Chemistry
Optical Isomerism
Optical isomerism is a type of stereoisomerism where molecules have the same molecular formula and connectivity but differ in the spatial arrangement of their atoms, leading to non-superimposable mirror images. These mirror-image molecules are called enantiomers and exhibit optical activity, meaning they rotate the plane of polarized light. Optical isomerism is important in fields such as pharmaceuticals and biochemistry due to its influence on biological activity.
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12 Key excerpts on "Optical Isomerism"
- (Author)
- 2014(Publication Date)
- Cuvillier Verlag(Publisher)
2 Theory This chapter presents the main theoretical background for the understanding of the thesis. A number of different optical effects are covered in the thesis and so we reserve the specific theoretical aspects for subsequent chapters. A short overview of the concepts of molecular chirality and optical activity are given as well as an introduction to the description of polarized light. A detailed analysis of both can be found in the books of Laurence Nafie [12] or Laurence Barron [3]. Here we will point out the main results. 2.1 Definitions of molecular chirality and optical activity 2.1.1 Definitions and sources for molecular chirality We will now take a closer look at the definition of chirality in the context of molecular geometry and symmetry. As mentioned before, a molecule is called chiral if it cannot be superimposed on its mirror image. This is the simplest definition of molecular chirality. A more rigorous definition can be given using group theory, i.e. the classification of symmetry operations. These include rotations, reflections, and improper rotations. A molecule posses symmetry if it is unchanged after one of those operations. A molecule is chiral, if it does not contain any improper rotation symmetry elements [3], like a center of inversion, reflection planes or rotation-reflection axis. Therefore chiral objects are not asymmetric, since they can posses symmetry. An example is a left-or right-handed helix, a chiral object which is symmetric with respect to a rotation axis through the mid point of the helix, perpendicular to its long axis (C 2 ). Molecular chirality is a special form of isomerism, which describes compounds with the same molecular formula but distinct structural compositions. There 'LHVHV :HUN LVW FRSULJKWJHVFKW]W XQG GDUI LQ NHLQHU )RUP YHUYLHOIlOWLJW ZHUGHQ QRFK DQ 'ULWWH ZHLWHUJHJHEHQ ZHUGHQ (V JLOW QXU IU GHQ SHUV|QOLFKHQ *HEUDXFK 8 2 Theory Figure 2.1: Overview about the different forms of isomerism.
- Morris Hein, Scott Pattison, Susan Arena, Leo R. Best(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
©Joachim Ladefoged/VII/CORBIS 26.1 Review of Isomerism 26.2 Plane-Polarized Light and Optical Activity 26.3 Fischer Projection Formulas 26.4 Enantiomers 26.5 Racemic Mixtures 26.6 Diastereomers and Meso Compounds STEREOISOMERISM The mirror image of this children’s ballet class is not superimposable on the class. C H A P T E R O U T L I N E M any of us grew up hearing such comments as “Can’t you tell your left from your right?” Have you ever watched a small child try to differentiate between a right and left shoe? Not surprisingly, the distinction between right and left is difficult. After all, our bodies are reasonably symmetrical. For example, both hands are made up of the same components (four fingers, a thumb, and a palm) ordered in the same way (from thumb through little finger). Yet there is a difference if we try to put a left-handed glove on our right hand or a right shoe on our left foot. Molecules possess similar, subtle structural differences, which can have a major impact on their chemical reactivity. For example, although there are two forms of blood sugar, related as closely as our left and right hands, only one of these structures can be used by our bodies for energy. Stereoisomerism is a subject that attempts to define these subtle differences in molecular structure. Stereoisomerism is an amazing phenomenon; it is where some compounds find a partner—their mirror image. C H A P T E R 26 672 CHAPTER 26 • Stereoisomerism TABLE 26.1 Why Are Stereoisomers Important to Biochemistry? Answer Comment #1 The great majority of biochemicals are stereoisomers. Understanding biochemical structures is difficult without a basic understanding of stereoisomerism. #2 Metabolism is stereospecific. For example, glucose (blood sugar) is easily metabolized while its enantiomer (see Section 26.4) is not useable. 26.1 REVIEW OF ISOMERISM Distinguish structural isomers from stereoisomers.
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
trans-1,2-dimethylcyclopentane H 3 C CH 3 H 3 C CH 3 cis-1,2-dimethylcyclopentane 878 Chemistry FIGURE 17.10 Relationships among isomers. (Note: This description excludes conformers, which can interconvert without breaking bonds.) Isomers different compounds with the same molecular formula. Bonds would have to be broken and reformed to convert from one to another. Constitutional isomers isomers with a different order of attachment of their atoms Stereoisomers isomers with the same order of attachment of atoms, but a different orientation of their atoms in space Enantiomers stereoisomers that are nonsuperimposable mirror images of each other Diastereomers stereoisomers that are not mirror images 17.2 Enantiomerism LEARNING OBJECTIVE 17.2 Recognise enantiomers as non-equivalent mirror images. As we have learned above, enantiomers are stereoisomers that are nonsuperimposable mirror images of each other. Except for inorganic compounds and a few simple organic compounds, the vast majority of molecules in the biological world show enantiomerism, including carbohydrates, lipids, amino acids and proteins, and nucleic acids. Further, approximately half of all pharmaceuticals show enantiomerism. To understand the significance of enantiomerism, recall that enantiomers have some different properties. While they have the same boiling points, melting points and solubilities, each of a pair of enantiomers reacts differently towards other chiral molecules. This is especially important in biology. For example, one form of thalidomide acts in the body to produce a sedative/hypnotic effect that controls the symptoms of morning sickness, whereas the other form acts to produce birth defects or to destroy some types of rapidly growing cancer cells. As the structure of thalidomide is relatively complicated, we will start by looking at a simpler example.
eBook - PDFPhosphorus
Chemistry, Biochemistry and Technology, Sixth Edition
- D.E.C. Corbridge(Author)
- 2016(Publication Date)
- CRC Press(Publisher)
established by special x-ray diffraction techniques 13.2.1 O PTICAL A CTIVITY AND E NANTIOMORPHISM Optically active materials rotate the plane of polarised light on passage through them Optical activ-ity can arise in an individual molecule when it cannot be superimposed on its mirror image The two non-superimposable forms (which are related to each other as a right hand is to a left hand) are known as enantiomers , and each is capable of rotating plane-polarised light in opposite directions The two enantiomorphic forms are designated dextro (d) and laevo (l) or ( + ) and ( − ) or (R) and (S) [13–15] Enantiomorphic molecules must lack an alternating axis of symmetry ( n ), and this includes the absence of a centre of symmetry ( l ) and a plane of symmetry (m) A molecule which is asymmetric and has no symmetry elements, or has only a simple axis of symmetry ( n ), is optically active since it cannot be superimposed on its mirror image The word dissymmetric is sometimes used to describe either of these two conditions Molecules which are dissymmetric and therefore optically active, are sometimes called diastereoisomers * Although racemisation corresponds to a part (50%) inversion, it is not necessarily the same process A racemic mixture may result from two reactions, one involving retention and the other an inversion, which take place simultaneously and each to the extent of 50% On the other hand the racemic mixture may result froma separate racemisation process which occurs after the formation of the product
- Frederick Bettelheim, William Brown, Mary Campbell, Shawn Farrell(Authors)
- 2019(Publication Date)
- Cengage Learning EMEA(Publisher)
413 CONTENTS 14.1 Enantiomerism How To . . . Draw Enantiomers 14.2 Specifying the Configuration of a Stereocenter 14.3 Possible Stereoisomers for Molecules with Two or More Stereocenters 14.4 Optical Activity and Chirality in the Laboratory 14.5 Significance of Chirality in the Biological World Chirality: The Handedness of Molecules 14 14.1 Enantiomerism In Chapters 11 through 13, we studied two types of stereoisomers, namely the cis -trans isomers of certain disubstituted cycloalkanes and appropri-ately substituted alkenes. Recall that stereoisomers have the same connec-tivity of their atoms but a different orientation of their atoms in space. and and CH 3 CH 3 CH 3 H 3 C cis -2-Butene trans -1,2-Dimethyl-cyclohexane cis -1,2-Dimethyl-cyclohexane C C H H CH 3 H 3 C trans -2-Butene C C H H CH 3 CH 3 In this chapter, we study the relationship between objects and their mirror images ; that is, we study stereoisomers called enantiomers and di-astereomers. Figure 14.1 summarizes the relationship among these isomers and those we studied in Chapters 11 through 13. The significance of enantiomers is that, except for inorganic compounds and a few simple organic compounds, the vast majority of molecules in the biological world show this type of isomerism. Furthermore, approximately one half of all medications used to treat humans display this property. As an example of enantiomerism, let us consider 2-butanol. As we discuss this molecule, we will focus on carbon-2, the carbon bearing the i OH group. Charles D. Winters Median cross section through the shell of a chambered nautilus found in the deep waters of the Pacific Ocean. The shell shows handedness; this cross section is a left-handed spiral. Copyright 2020 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
eBook - PDF- F Dwyer(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
II. Optical Isomerism and Stereochemistry The essential condition for Optical Isomerism in coordination compounds is the same as for organic substances: the structure must be nonsuperimpos-able on its mirror image. This does not mean, as pointed out by Jaeger (1930), that the structure need be asymmetric, for many complexes that have some elements of symmetry are resolvable. For these the description dissymmetric is more appropriate. The limiting symmetry conditions are that the molecule or ion should lack a plane or center of symmetry. The [M] = 96 TT N n 2 + 2 V R ba v* he ' 3 5. OPTICAL PHENOMENA IN METAL CHELATES 187 classical asymmetric carbon atom is obviously simulated by the metal atom in tetrahedral complexes of the form [ M ( A B C D ) ] , which, however, are difficult to prepare and probably too labile for successful resolution. Tetrahedral chelates derived from unsymmetrical bidentate ligands satisfy the symmetry conditions for Optical Isomerism, and are easily prepared. The bis(benzoylpyruvato)beryllate(II) anion (III) is one of the tetrahedral (ill) chelates that has been resolved (Mills and Gotts, 1926). The platinum atom in the octahedral complex [Pt(py)NH 3 ClBrIN0 2 ] 0 (IV) isolated as the Pt(rv) 'NH, NO a (rv) racemic pair of one of the many geometrical isomers (Gel'man and Essen, 1950) is also an asymmetric metal atom. In general, we are concerned with molecular asymmetry or dissymmetry in chelates (usually octahedral), and with the manner in which the ligand bridges the coordination positions. The dissymmetric tris (ethylenediamine)- 188 A. M. SARGESON cobalt (III) ion (Va) has one threefold and three twofold axes but lacks a plane or center of symmetry. If one ethylenediamine is replaced by two ammonia molecules (cis) (Vb) the complex is still dissymmetric but has lost some of its symmetry elements, namely, the threefold axis and two of the twofold axes.
eBook - PDF- David R. Klein(Author)
- 2020(Publication Date)
- Wiley(Publisher)
Such compounds are called stereoiso- mers, and we will explore the connection between stereoisomerism and drug action. This chapter will focus on the different kinds of stereoisomers. We will learn to identify stereoisomers, and we will learn several drawing styles that will allow us to compare stereoisomers. The upcoming chapters will focus on reactions that produce stereoisomers. 5.1 OVERVIEW OF ISOMERISM The term isomers comes from the Greek words isos and meros, meaning “made of the same parts.” That is, isomers are compounds that are constructed from the same atoms (same molecular formula) but that still differ from each other. We have already seen two kinds of isomers: constitutional isomers (Section 4.3) and stereoisomers (Section 4.14), as illustrated in Figure 5.1. FIGURE 5.1 The main categories of isomers. Isomers Stereoisomers Constitutional isomers Same molecular formula but different constitution (order of connectivity of atoms) Same molecular formula and constitution but different spatial arrangement of atoms DO YOU REMEMBER? Before you go on, be sure you understand the following topics. If necessary, review the suggested sections to prepare for this chapter. • Constitutional Isomerism (Section 1.2) • Tetrahedral Geometry (Section 1.10) • Drawing and Interpreting Bond-Line Structures (Section 2.2) • Three-Dimensional Representations (Section 2.6) Constitutional isomers differ in the connectivity of their atoms; for example: Ethanol Boiling point = 78.4°C C C O H H H H H H Methoxymethane Boiling point = –23°C C O C H H H H H H The two compounds above have the same molecular formula, but they differ in their constitution. As a result, they are different compounds with different physical properties. Stereoisomers are compounds that have the same constitution but differ in the spatial arrange- ment of their atoms.
- David R. Klein(Author)
- 2017(Publication Date)
- Wiley(Publisher)
Chapter 5 Stereoisomerism Review of Concepts Fill in the blanks below. To verify that your answers are correct, look in your textbook at the end of Chapter 5. Each of the sentences below appears verbatim in the section entitled Review of Concepts and Vocabulary. ______isomers have the same connectivity of atoms but differ in their spatial arrangement. Chiral objects are not superimposable on their ____________________. The most common source of molecular chirality is the presence of a _______________, a carbon atom bearing ______ different groups. A compound with one chiral center will have one non-superimposable mirror image, called its _______________. The Cahn-Ingold-Prelog system is used to assign the ______________ of a chiral center. A polarimeter is a device used to measure the ability of chiral organic compounds to rotate the plane of ____________________ light. Such compounds are said to be ____________ active. A solution containing equal amounts of both enantiomers is called a __________ mixture. A solution containing a pair of enantiomers in unequal amounts is described in terms of enantiomeric _________ (ee). For a compound with multiple chiral centers, a family of stereoisomers exists. Each stereoisomer will have at most one enantiomer, with the remaining members of the family being ______________. A ______ compound contains multiple chiral centers but is nevertheless achiral because it possesses reflectional symmetry. __________ projections are drawings that convey the configuration of chiral centers, without the use of wedges and dashes. Compounds that contain two adjacent C=C bonds are called ____________, and they are another common class of compounds that can be chiral despite the absence of a chiral center. The stereodescriptors cis and trans are generally reserved for alkenes that are disubstituted. For trisubstituted and tetrasubstituted alkenes, the stereodescriptors ____ and ____ must be used.
eBook - PDF- David R. Klein(Author)
- 2021(Publication Date)
- Wiley(Publisher)
208 CHAPTER 5 Stereoisomerism To help better visualize the relationship between these four compounds, we will use an analogy. Imagine a family with four children (two sets of twins). The first pair of twins are identical to each other in almost every way, except for the placement of one birthmark. One child has the birthmark on the right cheek, while the other child has the birthmark on the left cheek. These twins can be distinguished from each other based on the position of the birthmark. They are nonsuperimposable mirror images of each other. The second pair of twins look very different from the first pair. But the second pair of twins are once again identical to each other in every way, except the position of the birthmark on the cheek. They are nonsuperimposable mirror images of each other. In this family of four children, each child has one twin and two other siblings. The same rela- tionship exists for the four stereoisomers shown above. In this molecular family, each stereoisomer has exactly one enantiomer (mirror-image twin) and two diastereomers (siblings). Now consider a case with three chiral centers: OH Me Cl 1 2 3 Once again, each chiral center can have either the R configuration or the S configuration, giving rise to a family of eight possible stereoisomers: 1R, 2R, 3S 1S, 2S, 3R OH Me Cl OH Me Cl 1R, 2S, 3S 1S, 2R, 3R OH Me Cl OH Me Cl 1R, 2R, 3R 1S, 2S, 3S OH Me Cl OH Me Cl 1R, 2S, 3R 1S, 2R, 3S OH Me Cl OH Me Cl These eight stereoisomers are arranged above as four pairs of enantiomers. To help visualize this, imag- ine a family with eight children (four sets of twins). Each pair of twins are identical to each other with the exception of the birthmark, allowing them to be distinguished from one another. In this family, each child will have one twin and six other siblings. Similarly, in the molecular family shown above, each stereoisomer has exactly one enantiomer (mirror-image twin) and six diastereomers (siblings).
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2017(Publication Date)
- Wiley(Publisher)
Better yet, using different colored balls, make molecular models as you work this out. The compounds represented by structures 1–4 are all optically active compounds. Any one of them, if placed separately in a polarimeter, would show optical activity. The com- pounds represented by structures 1 and 2 are enantiomers. The compounds represented by structures 3 and 4 are also enantiomers. But what is the isomeric relation between the compounds represented by 1 and 3? We can answer this question by observing that 1 and 3 are stereoisomers and that they are not mirror images of each other. They are, therefore, diastereomers. • Diastereomers have different physical properties—different melting points and boiling points, different solubilities, and so forth. SOLVED PROBLEM 5.6 Draw all possible stereoisomers for 2-bromo-4-chloropentane. STRATEGY AND ANSWER: C2 and C4 are chirality centers in 2-bromo-4-chloropentane. We begin by drawing the carbon chain with as many carbons depicted in the plane of the paper as possible, and in a way that maximizes the symmetry between C2 and C4. In this case, an ordinary zig-zag bond-line formula provides symmetry between C2 and C4. Then we add the bromine and chlorine atoms at C2 and C4, respectively, as well as the hydrogen atoms at these carbons, resulting in formula I. To draw its enantiomer (II), we imagine a mirror and draw a reflection of the molecule. Mirror C4 C2 H Br H Cl II I C2 C4 H Br H Cl To draw another stereoisomer we invert the configuration at one chirality center by interchanging two groups at one chirality center, as shown for C2 in III. Then we draw the enantiomer of III by imagining its mirror reflection. Mirror C4 C2 H Br H Cl IV III C2 C4 H Br H Cl PRACTICE PROBLEM 5.19 4 Br H H Br 3 Br H H Br 2 H H Br Br 1 H H Br Br (a) If 3 and 4 are enantiomers, what are 1 and 4? (b) What are 2 and 3, and 2 and 4? (c) Would you expect 1 and 3 to have the same melting point? (d) The same boiling point? (e) The same vapor pressure?
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
the same analysis is true for the trans isomer. • Enantiomers only occur with compounds whose molecules are chiral. • A chiral molecule and its mirror image are called a pair of enantiomers. The rela- tionship between them is enantiomeric. The universal test for chirality of a molecule, or any object, is the nonsuperposability of the molecule or object on its mirror image. We encounter chiral and achiral objects throughout our daily life. Shoes are chiral, for example, whereas most socks are achiral. 198 CHAPTER 5 STEREOCHEMISTRY: Chiral Molecules The chirality of molecules can be demonstrated with relatively simple compounds. Consider, for example, 2-butanol: OH 2-Butanol Until now, we have presented the formula for 2-butanol as though it represented only one compound and we have not mentioned that molecules of 2-butanol are chiral. Because they are, there are actually two different 2-butanols and these two 2-butanols are enantio- mers. We can understand this if we examine the drawings and models in Fig. 5.3. PRACTICE PROBLEM 5.1 Classify each of the following objects as to whether it is chiral or achiral: (a) A screwdriver (d) A tennis shoe (g) A car (b) A baseball bat (e) An ear (h) A hammer (c) A golf club (f) A woodscrew PRACTICE PROBLEM 5.2 Construct hand-held models of the 2-butanols represented in Fig. 5.3 and demonstrate for yourself that they are not mutually superposable. (a) Make similar models of 2-bromopropane. Are they superposable? (b) Is a molecule of 2-bromopropane chiral? (c) Would you expect to find enantiomeric forms of 2-bromopropane? If model I is held before a mirror, model II is seen in the mirror and vice versa. Models I and II are not superposable on each other; therefore, they represent different, but isomeric, molecules. Because models I and II are nonsuperposable mirror images of each other, the molecules that they represent are enantiomers.
eBook - PDF- David R. Klein(Author)
- 2016(Publication Date)
- Wiley(Publisher)
These conformations are mirror images, yet they are not superimposable on one another. To see this more clearly, compare them in the following way: Traveling clockwise around the ring, the first chair has an axial methyl group first, followed by an equatorial methyl group. The second chair, going clockwise again, has an equatorial methyl group first, followed by an axial methyl group. The order has been reversed. These two conformations are distinguishable from one another; that is, they are not superimposable. The relationship between these two chair 210 CHAPTER 5 Stereoisomerism conformations is therefore enantiomeric; however, this compound is optically inactive, because the conformations rapidly interconvert at room temperature. The following procedure will be helpful for determining whether or not a cyclic compound is optically active: (1) Either draw a Haworth projection of the compound or simply draw a ring with dashes and wedges for all substituents and then (2) look for a plane of symmetry in either one of these drawings. For example, (cis)-1,2-dimethylcyclohexane can be drawn in either of the following ways: Internal plane of symmetry Internal plane of symmetry Me Me In each drawing, a plane of symmetry is apparent, and the presence of that symmetry plane indicates that the compound is not optically active. 5.9 Chiral Compounds That Lack a Chiral Center As we have seen throughout this chapter, the most common source of molecular chirality is the pres- ence of one or more chiral centers. However, the presence of a chiral center is not a necessary condi- tion for a compound to be chiral. Indeed, there are many examples of chiral compounds that lack a chiral center. In this section, we will explore a few such examples, and an additional example can be found in the end-of-chapter exercises. Atropisomers Substituted biphenyls, like the one shown below, exhibit severely restricted rotation about the bond that connects the two aromatic rings.
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