Conformational Analysis of Cyclohexane
Conformational analysis of cyclohexane involves studying the different spatial arrangements of its atoms. This includes examining the chair, boat, twist-boat, and other conformations. Understanding these conformations is important in organic chemistry as it affects the reactivity and stability of cyclohexane derivatives. The analysis helps in predicting and explaining the behavior of cyclohexane-based compounds in various chemical reactions.
6 Key excerpts on "Conformational Analysis of Cyclohexane"
eBook - ePub- Michael North(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
...In addition, all of the adjacent CH 2 groups would eclipse one another giving a large E ϕ. To avoid these unfavourable interactions, cyclohexane and its derivatives adopt non-planar structures. Benzene, however, is composed of sp 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 E θ. All the adjacent CH 2 groups are staggered with respect to one another as highlighted in the Newman projection 8.21a, so there is no torsional strain E ϕ. 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...
eBook - ePub- Graham Patrick(Author)
- 2004(Publication Date)
- Taylor & Francis(Publisher)
...Cyclohexane conformations (a) boat and (b) twist boat. This conformation is slightly more stable than the boat itself, but is still much less stable than the chair. The ability of a cyclohexane molecule to ring-flip is important when substituents are present. Each carbon atom in the chair structure has two C–H bonds, but these are not identical (Figure 13). One of these bonds is termed equatorial since it is roughly in the ‘plane’ of the ring. The other C–H bond is vertical to the ‘plane’ of the ring and is called the axial bond. Figure 13. (a) Equatorial C–H bonds; (b) axial C–H bonds. When ring flipping takes place from one chair to another, all the axial bonds become equatorial bonds and all the equatorial bonds become axial bonds. This does not matter for cyclohexane itself, but it becomes important when there is a substituent present in the ring. For example, methylcyclohexane can have two chair structures where the methyl group is either on an equatorial bond or on an axial bond (Figure 14). Figure 14. Ring flipping of methylcyclohexane. These are different shapes of the same molecule which are interconvertable due to rotation of C-C single bonds (the ring flipping process). The two chair structures are conformational isomers but they are not of equal stability. The more stable conformation is the one where the methyl group is in the equatorial position. In this position, the C–C bond connecting the methyl group to the ring has a torsional angle of 180° with respect to bonds 5–6 and 3–2 in the ring. In the axial position, however, the C–C bond has a torsional angle of 60° with respect to these same two bonds. This can be illustrated by comparing Newman diagrams of the two methylcyclohexane conformations (Figure 15). A torsion angle of 60° between C–C bonds represents a gauche interaction and so an axial methyl substituent experiences two gauche interactions with the cyclohexane ring whereas the equatorial methyl substituent experiences none...
eBook - ePub- Mukesh Doble, Anil Kumar Kruthiventi, Vilas Ganjanan Gaikar, Mukesh Doble, Anil Kumar Kruthiventi, Vilas Ganjanan Gaikar(Authors)
- 2004(Publication Date)
- CRC Press(Publisher)
...For example, if one considers an assembly of four atoms A, B, C, and D. One can envisage different types of connectivity, such as A-B-C-D, A-B-D-C, B-A-D-C, and so on. For any one of these molecules, which are constitutional isomers of A-B-C-D, even if we assume constant bond angles ABC and BCD, one can generate an infinite number of structures changing the torsion angle ABC/BCD. These structures are the different conformers of the molecule A-B-C-D. In spite of this infinite 3D structures possible for molecules, thankfully, since these structures differ in energies, the molecule exists in only a few (sometimes only in one) rapidly interchanging conformers. Thus, it is possible to predict the 3D structure of any molecule by studying the energy of the conformers. The lowest energy conformer is the state in which any molecule will exist in ground state. Thus, for cyclic structures, such as cyclopentane and cyclohexane (which we normally encounter in biologic systems), the probable low-energy conformers are the envelope form for the cyclopentane and the two chair forms for the cyclohexane (as shown in Fig. 2.11). One has to be cautious when applying these above principles because based on the substitutions on theses ring systems the lowest energy conformer may be different. However, these principles being the first principles are of great value in understanding the 3D structure of most of the biomolecules, thereby, their reactivity. FIGURE 2.11 Stable conformations of cyclopentane and cyclohexane. 2.4.3 Summary: Learning Outcome The representation of molecules as the structural formula. Given a molecular formula, there can be more than one compound having the same constituent atoms...
eBook - ePub- J Fisher, J.R.P. Arnold, Julie Fisher, John Arnold(Authors)
- 2020(Publication Date)
- Taylor & Francis(Publisher)
...The eclipsed conformation in which the methyl groups are in line with each other is labeled syn ; this is the highest energy conformation. The range of energies encompassed in conformational variations is readily depicted in an energy profile (Figure 4). Figure 4 The energy profile for conformational flexing in n-butane. Conformational isomerism is also possible in cyclic systems. For example, cyclohexane (C 6 H 12), can adopt a range of conformations via bond rotations which occur to relieve the strain energy (or torsional strain) that would be present if the ring was forced to be planar. The energy profile, with a representation of the distinct conformational isomers of cyclohexane is shown in Figure 5(a), together with the Newman projections of the (i) chair and (ii) boat conformations of cyclohexane (Figure 5(b)). Figure 5 (a) The energy profile for conformational flexing in cyclohexane, and (b) the Newman projections for the (i) chair and (ii) boat conformations. The possibility of conformational isomerism is very import antinbiology and canhave profound effects on the shape and functionality of biological macromolecules. For example, the cyclic sugar systems that are part of DNA and RNA molecules (Sections K2, M2, and M4) are flexible but preferentially adopt chair-like conformations referred to as C 2, -endo (or South) and C 3, -endo (or North), respectively (Figure 6). As a result, polymers of DNA are generally long and thin, whereas those of RNA are shorter and wider (Section M2). Figure 6 The conformational preferences of sugar rings found in (a) DNA and (b) RNA. Fischer projections The Fischer projection is another way to display, in two dimensions, the three-dimensional shape of a molecule. For example, the Fischer projection for butane is shown in Figure 7(a). Figure 7 (a) The Fischer projection for n-butane and (b) the three-dimensional shape it represents. The horizontal and vertical lines are reflecting the eclipsed conformation by convention...
eBook - ePub- Leslie H. Sperling(Author)
- 2015(Publication Date)
- Wiley-Interscience(Publisher)
...The trans and gauche positions are located 120° apart on an imaginary cone of rotation, where the preferred positioning avoids groups on the neighboring chain carbon atoms, the trans being the more extended conformation. The trans and gauche conformations are discussed further in Section 2.8. For C—C bond lengths of 0.154 nm, and C—C—C bond angles of 109°, the mer length of many addition polymers may be taken as 0.254 nm. See Section 6.3. 2.2 THEORY AND INSTRUMENTS Koenig (3) defines the microstructure of a polymer in terms of its conformation and configuration. The term conformation has taken on two separate meanings: (a) the long-range shape of the entire chain, which is discussed in Chapter 5, and (b) the several possibilities of rotating atoms or short segments of chain relative to one another, to be discussed later. The term configuration includes its composition, sequence distribution, steric configuration, geometric and substitutional isomerism, and so on, and is the major concern of this chapter. The several aspects of polymer chain microstructure have been studied by both chemical and physical methods. Koenig (3) describes several of these methods, which are summarized in Tables 2.1 and 2.2. Table 2.1 Chemical methods of determining polymer chain microstructure (3) Method Application Reference Elemental analysis Gross composition of polymers and copolymers, yielding the percent composition of each element; C, H, N, O, S, and so on. (a) Functional group analysis Reaction of a specific group with a known reagent. Acids, bases, and oxidizing and reducing agents are common. Example: titration of carboxyl groups. (b, c) Selective degradation Selective scissions of particular bonds, frequently by oxidation or hydrolysis...
eBook - ePub- Jeffrey Gaffney, Nancy Marley(Authors)
- 2017(Publication Date)
- Elsevier(Publisher)
...The simple cycloalkanes containing three to six carbon atoms are shown in Fig. 13.9. The structures shown in Fig. 13.9 are given in bonding notation and do not represent the actual structures of the molecules, which are not flat but are puckered in an attempt to achieve the109.5 degrees bonding angles of sp 3 hybridized alkanes. The smaller cycloalkanes, cyclopropane and cyclobutane, have significant ring strain since the bond angles of 60 and 90 degrees are forced to be much smaller than 109.5 degrees. This makes them much less stable than the larger cycloalkanes. Fig. 13.9 The chemical structure of the simplest cycloalkanes containing three to six carbon atoms. Cyclic alkenes are also common, but only in larger ring sizes since the geometry of an sp 2 hybridized carbon is normally linear and smaller rings with very small bond angles would be extremely unstable. The cyclic alkenes have two sites of unsaturation, one from the double bond and one from the ring formation. Since they have lost a total of four hydrogens from the unsaturated alkane, they have the same molecular formula as the alkynes. So they are isomeric with the corresponding alkyne hydrocarbon having the same molecular formula. Another very important class of hydrocarbons is the aromatic hydrocarbons ; cyclic hydrocarbons composed of σ and π bonds are arranged in such a way that the electrons in the π bonds become delocalized giving the molecule unusual stability. Aromatic hydrocarbons originally received their name because many of the compounds have a sweet or pleasant odor. The simplest of the aromatic compounds is benzene, with a molecular formula of C 6 H 6. All of the benzene carbon atoms have sp 2 hybridization, so the molecular geometry is planar. Benzene is a cyclohexane ring with three double bonds alternated with three single bonds...
