Conformational analysis cyclohexane

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... 

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-... 

Conformations of Cyclobutane and Cyclopentane Conformations of Cyclohexane 127 Axial and Equatorial Bonds in Cyclohexane 129 Conformational Mobility of Cyclohexane 131 Conformations of Monosubstituted Cyclohexanes Conformational Analysis of Disubstituted Cyclohexanes Boat Cyclohexane 140 Conformations of Polycyclic Molecules 141... 

Aspects of cyclohexane conformational analysis. A. Interconverting chair forms of cyclohexane, with axial and equatorial locations labeled. In the left structure letters x are axial while letters y are equatorial. Note that the chair flip moves axial substituents to the equatorial position and vice versa. B. Newman projections down the C1-C2/ C5-C4 bonds of methylcyclohexane. In the axial form, there is a gauche butane interaction between the methyl and C3. C. Views of cyclohexane, equatorial methylcyclohexane, and axial methylcyclohexane. Note that chair cyclohexane is a relatively disk-shaped molecule, and an equatorial methyl does little to disrupt this shape. In contrast, an axial substituent puts a "kink" into the structure. Also evident is the steric interaction between one methyl hydrogen and the two axial hydrogens on C3 and C5. 

The physical, chemical cind biological properties of a molecule often depend critically upo the three-dimensional structures, or conformations, that it can adopt. Conformational analysi is the study of the conformations of a molecule and their influence on its properties. Th development of modem conformational analysis is often attributed to D H R Bcirton, wh showed in 1950 that the reactivity of substituted cyclohexanes wcis influenced by th equatoricil or axial nature of the substituents [Beirton 1950]. An equcilly important reaso for the development of conformatiorml analysis at that time Wcis the introduction c analytic il techniques such as infreired spectroscopy, NMR and X-ray crystaillograph] which actucilly enabled the conformation to be determined. 

In this chapter we ll examine the confonnations of various alkanes and cycloalkanes, focusing most of our attention on three of them ethane, butane, and cyclohexane. A detailed study of even these three will take us a long way towar d understanding the main ideas of conformational analysis. 

The same kind of conformational analysis just carried out for cis- and fraus-l,2-dimethylcydohexane can be done for any substituted cyclohexane, such as as-l-tert-butyl-4-chlorocydohexane (see Worked Example 4.3). As you might imagine, though, the situation becomes more complex as the number of... 

Polycyclic compounds are common in nature, and many valuable substances have fused-ring structures. For example, steroids, such as the male hormone testosterone, have 3 six-membered rings and 1 five-membered ring fused together. Although steroids look complicated compared with cyclohexane or decalin, the same principles that apply to the conformational analysis of simple cyclohexane lings apply equally well (and often better) to steroids. 

The principles involved in the conformational analysis of six-membered rings containing one or two trigonal atoms, for example, cyclohexanone and cyclohexene are similar. The barrier to interconversion in cyclohexane has been calculated to be 8.4-12.1 kcal mol . Cyclohexanone derivatives also assume a chair conformation. Substituents at C2 can assume an axial or equatorial position depending on steric and electronic influences. The proportion of the conformation with an axial X group is shown in Table 4.4 for a variety of substituents (X) in 2-substituted cyclohexanones. 

An interesting case of conformational analysis comes to play when we consider a six-membered ring (cyclohexane). There are many conformations that this compound can adopt. You will see them all in your textbook the chair, the boat, the twist-boat. The most stable conformation of cyclohexane is the chair. We call it a chair, because when you draw it, it looks like a chair ... 

As is well known, the conformational properties of cyclohexane form one of the classical problems of conformational analysis (104) and have been the subject of numerous earlier computational studies (11, 41, 82,105-107). Cyclohexane shows a variety of very interesting conformational properties which make it an ideal candidate for illustrating key features of conformational calculations. 

Cyclohexaamylose-N-methylacetohydroxamic acid, preparation of, 23 254 Cyclohexadienes, 20 293 reaction with HCN, 33 19, 20 on silica, reactions of, 34 54-56-34 64 cracking, 34 55, 72 vibrational spectra, 42 243 Cyclohexadienyl radicals, ESR of, 22 300 1,4-Cyclohexanediols, conversion of ethers, mechanism, 35 361-364 Cyclohexanes, 33 101, 102, 103 autoxidauon of, 25 303 conformational analysis of, 18 9-17 dehydrogenation, 31 14, 21-22 benzene accumulation over platinum, 36 18... 

F. G. Morin and D. M. Grant, Conformational Analysis of Cyclohexanes, Cyclohexadienes and Related Hydroaromatic Compounds, P. W. Rabideau, ed., VCH, Weinheim, 1989. 

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... 

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