Conformational Isomers

10 Key excerpts on "Conformational Isomers"

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  eBook - ePub

  Organic Chemistry

  Concepts and Applications

  • Allan D. Headley(Author)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  constitutional isomers. Since structural isomers are different compounds, they have different properties, such as density and melting points.

  Problem 4.1

  1. Give structural formulas for all possible structural isomers of C5 H12 .
  2. Give IUPAC names for the isomers of question (i) above.

  4.3 Conformational Isomers of Alkanes

  At room temperature, rotation about a carbon–carbon single bond of molecules occurs freely. For some molecules that have bulky groups bonded to adjacent carbons of a carbon–carbon single covalent bond, rotation is a bit more difficult compared to molecules that have smaller groups bonded to similar adjacent carbon atoms. Owing to the restricted rotation that results around the carbon–carbon bond of such a molecule that has bulky groups, the bulky groups spend more time in specific regions of the molecule, typically as far away as possible from each other, compared to being closer to each other resulting in different conformations of the molecules. The terms conformers and rotamers are used to describe these types of isomers. By definition, conformers are isomers with the same atom connectivity but have different arrangements of specific groups about a single bond, which comes about due to rotation about a carbon–carbon single bond. The term isomeric conformer is used to describe conformers in which groups around a carbon–carbon single bond are in different locations. The illustration of this three‐dimensional concept on two‐dimensional paper can be challenging and specific representations must be used to convey the orientation of the different Conformational Isomers. There are two types of representations that are typically used to represent conformers in organic chemistry: dashed/wedge and the Newman projection

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  Chemistry for Pharmacy Students

  General, Organic and Natural Product Chemistry

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

    (Publisher)

  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. Configurational isomers include optical isomers, geometrical isomers, enantiomers and diastereomers.

  3.2.2.1 Conformational Isomers

  Atoms within a molecule move relative to one another by the rotation around covalent single bonds (σ bonds) and the 3D tetrahedral shape of the sp 3 ‐hybridized centres. Such rotation of covalent bonds gives rise to different conformations of a compound. Conformers may also result from restricted rotations in rings. The number of different conformers depends on the number of single bonds and on the number and size of the flexible rings. Each structure is called a conformer or conformational isomer.

  Generally, conformers rapidly interconvert at room temperature.

  cssStyle="font-style:italic;" 3.2.2.1.1 Conformational Isomers of Alkanes

  Conformational isomerism can be presented with the simplest example, ethane (C2 H6 ), which can exist as an infinite number of conformers by the rotation of C─C σ bond. Ethane has two sp 3 ‐hybridized carbon atoms and the tetrahedral angle about each is 109.5°. The most significant conformers of ethane are the staggered and eclipsed conformers. The staggered conformation is the most stable as it is of the lowest energy.

  cssStyle="font-style:italic;" 3.2.2.1.2 Visualization of Conformers

  There are four conventional methods for visualization of 3D structures on paper, that is, the ball and stick method, sawhorse method, wedge and broken line method and Newman projection method. Using these methods, the staggered and eclipsed conformers of ethane can be drawn as follows.

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  Stereochemistry

  Basic Concepts and Applications

  • M. Nógrádi(Author)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  Different conformers of a given compound can arise from stereoisomers through rotation around single bonds. It should be recalled that configuration and conformation are complementary concepts; configuration is a qualitative and conformation a quantitative term. A compound of a given configuration may assume many conformations each characterized by different sets of dihedral angles. Conformations, in turn, may be aehiral or chiral, and to the latter configurational symbols may be assigned. Energy relationships may be excluded from the discussionof configuration, whereas with conformation this is impossible. If this were not so, geometrical isomers which arise from rotation around double bonds, a process which involves considerable energy, would have to be classified as conformers. As stated above, conformations can be described in terms of torsion angles, and certain stable conformations are usually referred to as conformers. Special names are assigned to some conformers. One of the objects of conformational analysis is to select from the infinite number of possible conformations for a given compound those which are 93 relatively stable. In order that we may assess the conformational behaviour of complex molecules, some simple molecules such as ethane, n-butane, and cyclohexane will first be analyzed. 1.2.1 The conformation of ethane. Torsional strain When two non-bonding atoms approach each other a weak attraction is first set up between them, but once the distance is less than that correspond-ing to the sum of the van der Waals' radii the atoms experience a sharply increasing repulsion (cf. Fig. 1). For this reason the closest distance of ap-proach between non-bonding atoms is the sum of their van der Waals' radii.

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  DAT Prep Plus 2023-2024

  2 Practice Tests + Proven Strategies + Online

  • (Author)
  • 2023(Publication Date)
  • Kaplan Test Prep

    (Publisher)

  meso-tartaric acid has two chiral carbon atoms, the lack of optical activity is a function of the molecule as a whole.

  Conformational Isomers

  Conformational Isomers are compounds that differ only by rotation about one or more single bonds. These isomers represent the same compound in a slightly different position—analogous to a person who may be either standing up or sitting down. These different conformations can be seen when the molecule is depicted in a Newman projection, in which the line of sight extends along a carbon-carbon bond axis. Because the view of the second carbon atom is blocked by the first, the second carbon is depicted as a large circle at the back, and the first carbon is depicted as the intersection of its three other bonds in front. The different conformations are encountered as the molecule is rotated about the axis between the two carbons. The classic example for demonstrating conformational isomerism in a straight chain molecule is n-butane. In a Newman projection, the line of sight extends through the C−2-C−3 bond axis.

  Figure 40.13

  Straight-chain conformations

  The most stable conformation of n-butane is found when the two methyl groups (C–1 and C–4) are oriented 180° from each other. There is no overlap of atoms along the line of sight (besides C–2 and C–3), so the molecule is said to be in a staggered conformation. Specifically, it is called the anti conformation, because the two methyl groups are antiperiplanar to each other. This particular orientation is very stable and thus represents an energy minimum because all atoms are far apart, minimizing repulsive steric interactions.

  The other type of staggered conformation, called gauche, occurs when the two methyl groups are 60° apart. In order to convert from the anti to the gauche

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  Conformational Concept For Synthetic Chemist's Use: Principles And In Lab Exploitation

  Principles and in Lab Exploitation

  • Anatoly M Belostotskii(Author)
  • 2015(Publication Date)
  • WSPC

    (Publisher)

  Ignoring the contradiction with all conformational definitions, these chemical isomers are termed there conformers. This practice is absolutely prevalent; organic chemists consider interconversions, which are accompanied by disruption or formation of intramolecular Hbond(s), as conformational transformations. Pseudotropine (22a), biotine (22b), and calix[4]arenes 23 (Fig. 49) are simple demonstrative examples of numerous organic compounds, for which changes of molecular geometry are classified as conformational transformations, although these changes include formation or dissociation of H-bond(s). Intramolecular H bonds in peptides, proteins, oligosaccharides and nucleic acids are considered as a structural attribute of certain conformers or a family of similar conformers (in terms of bioorganic chemists, structures of the same conformational motif). For instance, intramolecular assembling or “melting” of two-strand domains (so-called hairpins) in single stranded RNAs or DNAs is caused by formation or dissociation, respectively, of many intramolecular H bonds between complementary nucleotide sequences that are fragments of the nucleic acid chain. The remarkable clover leaf structure of tRNAs (transfer RNAs; Fig. 49) is an example of such chemical structures. Nevertheless, it is frequently called conformation, in parallel to the correct term the secondary structure that means the biopolymer-specific chemical structure that results from chemically bonding molecular fragments separated by other fragments in the polymer chain. The mentioned flipping of the nucleobases in DNA duplexes (i.e., rotation around the glycoside bond in nucleotides that interconverts their anti and syn rotamers; Section 2.5) is also associated with dissociation or formation of H-bonds. As the reader may guess, biochemical literature uses the term conformational in characterizing this fast intramolecular transformation

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  Understanding Advanced Organic and Analytical Chemistry

  The Learner's ApproachRevised Edition

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

    (Publisher)

  Take, for instance, cyclopropane. For a carbon atom bonded to four substituents, repulsion between electron clouds is minimal at bond angles of 109.5° (as in a tetrahedral configuration). But when three of such carbon atoms are forced together into a ring, the C–C bond angles are acute and are less than the ideal 109.5° (What is the internal angle of an equilateral triangle?). In order to assume the triangular ring structure, the orbitals of two adjacent carbon atoms would not be able to overlap exactly head-on with each other to form the sigma bond. In chemical bonding terms, we say that the overlap is less effective. Consequently, the bond energy is less endothermic, indicating a weaker bond strength.

  Even when we talk about saturated alkanes such as ethane, rotation about a single bond is not smooth sailing.

  2.3.2 Conformational Isomerism

  Conformational isomerism is best understood when one acquires the skill of visualising the spatial orientation of groups of atoms around a central carbon atom using the Newman projection method. In this scheme, the spatial orientation of substituents about a carbon– carbon bond is visualized from the front-to-end view about this bond. Take ethane, for instance. Its Newman projection has C1 depicted as a point of intersection of three C–H bonds, and C2 is represented by the circle at the rear (see Fig. 2.1 ).

  Rotating only one of the C atoms of the carbon–carbon bond will bring the H substituents into different spatial orientation and hence different proximity to one another. On the left side of Fig. 2.1 is the staggered conformation and on the right, the eclipsed conformation. There are other intermediate conformations between these two, but our attention centers on the staggered and eclipsed conformations. These different but specific spatial orientations, obtained through rotation about a single bond, represent what is known as Conformational Isomers. The Newman projection is one approach for illustrating such isomers. Another approach is the Sawhorse representation, as shown in Fig. 2.1 .

  Fig. 2.1.

  Notice that the H atoms on both C atoms are much closer together in the eclipsed conformation? The inter-electronic repulsions between these H atoms are the greatest due to their close proximity. This structure is in a “tension” state and has a higher energy state than the staggered conformation. The staggered conformation is thus more stable than the eclipsed conformation, as shown in Fig. 2.2

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  Carbohydrate Chemistry

  Fundamentals and Applications

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

    (Publisher)

  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

  isomerism”). As the first two types are characteristic of carbohydrates, they will be emphasized in the following discussion.

  3.2.Constitutional Isomerism

  Constitutional isomers generally differ from one another only in the order of attachment of their atoms and the location of their bonds. In the functional group isomerism, the isomers have the same molecular formula, but their functional groups are different. The following compounds are examples of such isomers:

  Chain isomers have the same molecular formula, but the skeleton (usually carbon skeleton) differs by having branches or otherwise. The following compounds (C5 H12 ) are examples:

  The number of chain isomers increases very rapidly with the increase in the number of carbon atoms in the compound. Theoretically, for 6, 7, 8, 15, and 20 carbon atoms in an aliphatic hydrocarbon, the numbers of possible chain isomers are 5, 9, 18, 4347, and 366,319, respectively.

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  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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  The Elements of Polymer Science and Engineering

  An Introductory Text for Engineers and Chemists

  • Alfred Rudin(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  Chapter 4 Effects of Polymer Isomerism and Conformational Changes Three types of isomerism are important in macromolecular species. These involve constitutional, configurational, and conformational variations. These terms are defined and illustrated immediately below. Their usage in macromolecular science is very much the same as in micromolecular chemistry. 4.1 CONSTITUTIONAL ISOMERISM The constitution of a molecule specifies which atoms in the molecule are linked together and with what types of bonds. Isobutane (4-1) and n-butane (4-2) are familiar examples of constitu-C H , C H , -C -H C H 3 4-1 tional isomers. Each has the molecular formula C 4 H 10 but the C and Η atoms are joined together differently in these two molecules. In polymers C H 3 -C H 2 -C H 2 -C H 4-2 122 4.1 Constitutional Isomerism 123 the major types of constitutional differences involve positional isomerism and branching. 4.1.a Positional Isomerism Vinyl and vinylidene monomers are basically unsymmetrical because the two ends of the double bond are distinguishable (ethylene and tetra-fluorethylene are exceptions). One C of the double bond can be arbitrarily labeled the head and the other the tail of the monomer, as shown in the formula for vinyl fluoride (4-3). Η I head C H ? = C tail 4-3 In principle, the monomer can be enchained by head-to-tail linkages or head-to-head, tail-to-tail enchainments (4-4). Poly(vinyl fluoride) actually Η Η Η Η Η ι ι ι ι ι -C H -C -C H -C -C H — C —C — C H — C H - C 2 , 2 ι 2 | | 2 2 | F F F F F V y / ^ ν / HEAD -T O -T A I L HEA D-T O —HEA D TAIL -T O -T A I L 4-4 has about 15% of its monomers in the head-to-head, tail-to-tail mode. This is exceptional, however. Head-to-tail enchainment appears to be the pre-dominant or exclusive constitution of most vinyl polymers because of the influence of resonance and steric effects. Vinyl monomers polymerize by attack of an active center (4-5) on the double bond.

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  Essentials of Organic Chemistry

  For Students of Pharmacy, Medicinal Chemistry and Biological Chemistry

  • Paul M. Dewick(Author)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  Thus, there are two chiral centres, and the groups around each centre are the same. Again, we get only three stereoisomers rather than four, since the cis compound is an optically inactive meso compound. There is a plane of symmetry in this molecule, and it is easy to see that one chiral centre is mirrored by the other, so that we lose optical activity. Conformational mobility, such as we get in cyclohexane rings, makes the analysis more difficult, and manipulating molecular models provides the clearest vision of the relationships. Let us look at 1,2-dimethylcyclohexane as an example. Again, we have met the cis and trans isomers when we looked at conformational aspects (see Section 3.3.2). Here, we need to consider both configuration and conformation. In the trans compound, two mirror image enantiomeric forms can be visualized. These will be the (+)- and (−)- trans isomers. Note particularly that conformational changes may also be considered, but these do not change configuration, so we are only seeing different conformers of the same compound. The above scheme thus shows two interconvertible conformers (upper and lower structures) for each of the two non-interconvertible enantiomers (left and right structures). The cis compound provides the real challenge, however. If we draw version A, together with its mirror image C, they do not look capable of being superimposed. However, conformer A may be ring-flipped to an equal-energy conformer B, and this will have a corresponding mirror image version D. Now consider a 120° rotation of version A about the central axis; this will give D. A similar 120° rotation of version C about the central axis will give B. It follows, therefore, that if simple rotation of one structure about its axis gives the mirror image of a conformational isomer, then we cannot have enantiomeric forms but must have the same compound. These are thus two different conformers of an optically inactive meso compound

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