Positional Isomers

12 Key excerpts on "Positional Isomers"

• 

  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

  Basic Stereochemistry of Organic Molecules

  • Rose Marie O. Mendoza(Author)
  • 2020(Publication Date)
  • Arcler Press

    (Publisher)

  In addition to it, quantitative information on the conformational behavior of molecules in solution is also provided by the study of the temperature dependence of NMR spectra. 1.3. ISOMERS AND STEREOCHEMISTRY Isomers are defined as compounds with the same molecular formula but differ in structure. Isomers can be further classified as structural or constitutional and stereoisomers. Structural isomers are substances (with the same molecular formula) but differ in the arrangement or placement of atoms in the compound, which will make their condensed structure different from each other. The changes connectivity of the carbon atoms is also one of the distinct features of constitutional isomers. In stereoisomers, the atoms are connected sequentially in the same way, such that condensed formulas for the two molecules are identical. The isomers differ, however, in the way the atoms are arranged in space. There are two major sub-classes of stereoisomer; conformational isomers, which interconvert through rotations around single bonds, and configurational isomers, which are not readily interconvertible. Chiral compounds, on the other hand, are defined as the compounds which are having the same molecular and condensed structural formula, but vary in the orientation of four distinct groups that are around the atom of carbon. These are chiral compounds and geometric isomers or isomeric compounds that are present in the orientation of atoms on a double bond or ring system. Often, they are referred as stereoisomers since they belong to both categories (Figure 1.3). Basic Stereochemistry of Organic Molecules 8 Figure 1.3. Classification of isomer in stereochemistry . Source: Image by Wikimedia commons. Conformational analysis studies about the different orientations that can be achieved by the molecules and the way their energy changes as their atoms are rotated in space, therefore, becomes important since these will draw the distinction between one molecule over the other.

  Sign up to read

  Learn more about book
• 

  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.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Organic Chemistry

  A Mechanistic Approach

  • Penny Chaloner(Author)
  • 2014(Publication Date)
  • CRC Press

    (Publisher)

  221 7.1 INTRODUCTION We have previously discussed the existence of isomers of organic compounds—compounds that have the same molecular formula but a different construction. Isomers can be divided into two types—structural isomers and stereoisomers. For example, n-butane (7.1) and 2-methyl propane (7.2) are structural isomers—the atoms are joined together in a different way. Structural isomers are described as having a different constitution. 7.1 7.2 Stereoisomers have the same constitution, with all the same atoms joined together by the same type of bonds but a different shape. The existence of stereoisomers depends on the theory of organic structure and the fact that chemical bonds have directional properties. Stereoisomers can be further divided into two mutually exclusive groups: enantiomers and diastereoisomers (sometimes elided as diastereomers). Enantiomers are distinguishable molecules related as object and mirror image. Diastereoisomers are all other stereoisomers. Although there are many computer programs allowing you to generate molecules on screen and move them around in space, physical models are also useful in understanding stereochemistry. “Ball and stick” models are useful to get an overall picture of a molecule, including the relative sizes of the atoms, but they tend to be quite expensive (Figure 7.1). Skeletal models allow you to see just the bonds in a molecule but give no idea of steric effects. The advantage is that they are generally quite inexpen- sive (Figure 7.2). “Space-filling” models focus attention on the atoms, rather than the bonds; they are excellent for studying steric effects, giving a clearer view of atom size (Figure 7.3). These are sometimes known as Corey, Pauling, Koltun (CPK) models where atoms are represented as overlapping spheres, with radii genuinely representative of atomic radii. They are frequently used to study biological interac- tions, such as the docking of drugs at receptors.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Pharmacology

  A Handbook for Complementary Healthcare Professionals

  • Elaine Mary Aldred(Author)
  • 2008(Publication Date)
  • Churchill Livingstone

    (Publisher)

  Figure 6.2(i) ) occur because functional groups can branch off from the main carbon backbone. The number of carbon and hydrogen atoms remains the same but the structures are very different.

  Figure 6.2 The different forms of structural isomerism. (i) Chain (ii) positional (iii) mixture of chain and positional.

  Positional Isomerism

  In Positional Isomers (Figure 6.2(ii) ) the structure of the carbon backbone remains the same but the functional groups or side chains are moved around the backbone.

  Combinations

  It is possible to have a mixture of chain and positional isomerism (Figure 6.2(iii) ). Positional isomerism can occur around an aromatic ring (see Figure 5.3 , p. 31 ), hence the need for precise nomenclature.

  Stereoisomers

  It is at this point that thinking in three dimensions becomes important. Stereoisomers are isomers whose components are connected around the same point, but whose arrangement in space is different.

  Entaniomers

  Enantiomers are stereoisomers that are mirror images of each other and all thecomponents of the compound radiating from one point are different. The term forthis is chiral . The position of the compound around which this occurs is a chiral centre (see Figures 6.3 , and Figures 6.7 ). Take, for example, natural alanine, which is foundin only one isomeric form: L -alanine ( Figures 6.3 ). If alanine is produced synthetically, then both forms L and D alanine, are found. This is signifi cant when it comes togiving a patient nutritional supplementation, as the L isomer is more likely to fit intothe receptor site of an enzyme, which explains why natural products are favoured innutritional supplementation.

  Figure 6.3 The two isomers of alanine.

  • What exactly do D and L mean?

  Light normally vibrates in all directions, but if put through a polarizer it will vibrate in only one direction (Figure 6.4(i) ). If another polarizer is placed at exactly 90°, no light will get through. You can test this yourself with two pairs of good-quality sunglasses. Line them up, and then turn one through 90°. The lens will appear black because no light is getting through (Figure 6.4 (ii)

  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 - 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)

  The properties of the last two compounds are similar, but differ markedly from those of the first. The simple examples given above involve relatively few atoms per mole-cule. When larger numbers of atoms are involved, many more isomers may be possible. For example, C 6 H 14 has five isomers, while C 3 0 H 62 has sev-eral million. (a) (b) (c) I S O M E R I S M A N D F O R M U L A E 223 Stereoisomerism Stereoisomerism occurs when the same atoms can be bonded in the same way, but with different spatial arrangements in the different molecules. When a single covalent bond is formed between two atoms, the line of the bond can be considered as an axis of rotation. Rotation can occur about this axis, and part of a substance's kinetic energy is kinetic energy of rotation about the bonds in the molecules. Consider a three carbon unit with the carbon atoms bonded by single bonds, as shown in Fig. 50. If carbon atom C x is considered to be fixed, the effect of rotation about bonds (a) and (b) is that carbon atom C 3 can be anywhere on the dotted locus shown. Whether or not continuous rotation occurs depends on the kinetic energy of the system, and on the nature and size of the forces opposing the rotation. When a double covalent bond is formed between two atoms there is no free rotation about the line of the bonds. If two carbon atoms in the sp 2 hybrid state form a double bond, the resultant unit is planar, and because of the lack of free rotation stereoisomerism can occur. Consider the struc-The relative positions of the chlorine atoms in the two molecules is dif-ferent, and because they are fixed by the lack of rotation about the double bond, the two structures represent different molecules. Isomers of this type, which result from different arrangements of the atoms or groups of atoms bonded to carbon atoms joined by double bonds, are known as geometric isomers. Difference in the properties of this type of isomer is usually small.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Introduction to Chemical Nomenclature

  • R. S. Cahn, O. C. Dermer(Authors)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Stereoisomerism Stereochemistry is concerned with the arrangement in space of the constituent atoms of molecules, and those features of it that lead to existence of isomers constitute stereoisomerism^ Many diverse situations are involved but the nomenclature has been greatly simplified by the ability, acquired only recently, to assign absolute configurations^ and by the promulgation of the sequence rule^ which permits the names of stereoisomers to be differentiated in most situations in organic chemistry. Methods for handling many of the simpler steric relations have been codified by lUPAC* and have proved generally acceptable. However, a new dimension has been introduced by publication^ of methods recently introduced into CA Collective Subject Indexes; its most important features are included in this chapter (see p. 140). Full information on this and other aspects of stereochemical nomenclature can be found in the literature cited. First, however, we should define a few words that are not involved in actual names of compounds but are part of the required vocabulary. There have been variations but we shall follow IUP AC. The term 'structure' is allotted very wide application by lUPAC - to any aspect of the organization of matter ^cf atomic structure, electronic structure, structure of benzene). 'Constitution' is the term to be used to denote the nature and sequence of bonding of atoms in a molecule. Compounds having identical molecular formulas but differing in the nature or sequence of bonding or in arrangement of atoms in space are termed 'isomers'. Isomers differing in the nature or sequence of bonding are 'constitutional isomers' (e.g., 1/2); those differing only in the arrangement of their atoms in space are 'stereoisomers' (e.g., 3a/3b; 4a/4b). The adjective 'stereogenic' is applied to an atom or group in a molecule when interchange of two of the atoms or groups attached to it produces a non-identical compound. 'Stereoparent' is the name 128

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Foundations of Chemistry

  An Introductory Course for Science Students

  • Philippa B. Cranwell, Elizabeth M. Page(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  E and Z isomerism and chirality are both types of stereoisomerism. 12.3.1 Chain isomerism Chain isomerism occurs when the longest chain length in an alkane varies. For example, butane and 2-methylpropane both have the same molecular formula, C 4 H 10 , but the atoms have different connectivities (Figure 12.7). Structural isomers have the same molecular formula but a different spatial arrangement of atoms. 12.3 Isomers 413 In this case, the different arrangement of atoms has a large impact upon the physical properties of butane and 2-methylpropane, although their chemical reactivity is very similar. Butane has a melting point of -138 C and boiling point of -0.5 C, whereas 2-methylpropane has a melting point of -160 C and boiling point of -12 C. The reason for the difference is that in butane, there are stronger intermolecular forces between molecules because they can align much more closely (Figure 12.8). 12.3.2 Positional isomerism Positional isomerism means that, in a homologous series of compounds, the same functional group is present but is located in different positions along a chain. This type of isomerism is commonly encountered with alcohols. Figure 12.9 shows the two Positional Isomers for butanol: butan-1-ol and butan-2-ol. Although butan-1-ol and butan-2-ol are Positional Isomers, they are still members of the same homologous series (alcohols) and have similar chemical reactivity but different physical properties. 12.3.3 Functional group isomerism With functional group isomerism, the molecular formula is the same, but the functional groups present are different. This can have a major impact on both the chemical reactivity and physical properties of a compound. For example, two compounds have molecular formula C 2 H 6 O: ethanol and methoxy-methane, containing an alcohol and an ether, respectively (Figure 12.10). butane 2-methylpropane Figure 12.7 Chain isomers of C 4 H 10 . (a) (b) Figure 12.8 (a) Butane molecules; (b) 2-methylpropane molecules.

  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)

  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 CH 2 (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 sp 2 -hybridized carbon with two methyl groups and another one with two hydrogen atoms attached, whereas cis - and trans -2-butene have two sp 2 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. Figure 2.1 Examples of various types of isomers. Stereoisomerism at Saturated Centers In the preceding figure, cis - and trans -2-butene were shown to be stereoisomers by virtue of the spatial arrangement of groups around the double bonds. Stereoisomerism, however, can also occur at saturated centers. An example is illustrated in Figure 2.2. Structure A on the left has a carbon that is attached to H, OH, CH 2 OH, and CHO. As drawn, the central carbon, CHO, and CH 2 OH groups are in the plane of the paper, the H is behind the plane (depicted using a hashed wedge), and the OH is in front of the plane (solid wedge)

  Sign up to read

  Learn more about book