Geometrical Isomerism

7 Key excerpts on "Geometrical Isomerism"

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  eBook - ePub

  Carbohydrate Chemistry

  Fundamentals and Applications

  • Raimo Alén(Author)
  • 2018(Publication Date)
  • WSPC

    (Publisher)

  3. Isomerism

  3.1.General

  A general molecular formula (e.g., Cx Hy Oz Nn ) expresses the kind and number of the constituent atoms of a compound, but it insufficiently represents the structure of the compound in question. A molecular formula can thus correspond to several compounds (

  isomers

  ) that normally have different chemical and physical properties. Isomers can be defined as chemical compounds with identical molecular formulas (i.e., contain the same number of atoms of each element) that differ from one another in the arrangements of their atoms. This phenomenon is called “

  isomerism

  ” (in Greek, “isos” means “equal” and “meros” “part”), and it is divided into two main types (Fig. 3.1 ): (i)

  constitutional isomerism

  or structural isomerism and (ii)

  stereoisomerism

  or space isomerism. Upon examining certain isomers (such as aldoses with the same number of carbon atoms), one does not necessarily find differences based on constitutional isomerism, and finding the actual differences requires detailed comparison of the stereoisomeric properties of the structures.

  Constitutional isomerism can be divided into three subgroups: (i)

  functional group isomerism

  (“function isomerism”), (ii)

  chain isomerism

  (“skeletal isomerism”), and (iii)

  position isomerism

  (“regioisomerism”), which are discussed in the next chapters with the help of illustrative examples. The general name “structural isomerism” is traditionally used for “constitutional isomerism”. However, since the structure of the compounds can be thought to cause all isomerism, the use of the former term is not recommended.

  Fig. 3.1.

     The main types of isomerism and their subtypes.

  The branch of organic chemistry that examines the three-dimensional structures of molecules,

  stereochemistry

  , has gained importance when striving to understand the physical and chemical properties of various compounds. In carbohydrate chemistry, it is also essential to know the stereochemical structure of the compounds. Stereoisomerism can be seen to generally represent the form of isomerism where compounds with the same chemical structure (i.e., the order of attachment of the atoms involved and the location of the bonds between them) differ from each other only in the spatial direction of their atoms or atom groups. This isomerism is divided into (i)

  optical isomerism

  (“physical isomerism”), (ii)

  conformational isomerism

  , and (iii)

  geometric isomerism

  (“

  cis

  /

  trans

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  eBook - ePub

  Introductory Organic Chemistry and Hydrocarbons

  A Physical Chemistry Approach

  • Caio Lima Firme(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  Chapter Thirteen

  Isomerism

  ISOMERISM AND TYPES OF ISOMERISM

  Isomerism gives rise to isomers that are molecules with the same chemical formula but different structural parameters or different spacial structures (different type of branching or different type of functional group or different position of the same functional group or different arrangement of substituents or different absolute configuration of the asymmetric atom).

  As for structural isomerism, there are three types: chain isomerism, position isomerism (or regioisomerism), and functional isomerism.

  The chain isomerism is related to the different position of the branching in its main chain. For example, butane and 2-methyl-propane are isomers (C4 H10 ); pentane, 2-methyl-butane and 2,2-dimethyl-propane are isomers (C5 H12 ); but-1-ene and 2-methyl propene are isomers (C4 H8 ) as well (see Fig. 13.1(A) ).

  Figure 13.1 Bond line formula of (A) chain isomers, (B) regioisomers, and (C) functional isomers.

  The position isomerism (or regioisomerism) is related to a different position of the substituent group or functional group in the molecule. For example, but-1-ene and but-2-ene are isomers (C4 H8 ), pentan-2-one and pentan-3-one are isomers (C5 H10 O), 2-chloro-propane and 1-chloro-propane are isomers (C3 H7 Cl), orto-dichlorobenzene and para-dichlorobenzene are isomers (C6 H4 Cl2 ) as well (see Fig. 13.1(B) ).

  Functional isomerism is related to different functional groups with the same molecular formula. For example, propanone and propanal are isomers (C3 H6 O), hexan-1-ene and cyclohexane (C6 H12 ) are isomers as well (see Fig. 13.1(C) ).

  GEOMETRIC STEREOISOMERISM

  Stereoisomerism is related to specific arrangements of substituents where two isomers are differentiated by their spacial disposition. Stereo means spacial.

  Geometric stereoisomerm occurs in alkenes or derivatives and in substituted cycloalkanes. They generate two isomers called cis and trans or E and Z.

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  eBook - ePub

  Understanding Advanced Organic and Analytical Chemistry

  The Learner's ApproachRevised Edition

  • Kim Seng Chan, Jeanne Tan;;;(Authors)
  • 2016(Publication Date)
  • WS EDUCATION

    (Publisher)

  CHAPTER 2

  Isomerism in Organic Compounds

  2.1 Introduction

  If a molecule has the molecular formula C4 H8 , does it imply that all the C4 H8 molecules are identical? The alkene, but-1-ene, whose molecule is shown below, has the molecular formula C4 H8 .

  Yet, C4 H8 also represents the formula for cyclobutane, which belongs to the cycloalkane family:

  Compounds that have the same molecular formula but different structures are known as isomers. This phenomenon is known as isomerism. The two main types of isomerism are constitutional/ structural isomerism and stereoisomerism. These are further divided into subclasses of which some are discussed in this chapter. Isomers generally have different physical and chemical properties, but they can also have similar chemical properties if they contain the same functional groups. Each specific functional group possesses a characteristic set of chemical reactions.

  2.2 Constitutional/Structural Isomerism

  Constitutional/structural isomers are compounds with the same molecular formula but different structures or structural formulae. Both but-1-ene and cyclobutane constitute a pair of constitutional/structural isomers. The difference in structures can be attributed to either a difference in the arrangement of atoms or due to the presence of different functional groups.

  Based on the above definitions, constitutional/structural isomerism can be classified into three main types:

  • chain isomerism;

  • positional isomerism; and

  • functional group isomerism.

  2.2.1 Chain Isomerism

  Compounds that exhibit chain isomerism with each other have the same functional group but differ in the way the carbon atoms are connected in the mainskeletal carbon chain of their molecules. In other words, these molecules differ in the degree of branching, hence the term chain isomers

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  Drug Stereochemistry

  Analytical Methods and Pharmacology, Third Edition

  • Krzysztof Jozwiak, W. J. Lough, Irving W. Wainer, Krzysztof Jozwiak, W. J. Lough, Irving W. Wainer(Authors)
  • 2012(Publication Date)
  • CRC Press

    (Publisher)

  2 Stereochemistry—basic terms and concepts Krzysztof Jo ´z ´wiak INTRODUCTION According to the International Union of Pure and Applied Chemistry (IUPAC) definition (1), stereoisomerism is a type of isomerism that arises from the differences in the spatial arrangement of atoms without any differences in connectivity or bond multiplicity between the isomers. One of the most impor-tant branches of stereochemistry is related to molecular dissymmetry and to the study of chiral molecules. The terminology used to describe stereochemical relationships is often a maze of interchangeable terms (capital D ’s and L ’s, lowercase d ’s and l ’s, mixed in with R ’s, S ’s, ( þ )’s, and ( )’s, to name a few). It is therefore appropriate to address basic stereochemical terms and concepts to lay a foundation for the more technical discussions that follow. This is not meant to be an in-depth treatment of this topic; there are many fine texts on the subject (2,3), which may be consulted if more detailed understanding is required. The chapter is the update to work previously published by I.W. Wainer and A.A. Marcotte in the 2nd edition of this book. SYMMETRY AND DISSYMMETRY Symmetry or the lack of it is one of the interesting features of geometric figures with two or more dimensions. Dissymmetry is very common in real life: it may often be confronted without it being immediately apparent. On the other hand, dissymmetry may become painfully apparent when, for example, someone has to switch from driving on one side of the road to the other while passing the English Channel from France to England or vice versa. The Latin alphabet is very good example containing both symmetrical and asymmetrical two-dimensional letters, some of which have different appearances when they are reflected in a mirror. Six letters and their mirror images are presented in Figure 2.1.

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  Klein's Organic Chemistry

  • 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.

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  eBook - PDF

  Organic Chemistry

  • David R. Klein(Author)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  Such compounds are called stereoisomers, 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) Take the DO YOU REMEMBER? QUIZ in to check your understanding. 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.

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  Mathematical Chemistry and Chemoinformatics

  Structure Generation, Elucidation and Quantitative Structure-Property Relationships

  • Adalbert Kerber, Reinhard Laue, Markus Meringer, Christoph Rücker, Emma Schymanski(Authors)
  • 2013(Publication Date)
  • De Gruyter

    (Publisher)

  The decisive formulation is “if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself”. In mathematical terms this means that we cannot transform the molecule in space by a translation and/or a rotation into its mirror image. Translations and rotations are particular isometries , i.e. the distances between any two points in space are maintained. As translations do not really matter in our situa-tion, we restrict our attention to linear isometries, i.e. to isometries that fix the origin of 3D space. The reason is that we can embed a molecule in 3D space so that the ori-gin of the space coincides with the molecule’s barycenter, which is fixed under every symmetry operation. It turns out that linear isometries are compositions of rotations and reflections, orthogonal linear mappings where the representing matrix has deter-minant ±1 . The determinant of a proper rotation is +1 , that of an improper rotation −1 . Reflections are improper rotations. Applying two improper rotations to a molecule yields ‘the same molecule’, so we can speak of the mirror image as the result of reflection at any plane. The reason is that two improper rotations constitute a proper rotation, a linear mapping of determinant +1 , and, by definition, we consider molecules in space as equal if they arise from each other by a translation plus a proper rotation. Thus, chirality can be considered as an 92 | 3 Chirality equivalence relation that collects the molecules in equivalence classes consisting ei-ther of a single molecule (a self-enantiomorph ) or of two enantiomorphs , i.e. molecules that are mirror images of each other. Molecules belonging to equivalence classes of or-der 2 are called chiral , the elements of an equivalence class of order 1 are achiral . This needs a mathematical description and a discussion of the terms left and right in this context. Strictly speaking, chirality is this equivalence relationship.

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