Chemistry
Chirality
Chirality refers to the property of a molecule that makes it non-superimposable on its mirror image. In chemistry, chiral molecules have different spatial arrangements of atoms, leading to distinct chemical and biological properties. This property is important in fields such as drug development, as the two mirror-image forms, or enantiomers, can have different effects in the body.
Written by Perlego with AI-assistance
Related key terms
1 of 5
10 Key excerpts on "Chirality"
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
194 CHAPTER 5 STEREOCHEMISTRY: Chiral Molecules 5.1 Chirality AND STEREOCHEMISTRY Chirality is a phenomenon that pervades the universe. How can we know whether a particular object is chiral or achiral (not chiral)? • We can tell if an object is chiral by examining the object and its mirror image. Every object has a mirror image. Many objects are achiral. By this we mean that the object and its mirror image are identical—that is, the object and its mirror image are superposable one on the other.* Superposable means that one can, in one’s mind’s eye, place one object on the other so that all parts of each coincide. Simple geometrical objects such as a sphere or a cube are achiral. So is an object like a water glass. • A chiral object is one that cannot be superposed on its mirror image. Photo by Craig B. Fryhle The glass and its mirror image are superposable. *To be superposable is different than to be superimposable. Any two objects can be superimposed simply by putting one object on top of the other, whether or not the objects are the same. To superpose two objects (as in the property of superposition) means, on the other hand, that all parts of each object must coincide. The condition of superposability must be met for two things to be identical. Each of our hands is chiral. When you view your right hand in a mirror, the image that you see in the mirror is a left hand (Fig. 5.1). However, as we see in Fig. 5.2, your left hand and your right hand are not identical because they are not superposable. Your hands are chiral. In fact, the word chiral comes from the Greek word cheir meaning hand. An object such as a mug may or may not be chiral. If it has no markings on it, it is achiral. If the mug has a logo or image on one side, it is chiral. Photo by Michael Watson for John Wiley & Sons, Inc. FIGURE 5.1 The mirror image of a right hand is a left hand. Photo by Michael Watson for John Wiley & Sons, Inc. FIGURE 5.2 Left and right hands are not superposable.
- (Author)
- 2014(Publication Date)
- Cuvillier Verlag(Publisher)
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. It can be seen, that Chirality or enantiomerism is a special form of stereoisomerism (source: Wikimedia Commons) are many different classes of isomers which can be split into two main forms: structural isomerism and stereoisomerism (spatial isomerism) (Fig. 2.1). Structural or constitutional isomers are compounds where the connections/bonds of atoms and functional groups differ, which is in contrast to stereoisomers. Here the bond structure is the same, but the geome-trical orientation of atoms and functional groups in space is different. This class includes the enantiomers of chiral molecules as well as diastereomers. While structural isomers ty-pically have different chemical properties, stereoisomers behave often identically except in their interaction with other stereoisomers, a property already mentioned in chapter 1. Four distinct structural elements can impart Chirality in a molecule (Fig. 2.2) [12]: 1. Helix: It is the simplest embodiment of a chiral structure. Here the molecule posses a helical shape, where the sense of its Chirality is equal to the handedness of the helix and is classified as P (plus) for a clockwise and M (minus) for a counterclockwise orientation. Examples are the molecule hexahelicene or DNA. 2. Chiral Center: The chiral center is a central atom bonded to a set of different ligands (atoms or groups of atoms) in a spatial arrangement which is not superposable on its mirror image. In its simplest an most common form a chiral center is an atom with four unequal groups, usually a tetrahedral carbon. But also nitrogen, sulfur or phosphorus can be chiral centers.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
Photo by Craig B. Fryhle FIGURE 5.2 Left and right hands are not superposable. Photo by Michael Watson for John Wiley & Sons, Inc. 200 CHAPTER 5 Stereochemistry 5.1A The Biological Significance of Chirality The human body is structurally chiral, with the heart lying to the left of center and the liver to the right. Helical seashells are chiral and most are spiral, such as a right-handed screw. Many plants show Chirality in the way they wind around supporting structures. Honey- suckle winds as a left-handed helix; bindweed winds in a right-handed way. DNA is a chiral molecule. The double helical form of DNA turns in a right-handed way. Chirality in molecules, however, involves more than the fact that some molecules adopt left- or right-handed conformations. As we shall see in this chapter, it is the nature of groups bonded at specific atoms that can bestow Chirality on a molecule. Indeed, all but one of the 20 amino acids that make up naturally occurring proteins are chiral, and all of these are clas- sified as being left-handed. The molecules of natural sugars are almost all classified as being right-handed. In fact, most of the molecules of life are chiral, and most are found in only one mirror image form. Chirality has tremendous importance in our daily lives. Most pharmaceuticals are chi- ral. Usually only one mirror-image form of a drug provides the desired effect. The other mirror-image form is often inactive or, at best, less active. In some cases the other mir- ror-image form of a drug actually has severe side effects or toxicity (see Section 5.5 regarding thalidomide). Our senses of taste and smell also depend on Chirality. As we shall see, one mirror-image form of a chiral molecule may have a certain odor or taste while its mirror image smells and tastes completely different. The food we eat is largely made of molecules of one mirror-image form.
- Frederick Bettelheim, William Brown, Mary Campbell, Shawn Farrell(Authors)
- 2019(Publication Date)
- Cengage Learning EMEA(Publisher)
416 | Chapter 14 Chirality: The Handedness of Molecules 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). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. If an object and its mirror image are superposable, then they are identi-cal and enantiomerism is not possible. We say that such an object is achiral (without Chirality); that is, it has no handedness. Examples of achiral ob-jects include an undecorated cup, an unmarked baseball bat, a regular te-trahedron, a cube, and a sphere. To repeat, the most common cause of Chirality in organic molecules is a tetrahedral carbon atom with four different groups bonded to it. We call such a chiral carbon atom a stereocenter . 2-Butanol has one stereocenter; 2-propanol has none. As another example of a molecule with a stereocenter, let us consider 2-hydroxypropanoic acid, more commonly named lactic acid. Lactic acid is a product of anaerobic glycolysis. (See Section 27.2 and Chem-ical Connections 27A.) It is also what gives sour cream its sour taste. Figure 14.4 shows three-dimensional representations of lactic acid and its mirror image. In these representations, all bond angles about the cen-tral carbon atom are approximately 109.5° and the four bonds from it are directed toward the corners of a regular tetrahedron. Lactic acid displays enantiomerism or Chirality; that is, the original molecule and its mirror image are not superposable but rather are different compounds.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2017(Publication Date)
- Wiley(Publisher)
If the mug has a logo or image on one side, it is chiral. Photo by Michael Watson for John Wiley & Sons, Inc. FIGURE 5.1 The mirror image of a right hand is a left hand. Photo by Michael Watson for John Wiley & Sons, Inc. FIGURE 5.2 Left and right hands are not superposable. Photo by Craig B. Fryhle This mug is chiral because it is not superposable on its mirror image. 5.2 ISOMERISM: CONSTITUTIONAL ISOMERS AND STEREOISOMERS 195 5.1A The Biological Significance of Chirality The human body is structurally chiral, with the heart lying to the left of center and the liver to the right. Helical seashells are chiral and most are spiral, such as a right-handed screw. Many plants show Chirality in the way they wind around supporting structures. Honeysuckle winds as a left-handed helix; bindweed winds in a right-handed way. DNA is a chiral molecule. The double helical form of DNA turns in a right-handed way. Chirality in molecules, however, involves more than the fact that some molecules adopt left- or right-handed conformations. As we shall see in this chapter, it is the nature of groups bonded at specific atoms that can bestow Chirality on a molecule. Indeed, all but one of the 20 amino acids that make up naturally occurring proteins are chiral, and all of these are classified as being left-handed. The molecules of natural sugars are almost all classified as being right-handed. In fact, most of the molecules of life are chiral, and most are found in only one mirror image form.* Chirality has tremendous importance in our daily lives. Most pharmaceuticals are chiral. Usually only one mirror-image form of a drug provides the desired effect. The other mirror-image form is often inactive or, at best, less active. In some cases the other mirror-image form of a drug actually has severe side effects or toxicity (see Section 5.5 regarding thalidomide). Our senses of taste and smell also depend on Chirality.
eBook - PDF- Scott E. Denmark(Author)
- 2009(Publication Date)
- Wiley-Interscience(Publisher)
By a process of supermagnification, such iconic models26bring the unseen molecule into the familiar world of everyday experience: An object with a distinct shape that we can relate to, that we can handle, see, and enjoy. Indeed, the metaphor “molecular shape” works amazingly well: It encapsulates and conveys a great deal of useful chemical information, it is of undeniable heuristic and didactic value, and it serves as a powerful fount of inspiratiot1.2~ In this way the molecular property that we call Chirality becomes a vivid reality when we examine molecular models of enantiomers. But certain approximations are needed to yield this familiar picture of a molecule, that is, the classical model, and these need to be discussed next. Figure 2. A space-filling molecular representation is no more a molecule than Magritte’s pipe is a pipe. Reprinted with permission from B. P. Coppola, S. N. Ege, and R. G. Lawton, J. Chem. Educ. 1997, 74,84. Copyright 1997, American Chemical Society. 12 MOLECULAR Chirality I . The Classical Model Consider two enantiomorphous molecules, R (right-handed) and L (left-handed), isolated from their surroundings and from external fields but not from each other. That is, they are allowed to interact. In the quantum-mechanicaltreatment of this system, as two particles in a one-dimensional symmetric double-wellpotential, the two states are degenerate in energy and are related to wavefunctions YR and YL localized in the two potential wells. Superposition of these wavefunctions yields ground and first excited states Y+ and Y-: 1 Y+ = - (YR + Y,); a This superposition yields proper eigenstates of the underlying molecular Hamiltonian, that is, solutions of the time-independent Schrodinger equation ( Y R and YL are not eigenfunctions). A crucial difference between Yfl- and Y R W ~ is that the former are stationary states while the latter are not.
eBook - PDF- William H. Brown, Thomas Poon(Authors)
- 2017(Publication Date)
- Wiley(Publisher)
The signifi- cance of enantiomerism is that, except for inorganic and a few simple organic com- pounds, the vast majority of molecules in the biological world show this type of isomerism, including carbohydrates (Chapter 17), lipids (Chapter 19), amino acids and proteins (Chapter 18), and nucleic acids (DNA and RNA, Chapter 20). Further, approximately one‐half of the medications used in human medicine also show this type of isomerism. Enantiomers Stereoisomers that are nonsuperposable mirror images; the term refers to a relationship between pairs of objects. 6.1 6.2 6 . 2 What Are Enantiomers? 161 162 C H A P T E R 6 Chirality: The Handedness of Molecules As an example of a molecule that exhibits enantiomerism, let us consider 2‐butanol. As we go through the discussion of this molecule, we focus on carbon 2, the carbon bearing the OH group. What makes this carbon of interest is that it has four different groups bonded to it. The most common cause of enantiomerism among organic molecules is a carbon bonded to four different groups. OH CH 3 CHCH 2 CH 3 2-Butanol the four different “groups” bonded to this carbon are H, OH, CH 3 , and CH 2 CH 3 The structural formula we have just drawn does not show the shape of 2‐butanol or the orientation of its atoms in space. To do this, we must consider the molecule as a three‐ dimensional object. On the left are a ball‐and‐stick model of 2‐butanol and a perspective drawing of what we will call the “original” molecule. See Table 1.7 to review the meaning of the dashes and wedges in perspective drawings. Original OH C CH 2 CH 3 H CH 3 Mirror image C CH 3 CH 2 H CH 3 HO To the right in the preceding diagram is the mirror image of the original molecule. Every molecule and, in fact, every object in the world around us, has a mirror image.
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
mirror image of left hand left hand right hand One of the easiest ways to understand the concept of enantiomers is by looking at our hands. Figure 17.4 shows a left hand, its reflection in a mirror, and a right hand. Notice that the left hand’s reflection looks just like the right hand. That is, our left and right hands are mirror images of each other. Now look at your own hands. The left hand has four different fingers and a thumb in a particular order, the right hand has four different fingers and a thumb in the same order, but it is quite obvious that your left and right hands are not the same. Superimposability is the ability of two objects to fit ‘one within the other’ with no mismatch of parts. No matter how you move your hands, they are not superimposable. Just try putting a left-handed glove on your right hand (see figure 17.5). Objects that are nonsuperimposable on their mirror images, such as the shells in figure 17.6, are said to be chiral (pronounced ki-ral to rhyme with spiral, from the Greek word cheir meaning ‘hand’); that is, they show handedness. Chirality is encountered in three-dimensional objects of all sorts. A spiral binding on a notebook is chiral. A clockface is also chiral. As you examine the world around you, you will see that many objects are chiral (e.g. a ceiling fan, the thread of a screw, a computer keyboard). Even individual molecules can be chiral. Such molecules, stereoisomers that cannot be superimposed on their mirror images, are called enantiomers. 876 Chemistry FIGURE 17.5 A left-handed glove does not fit, that is, it is not superimposable, on the right hand. FIGURE 17.6 The next time you are at the beach, keep an eye out for a coiled shell. You will see that the coiling forms a spiral. Most coiled shells spiral in the same direction; with the shell apex pointing upwards, the shell’s opening is to the right, as in these examples.
eBook - PDF- William H. Brown, Thomas Poon(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
When we do this rotation, we see that all atoms and bonds of the mirror image fit exactly on the original. This means that the structures we first drew for the original and its mirror image are, in fact, the same molecule viewed from different perspectives: rotate about the C OH bond by 120° CH 3 C OH CH 3 H The original C OH CH 3 H CH 3 The mirror image rotated by 180° CH 3 C OH CH 3 H The mirror image 180° every single group bonded to this atom matches up with the corresponding groups in the mirror image (i.e., they are superposable) after manipulating the mirror image in space, we see that it is superposable on the original If an object and its mirror image are superposable, then the object and its mirror image are identical, and there is no possibility of enantiomerism. We say that such an object is achiral (without Chirality). Chiral From the Greek cheir, meaning hand; chiral objects are not superposable on their mirror images. Achiral An object that lacks Chirality; an object that has no handedness and is superposable on its mirror image. Charles D. Winters Left‐ and right‐handed sea shells. If you cup a right‐ handed shell in your right hand with your thumb pointing from the narrow end to the wide end, the opening will be on your right. 164 C H A P T E R 6 Chirality: The Handedness of Molecules An achiral object has at least one plane of symmetry. A plane of symmetry (also called a mirror plane) is an imaginary plane passing through an object and dividing it so that one‐ half of the object is the reflection of the other half. The beaker shown in Figure 6.2 has a single plane of symmetry, whereas a cube has several planes of symmetry. 2‐Propanol also has a single plane of symmetry. Plane of symmetry An imaginary plane passing through an object and dividing it such that one half is the mirror image of the other half. (a) (b) (c) Plane of symmetry Planes of symmetry CH 3 CH 3 OH H FIGURE 6.2 Planes of symmetry in (a) a beaker, (b) a cube, and (c) 2‐propanol.
eBook - ePub- C. Brown(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
1Chirality: Pharmacological Action and Drug Development
MARTHA HYNECK, JOHN DENT and JERRY B. HOOK, Research and Development, Smith Kline Beecham Pharmaceuticals, 709 Swedeland Road, King of Prussia, Pennsylvania 19406–2799, USAPublisher Summary
This chapter discusses that optical activity is caused by molecular asymmetry and that nonsuperimposable mirror-image structures results from this molecular asymmetry. There is a hypothesis that the chiral nature of compounds is because of the fact that carbon constituents can have a non-planar spatial arrangement giving rise to nonsuperimposable mirror images. Most naturally occurring medicinal agents exist in their optically active or single isomer form, such as quinidine and quinine, (-)-morphine, and (+)-digitoxin. However, many synthetic chemicals are produced as the optically inactive racemate. Because of potential pharmacological, pharmacokinetic, and toxicological issues, some scientists suggest that only single isomers should be considered for drug development and regulatory approval. Pharmacokinetic investigations into the disposition of enantiomers have enhanced the understanding of racemic drug action and have helped to understand previously inexplicable pharmacodynamic outcomes following administration of racemates to patients.Introduction
The field of stereochemistry has been developing since the early 1800s when Jean-Baptiste Biot, a French physicist, discovered optical activity in 1815. By the middle of the 19th century, Louis Pasteur had performed the first resolution of a racemic mixture, d - and l -tartaric acid. From this work Pasteur made the remarkable proposal that optical activity was caused by molecular asymmetry and that nonsuperimposable mirror-image structures resulted from this molecular asymmetry. Despite considerable scepticism within the community of chemists, scientists from several countries continued exploring this new field and with each new scientific contribution the relationship between optical activity and molecular asymmetry unfolded. By the end of the 18th century, Van’t Hoff of Holland and Le Bel of France strengthened Pasteur’s proposal by hypothesizing that the chiral nature of compounds was due to the fact that carbon constituents could have a non-planar spatial arrangement giving rise to nonsuperimposable mirror images (Drayer, 1988a
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
