Diastereomers
Diastereomers are a type of stereoisomer that have different spatial arrangements of atoms and are not mirror images of each other. They have different physical and chemical properties, such as melting points and solubilities. Diastereomers arise when a molecule has two or more stereocenters and can have multiple non-superimposable structures.
7 Key excerpts on "Diastereomers"
eBook - ePub- Graham Patrick(Author)
- 2004(Publication Date)
- Taylor & Francis(Publisher)
...Such a molecule has a plane of symmetry and cannot be chiral. The mirror images are superimposable and optical isomers are not possible. Diastereomers Molecules containing more than one asymmetric center can have several stereoisomers, that is, different configurational isomers. For each asymmetric center present (n) in a molecule, there are 2 n possible stereoisomers. Every stereoisomer has a mirror image and so there are 2 n -1 sets of enantiomers. Each set of enantiomers is called a diastereomer. Diastereomers are different molecules having different chemical and physical properties. Related topic (D2) Configurational isomers — alkenes and cycloalkanes Definition Optical isomerism is another example of configurational isomerism and is so named because of the ability of optical isomers to rotate plane-polarized light clockwise or counterclockwise. The existence of optical isomers has very important consequences for life, since optical isomers often have significant differences in their biological activity. This is because optical isomers may interact differently with other optical isomers, such as proteins. It is also possible to distinguish between optical isomers in a reaction if another molecule is present as an optical isomer, causing one optical isomer to react in preference to the other. Such a reaction is termed an asymmetric reaction. Otherwise, optical isomers have identical chemical and physical properties. Asymmetric molecules A molecule such as chloroform (CHCl 3) is tetrahedral and there is only one way of fitting the atoms together. This is not the case for a molecule such as lactic acid. There are two ways of constructing a model of lactic acid, such that the two structures obtained are nonsuperimposable and cannot be interconverted without breaking covalent bonds. As such, they represent two different molecules which are configurational isomers...
- M. O. Faruk Khan, v Philip(Authors)
- 2018(Publication Date)
- Bentham Science Publishers(Publisher)
...For example, the oral bioavailability of D -methotrexate is only 2.5% that of the L -isomer, which may account for its differential absorption, protein binding and metabolism. Thus, the chirality or stereochemistry of drugs is an important factor to be considered for therapeutic agents. The following sections consider different stereochemical features of drugs. Definitions and illustrations are included to aid in understanding the different conformational and configurational aspects of drugs. Stereochemical Definitions and Illustrations Isomers are molecules with identical molecular formulas but different structural formulas or different stereochemical formulas and hence different physical and/or chemical properties. Molecules having identical molecular formulas but different structural formulas are termed structural isomers. For example, ethanol (CH 3 CH 2 OH) and methyl ether (CH 3 OCH 3) are structural isomers as both have the same molecular formula (C 2 H 6 O), but different connectivity of atoms. Structural isomers would be expected to have very different physical/chemical characteristics (i.e. color, melting point, solubility, etc.). Stereoisomers come about from chirality or from restricted rotation such as in a ring or olefinic (double) bond. There are two obsolete terms that have been used in the past with respect to stereoisomers. The first, optical isomers, is used to describe stereoisomers with different optical properties. These optical properties arise from chirality and will be discussed under that topic. The second, geometric isomers, is used to describe cis-trans isomerism. Chirality is a geometric property whereby a molecule (or any rigid object for that matter) is not superimposable on its mirror image. Chirality arises either from chiral centers or from restricted rotation, referred to as axial chirality. Stereoisomers have identical molecular and structural formulas but they have different spatial arrangements of the same groups...
eBook - ePub- Mukesh Doble, Anil Kumar Kruthiventi, Vilas Ganjanan Gaikar, Mukesh Doble, Anil Kumar Kruthiventi, Vilas Ganjanan Gaikar(Authors)
- 2004(Publication Date)
- CRC Press(Publisher)
...All of which are isomers. The isomers can differ based on connectivity of the constituent atoms (the constitutional isomers) or based on the three-dimensional placement of the atoms (the stereoisomers). The energies of the various conformers are not the same, and the molecule exists in its lowest enery conformation. 2.5 EXPLANATION OF TERMS ENCOUNTERED IN DESCRIBING STEREOCHEMISTRY Chiral: This term can be applied to molecules, conformations, macroscopic objects, crystals, etc. When an entity is nonsuperimposable with its mirror image, this term is used. Achiral: If an entity is superimposable with its mirror image, it is termed achiral. Chiral center: In a tetrahedral (Xabcd) or trigonal pyramidal (Xabc) structure, the atom (X) to which different ligands (atoms or molecules) are attached is termed the chiral center. R and S notations based on the Cahn-Ingold-Prelog system (CIP system), which are chirality descriptors can be assigned to this center. Enantiomers: Mirror-image stereoisomers. These normally have similar physical and chemical properties. Enantiomer: The pair of molecular species, which are stereoisomers, that are mirror image to each other and are nonsuperimposable. FIGURE 2.12 Diastereoisomers. FIGURE 2.13 Enantiotopic atoms in a prochiral group. Diastereoisomers: Stereoisomers that are not mirror images. These differ in their physical and chemical properties. These can usually be obtained by the presence of more than one chiral center in the molecule, for example, the erythose and threose (tetroses) sugars, as shown in Fig. 2.12. Prochirality: This is a term used to describe atoms, groups, molecules, or even faces of a particular molecule. Prochiral atoms and groups: In a tetrahedral carbon (sp 3), when the replacement of one of the two similar ligands leads to a chiral product, the two similar ligands are called as prochiral atoms or ligands. Pro-R, pro-S descriptors are used to differentiate these two similar ligands. For example, in Fig...
eBook - ePub- Michael North(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
...Thus any physical property that depends on molecular packing and/or intermolecular interactions will also be unable to distinguish between enantiomers (samples which are not composed of a single enantiomer of a compound may have different physical properties as will be discussed in Chapter 3, section 3.4). Examples of such techniques include melting point, boiling point and density. Thus enantiomers not only have identical chemical properties (unless reacting with other enantiomers as will be discussed in Chapter 3, section 3.3), they also have identical physical properties, with one important exception which will be introduced in Chapter 3, section 3.4.1. The two stereoisomers 1.6 and 1.7 introduced at the beginning of this chapter, however, are not mirror images of one another and so cannot be enantiomers. By definition, any pair of stereoisomers which are not enantiomers of one another are called Diastereomers. Table 1.4, lists the bond lengths, bond angles and torsional angles for a pair of Diastereomers 1.45 and 1.46. Comparison of the data in Table 1.4 with that in Table 1.3 illustrates the important difference between enantiomers and Diastereomers. Diastereomers have different bond lengths, bond angles and torsional angles. Hence they have different energies and will have different chemical properties. Often, Diastereomers will have similar chemical properties since they contain the same functional groups, but will undergo reactions at a different rate. Occasionally, however, the Diastereomers will have completely different chemical properties as shown in Scheme 1.1. Thus alkene 1.47 when heated to 180°C, loses water and forms a cyclic anhydride 1.49. Alkene 1.48 which is a diastereomer of compound 1.47 however, cannot be dehydrated to a cyclic anhydride. Table 1.4 Bond lengths and angles for diasterereomers 1.45 and 1.46 Diastereomers have different molecular shapes and volumes, as is particularly apparent for isomers 1.6 / 1.7 or 1.45 / 1.46...
- Ahindra Nag, Ahindra Nag(Authors)
- 2018(Publication Date)
- CRC Press(Publisher)
...1 Basic Stereochemical Approaches to Natural Products and Drugs Ahindra Nag CONTENTS 1.1 Basic Concept of Chirality 1.2 Meso Compounds 1.3 Tautomerism and Valance Tautomerism 1.4 Conformation 1.5 Fischer Projection and Absolute Configuration 1.6 Chiral Resolution 1.6.1 Crystal Picking 1.6.2 Chemical Separation 1.6.3 Biochemical Separation 1.6.4 Chromatographic Separation 1.7 Application of Enantiomers in Drugs and Natural Products Problems Answers References 1.1 Basic Concept of Chirality An asymmetric carbon atom (known as the stereogenic center) is attached to four different groups 1 termed a chiral (pronounced as kiral). The word chiral derives from the Greek word cheira meaning hand, which is closely related to optical activity. For a molecule to have chirality, it must not possess a plane, a center, or a fourfold alternating axis of symmetry. Molecules which are mirror images of each other are termed enantiomers (from the Greek entatios meaning opposite) and need chiral recognition to be separated. Enantiomers react 1, 2, 3, 4, 5, 6 at different rates with other chiral compounds and may have different solubilities in the presence of an optically active solvent. They may display different absorption spectra under circulatory polarized light. Enantiomers may have different optical rotations, which could be either (+), that is, dextrorotatory (clockwise), or (−), that is, levorotatory (anticlockwise), and can be determined by a polarimeter. The optical purity of a mixture of enantiomers is given by % Optical purity of sample = 100 ∗ Specific rotation of sample Specific rotation of a pure enantiomer Specific rotation α D = α obs /c l where α obs is the experimentally observed rotation c is the concentration in g/mL l is the path length of the cell used, expressed in dm (=10 cm) Enantiomeric excess 7 is one of the indicators of the success of an asymmetric synthesis...
eBook - ePub- John Andraos, Albert S. Matlack(Authors)
- 2022(Publication Date)
- CRC Press(Publisher)
...10 Stereochemistry DOI: 10.1201/9781003033615-10 10.1 Importance of Optical Isomers Most organic molecules in living organisms contain asymmetrical centers (i.e., they are chiral). (The terminology of stereochemistry has been reviewed by Moss. 1) For example, amino acids that are incorporated into proteins are L and sugars in carbohydrates are D. It is understandable that the three-dimensional structures of the receptors in proteins for small molecules will favor only one optical isomer (i.e., the one that fits sterically, hydrogen bonds properly, and so on). Most compounds made for use by plants and animals will have to be single optical isomers. These include agricultural chemicals, drugs, flavors, food additives, fragrances, and such. This is especially important with drugs, 2 for which the unwanted isomer may produce toxic side effects. There are several possibilities: (a) The unwanted isomer may be inert, in which case its synthesis is a waste, for it must be discarded or recycled, if this is possible. (b) The unwanted isomer is useful for another entirely different purpose. (c) The unwanted isomer can ruin the effects of the desired one and may prove to be toxic. 3 (d) The isomers may racemize in the body so that the racemate may be used. Sheldon has listed some examples (Scheme 10.1) that illustrate these cases. 4 SCHEME 10.1 (a) Enantiomers of carvone; (b) enantiomers of aspartame. The reader will appreciate the difference in flavors of the two terpene isomers. Aspartame is the popular artificial sweetener found in many soft drinks and other foods. Dramatic differences can also be found in the isomers of drugs (Scheme 10.2). SCHEME 10.2 Enantiomers of selected pharmaceuticals along with their biological properties. Some drugs may not need to be single isomers because they racemize in the body. The analgesic ibuprofen (Scheme 10.3) is an example...
eBook - ePub- J Fisher, J.R.P. Arnold, Julie Fisher, John Arnold(Authors)
- 2020(Publication Date)
- Taylor & Francis(Publisher)
...There are two basic types of configurational isomer, geometric isomers and optical isomers. Related topics (B3) Shapes of some small molecules (B4) Drawing chemical structures (E2) Stereoisomerism Definition One of the key routines performed in the investigation of an unknown compound is elemental analysis. A knowledge of the ratio of carbon to hydrogen to oxygen, for example, together with the molecular mass of a compound (determined by mass spectrometry (Section Q8)), enables the determination of the molecular formula (Section B4). However, in all but the simplest cases, this is not sufficient to determine the nature of the compound present. This is because, while obeying the rules of valence (Section A2), it is generally possible to write more than one structure for a particular molecular formula. For example, the molecular formula C 2 H 6 O is that of ethanol (CH 3 CH 2 OH) and dimethylether (CH 3 OCH 3). Although ethanol and dimethylether have the same elemental composition they are not the same compound, they are isomers (structural or constitutional isomers to be precise). There are two basic classes of isomer, structural and stereoisomer (see Section E2 for labeling of such isomers); conformational and configurational isomers are subclasses of the latter, and it is important that these are understood to identify an unknown unambiguously. Structural isomers The term structural isomer is the most general term that may be used to describe any compound that has the same molecular formula as another compound, but different physical properties. The example of ethanol and dimethylether has already been provided; these are in different chemical classes. Propan-1-ol (CH 3 CH 2 CH 2 OH, boiling point 97.4°C) and propan-2-ol (CH 3 CH(OH)CH 3, boiling point 82.4°C) are both C 3 H 8 O alcohols but are structural isomers. They cannot be interconverted by bond rotation, and they are not related as mirror images...
