposable means the structure cannot be rotated to match the original molecule, no matter how it is turned.

Enantiomers always have the same connectivity but opposite R, S designations. For example, the enantiomer of (2R, 3R)-tartaric acid is (2S, 3S)-tartaric acid. A 50:50 mixture of a pair of enantiomers is called a racemic mixture, which is optically inactive because the rotations produced by each enantiomer cancel each other out. If there is an excess of one enantiomer, the optical purity of a sample can be determined by measuring the rotation and comparing it to that of a pure enantiomer.

In summary, enantiomers are stereoisomers that are non-superimposable mirror images. They have the same atomic connectivity but differ in their 3D arrangement, making them mirror images that cannot be superimposed without breaking and remaking bonds. The existence of optical isomers in enantiomers can be determined using the mirror image method, and their optical purity can be assessed by comparing rotations with pure enantiomers.

Explore related products

BIOACTIVE ANALYTES, Including CNS Drugs, Peptides, and Enantiomers (Methodological Surveys in Biochemistry and Analysis, 16 A)

$76.01 $99

Pharmacokinetics of enantiomers of profens

$10

Essentials in Modern HPLC Separations

$195 $195

(S,S) | Left but not Right T-Shirt

$19.99

Diastereomers are not non-superimposable mirror images

Diastereomers are a type of stereoisomer. Stereoisomers are isomers that have the same molecular formula and connectivity but differ in the spatial arrangement of atoms. They are distinct chemical entities with different properties.

Enantiomers, on the other hand, are stereoisomers that are non-superimposable mirror images. They have the same connectivity but opposite R, S designations. For example, the enantiomer of (2R, 3R)-tartaric acid is (2S, 3S)-tartaric acid.

To determine whether two molecules are enantiomers or diastereomers, one can assign the absolute configuration (R or S). If the connectivity of the two molecules is the same, but the configurations of all the chiral centers are different, then they are enantiomers. If the molecules share the same configuration on at least one but not all chiral centers, then they are diastereomers.

Another method is to build the two molecules using a model kit and compare them by rotating one molecule until it becomes a mirror image of the other.

Assigning absolute configuration (R or S) to determine enantiomers or diastereomers

To identify enantiomers or diastereomers, it is essential to determine the absolute configuration of molecules, which can be assigned as either R or S. This process involves examining the arrangement of atoms around a chiral centre, also known as a stereocenter or chirality centre. The R/S nomenclature, also known as the Cahn-Ingold-Prelog (CIP) rules, provides a standardised method for naming enantiomers based on their "handedness" or "right-handed" and "left-handed" configurations.

To assign the absolute configuration, the first step is to locate the carbon atom(s) with four distinct groups or atoms attached to it within the molecule. These carbon atoms are referred to as chirality centres. Once these centres are identified, the next step is to determine the priorities of the atoms attached to the chiral centre. The Cahn-Ingold-Prelog rules provide guidance on prioritisation, primarily based on the atomic numbers of the atoms connected to the chiral carbon. The atom with the highest atomic number takes precedence. In the event of a tie, the next atom(s) along the chain are considered until a difference is observed.

After assigning priorities to the atoms, a curved arrow is drawn from the highest priority atom to the lowest priority atom. If the arrow follows a counterclockwise direction, moving left when leaving the 12 o'clock position, the configuration is assigned as S (derived from the Latin word "Sinister" meaning left). Conversely, if the arrow moves in a clockwise direction, the configuration is labelled as R (derived from the Latin word "Rectus" meaning right). The R or S designation is then added as a prefix to the name of the enantiomer. For example, (R)-2-Bromobutane and (S)-2,3-Dihydroxypropanal.

It is important to note that enantiomers have the same connectivity but differ in their R and S designations. For instance, the enantiomer of (2R, 3R)-tartaric acid is (2S, 3S)-tartaric acid. On the other hand, diastereomers share the same configuration at one or more chiral centres but not all. For example, a diastereomer of (2R, 3R)-tartaric acid is (2R, 3S)-tartaric acid.

By following these steps and considering the R/S nomenclature, one can effectively determine the absolute configuration of molecules and identify enantiomers or diastereomers. This process is crucial in understanding the stereochemistry and isomerism of organic compounds.

Frequently asked questions