Conformations cyclohexane

Newman projection showing relief of eclipsing of bonds 

The conformation of cyclopentane is slightly folded, like the shape of an envelope. This puckered conformation reduces the eclipsing of adjacent CHa groups. 

We will cover the conformations of cyclohexane in more detail than other cycloalkanes because cyclohexane ring systems are particularly common. Carbohydrates, steroids, plant products, pesticides, and many other important compounds contain cyclohexane-like rings whose conformations and stereochemistry are critically important to their reactivity. The abundance of cyclohexane rings in nature is probably due to both their stability and the selectivity offered by their predictable conformations. 

The chair conformation of cyclohexane has one methylene group puckered upward and another puckered downward. Viewed from the Newman projection, the chair has no eclipsing of the carbon-carbon bonds. The bond angles are 109.5°. 

In the symmetrical boat conformation of cyclohexane, eclipsing of bonds results in torsional strain. In the actual molecule, the boat is skewed to give the twist boat, a conformation with less eclipsing of bonds and less interference between the two flagpole hydrogens. 

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The various conformations of cyclohexane are m rapid equilibrium with one another but at any moment almost all of the molecules exist m the chair conformation Not more than one or two molecules per thousand are present m the skew boat confer matron Thus the discussion of cyclohexane conformational analysis that follows focuses exclusively on the chair conformation... 

Figure 3 Shapes and relative energies of cyclohexane conformations...

Note that we haven t used the words ch and turns in this discussion of cyclohexane conformation. Two hydrogens on the same face of the ring are always cis, regardless of whether they re axial or equatorial and regardless of whether they re adjacent. Similarly, two hydrogens on opposite faces of the ring are always trans. 

Fig. 7.17 RMS fits of two similar phenyl-cyclohexane conformations. The structural diagram indicates the two independent symmetry axes (dotted lines). The left-hand...

If one methyl group is replaced by phenyl, the a-ICSs seem to tend to stronger shieldings in the cyclohexane conformer 143 a value of ca. — 7 can be estimated... 

Ligation in Belted lonophores, Biasing Cyclohexane Conformation,... 

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

The cyclohexane conformers can be isolated and identified. It is explained it with a suitable example ... 

H-NMR studies on 2,6-dimethylmorpholine314 and N-substituted 2,6-dimethylmorpholines315 are consistent with the single diequatorial conformation 391 for the cis isomers and with the equilibrium 392 393 for the trans isomers. The 13C-NMR spectrum of the monomethyl-, 2,3-, 2,5-, 2-6-, 3-5-dimethyl-, 2,3,4-, 2,3,6-trimethyl-, and 2,3,5,6-tetramethylmorpholines are largely consonant with expectations based on conformational principles derived from cyclohexane conformational analysis.316 The all-cis 2,3,5,6-isomer, which did not prove amenable to analysis by H-NMR spectro-... 

Quantitatively, the anomeric effect is defined as the difference in conformational free energy for the process shown in Scheme 28 and the corresponding process in cyclohexane. Conformational energies for a range of substituents are available (82JA3635). 

Other substituted cyclohexanes are similar to methylcyclohexane. Two chair conformations exist in rapid equilibrium, and the one in which the substituent is equatorial is more stable. The relative amounts of the two conformations depend on the effective size of the substituent. The size of a substituent, in the context of cyclohexane conformations, is related to the degree of branching at the atom connected to the ring. A single... 

X-ray studies of crystalline a- and /3-D-glucose show that these molecules have their atoms arranged in space as correspond to 16 and 17. This is what we would expect from our studies of cyclohexane conformations (Sections 12-3A to 12-3D), because for the j3 form, all of the substituents on the oxacyclohexane ring are in equatorial positions, and for the a form, all except the hydroxyl at the anomeric carbon (Cl) are equatorial. 

Draw the conformational isomers of cis-1,2-dimethylcyclohexane and cis-3,4-dimetliylcyclohexanone. While the cyclohexane conformers are of equal energy, the cyclohexanone conformers are not. Indicate which con-former is favored and explain why. 

Fig. 5.45 One might have guessed that the chair cyclohexane conformations 1 and I are connected by a boat-shaped intermediate 2. However, this C2v structure shows an imaginary frequency it is a transition state which wants to twist toward 3 (arrows) or 3 (arrows in opposite directions, not shown), which are the actual intermediates (no imaginary frequencies) between 1 and 1. The chair conformation reaches the twist via a half-chair 4...

Summary Rules for Naming Alkanes 94 3-4 Physical Properties of Alkanes 95 3-5 Uses and Sources of Alkanes 97 3-6 Reactions of Alkanes 99 3-7 Structure and Conformations of Alkanes 100 3-8 Conformations of Butane 104 3-9 Conformations of Higher Alkanes 106 3-10 Cycloalkanes 107 3-11 Cis-trans Isomerism in Cycloalkanes 109 3-12 Stabilities of Cycloalkanes Ring Strain 109 3-13 Cyclohexane Conformations 113... 

Q Draw accurate cyclohexane conformations, and predict the most stable conformations of substituted cyclohexanes. Explain why large groups are more stable in equatorial positions rather than in axial positions. Problems 3-43,44,45, and 49... 

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