Conformational Analysis of Cyclohexane

9 Key excerpts on "Conformational Analysis of Cyclohexane"

• 

  eBook - PDF

  Stereochemistry

  Basic Concepts and Applications

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

    (Publisher)

  The importance of these results only became ap-parent to organic chemists in 1951 when Barton pointed out that the non-planarity of cyclohexane has important consequences for the reactivity of its derivatives. X x (121) (122) Unsubstituted cyclohexane can take up two basic types of conformational states. One of these is the well-known highly symmetrical (D 3flt ) chair form C (Fig. 62a), which is relatively rigid and which can be distorted from this shape only by the application of considerable angle strain**. The other state cannot however be characterized by a single conformation, and for this reason it is not reasonable to compare the chair form exclusively with the + The study of various conformations is relatively easily accomplished by the use of molecular models in which rotation around single bonds is free but resistance against angle distortion can be readily perceived (e .g. on Dreiding or Fieser models). 101 boat form. The proper boat form of C 2v symmetry is only one, and in fact an energetically less favoured, member of a family of conformers which differ only slightly in energy from each other and can be easily interconverted virtually without the generation of angle strain through a concerted change of dihedral angles. This sort of motion which avoids angle strain in cyclic compounds is called pseudorotation. In such pseudorotation complete turns around dihedral angles are excluded, i.e. there is no real rotation as in the case of ethane but only an oscillation of dihedral angles between certain limits (for further details see Section 2.5). Among the flexible conformations of cyclohexane, two of higher symmetry have specific names — the boat form (B) already mentioned, and the chiral twist boat forms (TB and TB*) with D 2 symmetry (Fig. 62b) + . The chair form C is completely free from any kind of strain : all the bonds are staggered (E

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Principles and Applications of Stereochemistry

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

    (Publisher)

  2 hybridized carbon atoms for which the minimum energy bond angle is 120°. This is one reason why benzene does adopt a planar conformation. The various conformations which cyclohexane and its derivatives may adopt will be examined in some detail, both because these conformations are well defined and because a very large number of organic compounds contain six-membered rings, so the analysis of the conformations of these rings is of some importance.

  8.7.1  The chair conformation

  The chair conformation of cyclohexane is shown in structure

  8.21

  . In this conformation, all the C–C–C bond angles are approximately 109°, so there is no angle strain Eθ . All the adjacent CH2 groups are staggered with respect to one another as highlighted in the Newman projection

  8.21a

  , so there is no torsional strain Eϕ . In addition, there are no close interactions, so there is no strain energy at all in the chair form of cyclohexane.

  The name ‘chair conformation’ arises because (with a little imagination) the structure resembles a chair. Four of the carbon atoms are coplanar and form the seat of the chair whilst the other two are located above and below this plane respectively. The carbon atom which is displaced above the plane of the seat of the chair can be imagined as forming a head rest, whilst the carbon atom which is displaced below the plane forms a foot stool. One important point that must be realized, however, is that all six carbon atoms in the chair conformation of cyclohexane are equivalent. Depending upon the direction from which the molecule is viewed, any of the six carbon atoms may be considered as being part of the seat, head rest or foot stool of the chair. A molecular model will be of great help in appreciating this.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Introduction to Stereochemistry

  • Andrew Clark, Russ Kitson, Nimesh Mistry, Paul Taylor, Matthew Taylor, Michael Lloyd, Caroline Akamune(Authors)
  • 2020(Publication Date)
  • Royal Society of Chemistry

    (Publisher)

  As we said, every carbon in methyl cyclohexane is tetrahedral, meaning that it cannot adopt a planar conformation. However, it can form a chair or a boat conformation. Of the two possible conformations, the chair is more stable as it avoids unfavourable eclipsing interactions.

    Key Learning Point

  The chair conformation of cyclohexane is the lowest-energy conformation, more stable than the boat or other possible conformations.

  5.1.2 Substituents Adopt Either Axial or Equatorial Positions

  If we add the hydrogens to the chair conformation (Figure 5.8 ), we can see that the hydrogens are pointing in different directions relative to the ring.

  Figure 5.8 Hydrogens on cyclohexane.

  Some hydrogens point either straight up or straight down above and below the ring. These are called axial hydrogens. If we look at these axial hydrogens more closely, we can see that they alternate between pointing up and pointing down as we go around the ring (Figure 5.9 ).

  Figure 5.9 Axial and equatorial hydrogens on cyclohexane.

  Other hydrogens point to the side of the ring. These are called equatorial hydrogens. In fact, each carbon has one axial hydrogen and one equatorial hydrogen. As you move around the ring, the type of hydrogen that is on the top and bottom alternates between being axial and equatorial.

  Imagine the chair conformations superimposed on a globe. The axial hydrogens will point towards the axes (i.e. north and south pole) and the equatorial hydrogens will point towards where the equator will be.

  5.1.3 Chairs Can Ring Flip (Invert)

  So far, we've introduced one form of chair conformation, where the carbon on the left is pointing up and the carbon on the right is pointing down. However, another chair conformation can exist where the left and right carbons point in opposite directions. These two chair conformations can actually interconvert with one another in a process called ring flipping (Figure 5.10

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Thermodynamics

  Kinetics of Dynamic Systems

  • Juan Carlos Moreno Piraján(Author)
  • 2011(Publication Date)
  • IntechOpen

    (Publisher)

  On the contrary, the entropy counterpart is strongly dependent of the frequency, particularly in the region of 0-200 cm -1 , therefore, the treatment of low frequency modes definitively has a pronounced effect on the entropy evaluation. 4. Conformational analysis of cycloalkanes Despite a rather simple carbon–hydrogen cyclic skeleton structure, the cycloalkanes have indeed attracted the interest of several research investigations in the experimental and theoretical fields. These studies are mainly concerned with the conformational analysis as a Thermodynamics – Kinetics of Dynamic Systems 32 Fig. 4. Thermodynamic energy or internal thermal energy ( ܧ ௜௡௧ǡ௩௜௕ ) and entropic ( ܶܵ ௩௜௕ ) vibrational contributions (in units of kcal mol -1 ) represented as a function of the vibrational frequency, calculated with the aid of the statistical thermodynamics formulae, within the harmonic oscillator (HO) approximation (HO vibrational partition function), at room temperature and normal pressure. function of the temperature and pressure conditions. Electron diffraction experiments have been of great aid to provide population data for cycloalkanes for gas phase samples, as reported for cycloheptane (Dillen & Geise, 1979), cyclooctane (Dorofeeva et al., 1985), cyclodecane (Hilderbrandt, Wieser & Montgomery, 1973) and cyclododecane (Atavin et al., 1989). For solution and solid state samples NMR spectroscopy have provided valuable information for temperature-dependent conformational analysis as given for cyclononane (Anet & Krane, 1980), cyclodecane (Pawar et al., 1998), cycloundecane (Brown, Pawar & Noe, 2003), and cyclododecane (Anet & Rawdah, 1978). In all these experimental investigation a population conformation with an uncertainty of ±5% was reported, and so the preferred conformation for each cycloalkane containing 7 to 12 carbon atoms precisely determined.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Organic Chemistry

  • T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  Methylcyclohexane (Section 4.12) exists mainly in the chair conformation with its methyl group equatorial for the same reason. Disubstituted cycloalkanes (Section 4.13) prefer a conformation with both sub- stituents equatorial if this is possible, and, if not, they prefer a conformation with the larger group equatorial. The preferred conformation in each instance is the one of lowest potential energy. Another effect that we encountered in this chapter, and one we shall see again and again, is how steric factors (spatial factors) can affect the stability and reactivity of molecules. Unfa- vorable steric interactions between groups are central to explaining why certain conforma- tions are higher in energy than others. But fundamentally this effect is derived itself from another familiar principle: like charges repel. Repulsive interactions between the electrons of groups that are in close proximity cause certain conformations to have higher potential energy than others. We call this kind of effect steric hindrance. PRACTICE PROBLEM 4.22 Carbonyl groups also count for a unit of hydrogen deficiency. What are the indices of hydrogen deficiency for the reactant and for the product in the equation shown at the beginning of Section 4.16 for synthesis of a perfume ingredient? Why Do These Topics Matter? Pushing the Boundaries of Bonding, All Within the Rules In this chapter we have learned many of the rules of bond formation and of conformation. Although there are only a few kinds of bonds in organic mole- cules, they can be combined in an infinite number of ways, sometimes leading to molecules whose existence defies our expectations. For example, using just C C and C H bonds, chemists have been able to synthesize structures such as cubane, prismane, and bicyclo[1.1.0]butane, materials that have incredible strain built into their structures.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Organic Chemistry

  • T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  Disubstituted cycloalkanes (Section 4.13) prefer a conformation with both substituents equatorial if this is possible, and, if not, they prefer a conformation with the larger group equatorial. The preferred conformation in each instance is the one of lowest potential energy. Another effect that we encountered in this chapter, and one we shall see again and again, is how steric factors (spatial factors) can affect the stability and reactivity of mol- ecules. Unfavorable spatial interactions between groups are central to explaining why certain conformations are higher in energy than others. But fundamentally this effect is derived itself from another familiar principle: like charges repel. Repulsive interactions between the electrons of groups that are in close proximity cause certain conformations to have higher potential energy than others. We call this kind of effect steric hindrance. 4.18 APPLICATIONS OF BASIC PRINCIPLES 187 In this chapter we have learned many of the rules of bond formation and of conformation. Although there are only a few kinds of bonds in organic molecules, they can be combined in an infinite number of ways, sometimes leading to molecules whose existence defies our expectations. For example, using just C − C and C − H bonds, chemists have been able to synthesize structures such as cubane, prismane, and bicyclo[1.1.0]butane, materials that have incredible strain built into their structures. Strained compounds are also found in nature, with one recent discovery being pentacycloanammoxic acid, a material isolated from a particular bacte- rial strain. This compound is also known as a ladderane because it has a connected set of five 4-membered rings that exist in three-dimensional space like a ladder, or staircase.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  VCD Spectroscopy for Organic Chemists

  • Philip J. Stephens, Frank J. Devlin, James R. Cheeseman(Authors)
  • 2012(Publication Date)
  • CRC Press

    (Publisher)

  81 5 Conformational Analysis The objectives of the conformational analysis of a molecule are to determine the num-ber of minima on the potential energy surface (PES) of the molecule, i.e., the number of stable conformations, and the geometries and energies of each stable conforma-tion. The number, geometries, and energies of the stable conformations of a molecule depend on the methodology used to calculate the PES. Ideally, when infrared (IR) and vibrational circular dichroism (VCD) spectra are to be calculated using density func-tional theory (DFT), conformational analysis should also be carried out using DFT. For very small molecules this is straightforward. For example, consider the chiral substituted oxiranes, methyl-oxirane, 1 , and phenyl-oxirane, 2 . O C O H 3 H 1 H 2 1 2 3 1 2 3 4 S -1 S -2 H H H H H H The oxirane ring is conformationally rigid, as are also the methyl and phenyl sub-stituents. However, in 1 the methyl group can rotate about the C–C bond, connecting it to the oxirane ring, and in 2 , the phenyl group can rotate about the C–C bond, connecting it to the oxirane ring. To determine the number of stable conformations of 1 and 2 , relaxed scans of their PESs with respect to rotation of the CH 3 and C 6 H 5 groups are carried out. Specifically, the dihedral angles C 1 C 2 C 3 H 1 of 1 and C 1 C 2 C 3 C 4 of 2 are varied from 0 to 360°. For each value of the dihedral angle, optimization of the molecule is carried out, giving the relaxed energy. The plot of the relaxed ener-gies vs. the dihedral angle values is the relaxed PES scan. The relaxed PES scan of S -1 calculated at the B3LYP/6-31G* level with C 1 C 2 C 3 H 1 being varied in steps of 10° is shown in Figure 5.1. Three valleys in the PES are observed, and therefore, three stable conformations are predicted. In Table 5.1, C 1 C 2 C 3 H 1 dihedral angles and the relative energies of the lowest-energy structures are listed.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Solomons' Organic Chemistry

  • T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  All groups are equatorial. Much more stable conformation. All groups axial. Much less stable conformation. CH 3 CH 3 CH 3 H 3 C H 3 C H 3 C ring flip PRACTICE PROBLEM 4.17 Provide an explanation for the surprising fact that all-trans-1,2,3,4,5,6-hexaisopropylcy- clohexane is a stable molecule in which all isopropyl groups are axial. (You may find it helpful to build a handheld molecular model.) PRACTICE PROBLEM 4.18 Write the structures of two chair conformations of 1-tert-butyl-1-methylcyclohexane. Which conformation is more stable? Explain your answer. Chemical Abstracts Service (CAS) determines the number of rings by the formula S − A + 1 = N, where S is the number of single bonds in the ring system, A is the number of atoms in the ring system, and N is the calculated number of rings (see Problem 4.30). HINT 4.14 BICYCLIC AND POLYCYCLIC ALKANES 182 CHAPTER 4 NOMENCLATURE AND CONFORMATIONS OF ALKANES AND CYCLOALKANES In cis-decalin the two hydrogen atoms attached to the bridgehead atoms lie on the same side of the ring; in trans-decalin they are on opposite sides. We often indicate this by writ- ing their structures in the following way: H H trans-Decalin cis-Decalin H H Simple rotations of groups about carbon–carbon bonds do not interconvert cis- and trans-decalins. They are stereoisomers and they have different physical properties. Adamantane is a tricyclic system that contains a three-dimensional array of cyclohex- ane rings, all of which are in the chair form. Adamantane In the chapter closer we shall see several examples of other unusual and highly strained, cyclic hydrocarbons. 4.15 CHEMICAL REACTIONS OF ALKANES Alkanes, as a class, are characterized by a general inertness to many chemical reagents. Carbon–carbon and carbon–hydrogen bonds are quite strong; they do not break unless alkanes are heated to very high temperatures.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Topics in Stereochemistry

  • Norman L. Allinger, Ernest L. Eliel(Authors)
  • 2009(Publication Date)
  • Wiley-Interscience

    (Publisher)

  CONJUGATED CYCLOHEXENONES 161 the appropriate physical measurements of circular dichroism, optical rotatory dispersion, nuclear magnetic resonance, etc. However, it has to be said that the frequent existence of several conformers in solution does not simplify the problem. We assume that the most stable conformations of cyclohexenones are analogous to those of the corresponding cyclohexene derivatives, taking into account a further flattening of the cyclohexene ring under the influence of the additional trigonal carbon. Concerning the con- formations of cyclohexene, it was first noted by Bucourt and Hainaut (19) that only the half-chair form corresponds to an energy minimum, while the other forms correspond to pseudo-rotational motions but not to energy minima. When the cyclohexene is fused to another ring, there are ordinarily found two energy minima which correspond to the half-chair and the 1,2 diplanar forms. Although frequently mentioned in the literature, the boat (usually called half-boat) is the least stable form in almost all cases, because it is not located at an energy minimum but occurs on the side of the pseudorotational potential curve. In the following we adopt for the conformers of equatorially substituted cyclohexenones, the order of decreasing stabilities that was given by Bucourt and Hainaut (19) for the various conformations of the un- substituted cyclohexene (Table 11), namely : monoplanar, 1,2 diplanar (half-chair) (also called envelope, sofa) > lB3 > lS4 (boat) For example, in the case of 3-keto-A4-unsaturated steroids the conformational equilibrium in solution will involve (Fig. 3, Sect. II- B-2) a population of half-chair (major component) and 1,2 diplanar form (minor component) and only minute amounts, if any, of either 1,3 diplanar or boat forms. Because of their high energetic content (19) the 1,3 diplanar form and especially the boat can be neglected, es- pecially in our simplified treatment (20).

  Sign up to read

  Learn more about book