Cyclohexane molecular model

In keeping with the bent bond de scription of Figure 3 10 the carbon-carbon bond distance in cycio propane (151 pm) is slightly shorter than that of ethane (153 pm) and cyclohexane (154 pm) The calculated val ues from molecular models (see Learning By Modeling) reproduce these experimen tal values... 

Use molecular models to compare —C=CH —CH=CH2 and —CH2CH3 with respect to their preference for an equatonal onentation when attached to a cyclohexane nng One of these groups IS very much different from the other two Which one" Why" ... 

Make molecular models of the two chair conformations of cis 1 tert butyl 4 phenyl cyclohexane What is the strain energy calculated for each conformation by molecular mechanics Which has a greater preference for the equatorial onentation phenyl or tert butyD... 

The most stable conformation of cyclohexane is the chair. Electron diffraction studies in the gas phase reveal a slight flattening of the chair compared with the geometry obtained when tetrahedral molecular models are used. The torsion angles are 55.9°, compared with 60° for the ideal chair conformation, and the axial C—H bonds are not perfectly parallel but are oriented outward by about 7°. The length of the C—C bonds is 1.528 A, the length of the C—H bonds is 1.119 A, and the C—C—C angles are 111.05°. ... 

Make a molecular model of the chair conformation of cyclohexane, and turn it so that you can look down one of the C—C bonds. 

Construct a molecular model of rra i-2-bromocyclohexanol in its most stable conformation. This conformation is ill-suited to undergo epoxide formation on treatment with base. Why What must happen in order to produce 1,2-epoxy cyclohexane from rra i-2-bromocyclohexanol ... 

The easiest way to visualize chair cyclohexane is to build a molecular model. (In fact do it now.) Two-dimensional drawings like that in Figure 4.7 are useful, but there s no substitute for holding, twisting, and turning a three-dimensional model in your own hands. The chair conformation of cyclohexane can be drawn in three steps. 

Conformational mobihty, 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. 

Dynamic NMR gives information on the number and symmetries of conformations present in solution and on the energy barriers separating these conformations. This is particularly true for systems with barriers between about 25 and 90 kJ mol-1, a situation which often occurs in the medium ring. The interpretation of the NMR data can be carried out by the examination of molecular models, but this is a relatively crude and sometimes misleading method. Empirical force field (or molecular mechanics) calculations are much superior, even though the parametrization of heteroatoms may be open to question. Quantum mechanical calculations are not very suitable the semiempirical type, e.g. MINDO, do not reproduce conformational properties of even cyclohexane satisfactorily, and the ab initio... 

The same distortion, however, is not required for chelation of transA, -cyclohexanediamine (chxn) molecular models show that the two NH2 groups are now ideally situated for bidentate formation to a metal ion (Fig. 8). 7rart -l,2-cyclohexanediamine differs from the ethylenediamine system in that, when it is coordinated, only one chair conformation of the cyclohexane ring is possible. If the other conformer is assumed, the NH2... 

Deuterium atom was neatly incorporated at the bridgehead position C-l in ketone 464, the only compound in which the cyclohexane ring is locked in a boat conformation. Examination of molecular models indicates that the cyclohexanone ring can easily adopt a boat form in 460 and 461. It appears to be more difficult with ketone 462 and almost impossible with ketone 463. 

FIGURE 5.41 Molecular models showing diastereomers of the A-sym-cis-R,R-cdda complex with respect to the absolute configurations of the coordinated asymmetric nitrogens and the normal axial (a) and abnormal equatorial (b) orientations of the terminal acetate rings. The solid circles in (a) refer to the acetate and the cyclohexane methylene of which the protons suffer severe steric contact as shown by arrows.170... 

It is a small step from van der Waals, electron-domain models of the C—H bonds of, e.g., biphenyl, cyclohexane, or methane (Figs. 1—3), to molecular models in which to a first, and useful, approximation each valence-shell electron-pair is represented by a spherical, van der Waals-like domain 7h (Non-spherical domains may be useful for describing, e.g., lone pairs about atoms with large atomic cores, -electrons, and the electron-pairs of multiple bonds vide infra.)... 

For the PIB/PS pair, we take X12 equal to ca. 40 J/cm3 (9.6 cal/cm3), since four systems consisting of an aromatic and an aliphatic component were found to give similar values of Xi2 42 J/cm3 for PIB/benzene (16), 42 J/cm3 for cyclohexane/benzene (17), 40 J/cm3 for n-heptane/benzene (17), and 42 J/cm3 for cyclohexane/PS (18). The ratio Si/s2 can be determined by examination of the molecular models of respective polymer segments. To avoid introducing any additional parameters which could be construed as adjustable, we obtained the S /s2 ratio of 1.24 for PIB/PS by multiplying the S /s2 ratios 0.62 for PIB/cyclohexane (19) and 2.0 for cyclohexane/PS (18), which were determined in the previous studies of these polymer-solvent systems. 

Cyclohexane achieves tetrahedral bond angles and staggered conformations by assuming a puckered conformation. The most stable conformation is the chair conformation shown in Figure 3-19. Build a molecular model of cyclohexane, and compare its shape... 

Norbornane has a conformationally locked boat cyclohexane ring (Section 4.5) in which carbons 1 and 4 are joined by an additional CH2 group. Note how, in drawing this structure, a break in the rear bond indicates that the vertical bond crosses in front of it. Making a molecular model is particularly helpful when trying to see the three-dimensionality of norbornane. 

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