Structural Isomerism

10 Key excerpts on "Structural Isomerism"

• 

  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

  Sign up to read

  Learn more about book
• 

  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

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Chemistry for Pharmacy Students

  General, Organic and Natural Product Chemistry

  • Lutfun Nahar, Satyajit Sarker, Professor Satyajit D. Sarker(Authors)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  Louis Pasteur, a well‐known French biologist, chemist, and microbiologist, is the first stereochemist, who observed that salts of tartaric acid collected from wine production vessels could rotate plane polarized light, but that salts from other sources did not; this observation was made in 1849. Before we go into further details, let us have a look at different types of isomerism that may exist in organic molecules.

  3.2 ISOMERISM

  Compounds with the same molecular formula but different structures are called isomers. For example, 1‐butene and 2‐butene have the same molecular formula, C4 H8 , but structurally they are different because of the different position of the double bond. There are two types of isomers: constitutional isomers and stereoisomers.

  3.2.1 Constitutional Isomers

  When two different compounds have the same formula but different connections between atoms, they are called constitutional isomers. To determine whether two molecules are constitutional isomers, simply count the number of each atom in both molecules and see how the atoms are arranged. If both molecules possess the same count for all of the different atoms, and the atoms are arranged in different ways, the molecules will be considered as constitutional isomers. For example, ethanol and dimethylether have the same molecular formula, C2 H6 O, but they differ in the sequence of bonding. Similarly, butane and isobutane are two constitutional isomers. Constitutional isomers generally have different physical and chemical properties.

  3.2.2 Stereoisomers

  Stereoisomers are compounds that have the same molecular formula and the same connection between atoms, but differ in the arrangement of atoms in the space, that is, in three dimensions (3D). For example, in α‐glucose and β‐glucose, the atoms are connected in the same order, but the 3D orientation of the hydroxyl group at C‐1 is different in each case. Similarly, cis‐ and trans‐cinnamic acid only differ in the three dimensional orientation of the atoms or groups.

  There are two major types of stereoisomers: conformational isomers and configurational isomers

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Principles and Applications of Stereochemistry

  • Michael North(Author)
  • 2017(Publication Date)
  • Routledge

    (Publisher)

  1.5 respectively. In general, these three isomers will undergo the same types of chemical reaction (benzylic oxidation for example), but will do so at different rates, again illustrating their different chemical properties.

  There is, however, another type of isomerism, one in which all of the atoms in the two isomers do have the same connectivity. A familiar example is found in 1,2-disubstituted alkenes such as compounds 1.6 and 1.7 . In both of these isomeric compounds, the order in which the carbon atoms are joined together is C𝟙–C𝟚=C𝟛–C𝟜 and the only difference between them is that in isomer 1.6 the two methyl groups are on the same side of the double bond, whilst in isomer 1.7 the two methyl groups are on opposite sides of the double bond. Any pair of isomers which have the same connectivity of their atoms but which differ in the relative orientation of those atoms are called stereoisomers.

  Stereoisomers are the topic of this book and the following chapters will investigate the different structural features which are responsible for stereoisomerism, and discuss the chemical, biological and physical consequences of the formation of stereoisomers. Both organic and inorganic compounds can exhibit stereoisomerism, and examples of each will be found throughout this book. Essentially, stereochemistry is concerned with the shapes of molecules, and the consequences of a molecule adopting a particular shape.

  Later in this chapter, the way in which the shape of a molecule may be predicted using Valence Shell Electron Pair Repulsion Theory (VSEPR) will be introduced, and the nature of the bonding found in the most common chemical structures will be discussed. At the end of this chapter, the various classifications of stereoisomers will be introduced and these will be discussed in more detail throughout the remainder of this book. However, many of the structures seen later in this chapter are three dimensional, and before they are discussed it is necessary to understand the conventions used when representing three dimensional structures on a two dimensional piece of paper.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Introduction to General, Organic, and Biochemistry

  • Morris Hein, Scott Pattison, Susan Arena, Leo R. Best(Authors)
  • 2014(Publication Date)
  • Wiley

    (Publisher)

  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. Isomerism is the phenomenon of two or more compounds having the same number and kinds of atoms. (See Section 19.7.) There are two types of isomerism. In the first type, known as struc- tural isomerism, the difference between isomers is due to different structural arrangements of the atoms that form the molecules. For example, butane and isobutane (C 4 H 10 ), ethanol and dimethyl ether, (C 2 H 6 O) and 1-chloropropane and 2-chloropropane (C 3 H 7 Cl) are structural isomers: CH 3 CH 2 CH 2 CH 3 CH 3 CH 2 OH CH 3 CH 2 CH 2 Cl butane ethanol 1-chloropropane CH 3 OCH 3 CH 3 CHClCH 3 methoxymethane 2-chloropropane (dimethyl ether) In the second type of isomerism, the isomers have the same structural formulas but differ in the spatial arrangement of the atoms. This type of isomerism is known as stereoisomerism. Thus, compounds that have the same structural formulas but differ in their spatial arrange- ment are called stereoisomers. There are two types of stereoisomers: cis–trans or geometric isomers, which we have already considered, and optical isomers, the subject of this chapter. One outstanding feature of optical isomers is that they have the ability to rotate the plane of plane-polarized light. Stereoisomerism exists in all biological systems. (See Table 26.1.) KEY TERMS stereoisomerism stereoisomer LEARNING OBJECTIVE LEARNING OBJECTIVE CH 3 CHCH 3 CH 3 2-methylpropane (isobutane) 26.2 PLANE-POLARIZED LIGHT AND OPTICAL ACTIVITY Describe plane-polarized light and how chiral carbons demonstrate optical activity. Plane-polarized light is light that is vibrating in only one plane.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  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.

  Sign up to read

  Learn more about book
• 

  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.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Chemistry for Technologists

  The Commonwealth and International Library: Electrical Engineering Division

  • G. R. Palin, N. Hiller(Authors)
  • 2014(Publication Date)
  • Pergamon

    (Publisher)

  SECTION V ORGANIC CHEMISTRY This page intentionally left blank CHAPTER 13 Isomerism and Formulae Multiplicity of Organic Compounds The ability of carbon atoms to bond with one another in large numbers, as well as forming bonds with atoms of other elements, has been mentioned in Chapter 1. It was also mentioned that this results in there being an enormous number of carbon containing compounds. Where oxygen only forms two stable compounds with hydrogen, carbon forms millions. Where hydrogen, nitrogen and oxygen form three stable compounds, hydro-gen, nitrogen and carbon form millions, and so on. This has led to the separation of the study of carbon containing compounds from the study of the compounds of the other elements. It has also led to a somewhat different technique for naming compounds and writing formulae, which is necessary because of the widespread occurrence of isomerism amongst organic compounds. Isomers can be defined as compounds with the same molecular formula, but differing in one or more of their chemical or physical properties. Iso-merism does occur in inorganic compounds, but only to a limited extent, and the number of isomers corresponding to a given molecular formula is small. Isomerism occurs in all but the simplest organic compounds, and the number of isomers can be very large. For instance, the relatively simple molecular formula C 8 H 18 applies to eighteen different isomers. Structural Isomerism Structural Isomerism occurs when the chemical bonds holding the atoms together in the different molecules, are different. One of the simplest examples of this type of isomerism occurs in the two compounds having the molecular formula C 2 H e O . In both, the carbon is in the sp 3 hybrid state, i.e. it forms four single covalent bonds, tetrahedrally orientated. The oxygen atom forms two covalent bonds, involving the two unpaired 2p electrons, and each hydrogen atom forms one covalent bond, involving the unpaired Is electron.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Organic Chemistry Concepts and Applications for Medicinal Chemistry

  • Joseph E. Rice(Author)
  • 2014(Publication Date)
  • Academic Press

    (Publisher)

  Chapter 2

  The Three-Dimensional Structure of Organic Compounds

  Abstract

  This chapter opens with a discussion of the various types of isomers leading into a more detailed look at stereoisomerism. The concept of optical isomers at saturated asymmetric centers is then introduced along with a discussion of the properties of enantiomers and the methods used to designate absolute configuration at such stereocenters. Diastereomers are then discussed as being stereoisomers that are not enantiomers. The idea of prochirality is introduced and examples of saturated prochiral centers and unsaturated prochiral faces are given. Stereoisomerism at unsaturated centers is then discussed along with a system to designate those isomers. The last section deals with conformations, beginning first with acyclic systems and then progressing to cyclohexanes, and finally to five-, four-, and three-membered saturated rings.

  Keywords

  Absolute configuration ; Chiral center ; Conformation ; Diastereomer ; Dihedral angle ; Eclipsed ; Enantiomer ; Isomer ; Nonbonded interactions ; Prochiral

  One of the critical factors associated with drug–receptor interactions is the three-dimensional structure of both the drug and receptor molecules. Efficient interaction between a drug and its receptor depends not only on the absolute and relative positioning of groups in space but also on the ability of the molecule to modify its shape by rotating about single bonds. These aspects of chemical structure are known as stereochemistry and conformation and are the topics covered in this chapter.

  Isomers

  Two or more compounds that have the same empirical formula but differ with respect to how the various atoms are joined are called isomers . Isomers are broken down into two broad categories. The first is constitutional isomers —compounds that differ in constitution or make-up. Thus cyclopropane and propylene (Figure 2.1 ) are constitutional isomers because cyclopropane is composed of three CH2 (methylene) groups arranged into a three-membered ring, whereas propylene is acyclic with a vinyl and a methyl group. The alcohols n -propanol and i -propanol are also constitutional isomers that are similar chemically but differ in the position to which the hydroxyl group is attached (also called positional isomers ). Thus n -propanol has the hydroxyl group attached to methylene with an ethyl group attached to the same carbon. In contrast, i -propanol has the OH group attached to a CH (methine) that is also attached to two methyl groups. Another example of this is seen with isobutylene and cis - or trans -2-butene. Isobutylene has one sp2 -hybridized carbon with two methyl groups and another one with two hydrogen atoms attached, whereas cis - and trans -2-butene have two sp2 carbons each with one methyl group and one hydrogen atom. Thus, isobutylene and cis - or trans -2-butene are constitutional isomers. But what is the relationship between cis - and trans -2-butene? Since they have the same constitution but differ in the special arrangement of the various groups, they are called stereoisomers

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Environmental Chemistry

  • John Wright(Author)
  • 2004(Publication Date)
  • Routledge

    (Publisher)

  trans.
• A chiral centre is one containing an asymmetric carbon atom and gives rise to stereoisomerism called optical isomerism. Optical isomers are optically active.
• Optically active chemicals can rotate the plane of polarised light. Optical isomers are non-superimposable on each other. By convention (+) isomers rotate the plane of polarised light clockwise, and (-) isomers rotate it to the left.
• Organic molecules can undergo a range of chemical reactions according to the functional groups present, e.g. oxidation, reduction, addition, condensation, hydrolysis and substitution reactions.

Questions

1. From the pairs of molecules listed below, which
   1. (a) belong to the same homologous series
   2. (b) are position isomers
   3. (c) are functional isomers
   4. (d) are skeletal isomers
   5. (e) are identical molecules
   6. (f ) are none of the above.
      1. (i) CH3 CH2 CH2 OH
      2. (ii) CH3 CHOHCH3
      3. (iii) CH3 CHOHCH3
      4. (iv) (CH3 )2 CHOH
      5. (v) CH3 OCH3
      6. (vi) CH3 CH2 OH
      7. (vii) (CH3 )2 CHCH3
      8. (viii) CH3 CH2 CH2 CH3
      9. (ix) CH3 CH2 CH2 OH
      10. (x) CH3 OCH3
      11. (xi)(CH3 )2 CHCH2 Br
      12. (xii)CH3 CH2 CHCH3 CH2 Br
2. An organic material, containing one carbon–carbon double bond has the molecular formula C3 H5 Cl. Draw all the possible structural formulae.
3. Using the compound from (2) above, identify the one compound that can exhibit geometric isomerism.
4. An ester, X, has the molecular formula C4 H6 O2 . It is suspected to have one carbon–carbon double bond in its structure.
   1. (a) Suggest a test that could be used to show that this compound contains a carbon–carbon double bond.
   2. (b) There are five possible structures for ester X. One of these structures exists as a pair of geometric isomers. Draw the five structures, showing which one exists as a pair of geometric isomers.
   3. (c) If the five possible esters are hydrolysed, what five alcohols will be formed? Which pair of alcohols shows positional isomerism?