Chair form of cyclohexane

Chair cyclohexane (Section 4.5) A three-dimensional conformation of cyclohexane that resembles the rough shape of a chair. The chair form of cyclohexane is the lowest-energy conformation of the molecule. 

In chair form of cyclohexane ring, there are two possible kinds of bonds, those pointing up and down are called axial and those pointing sideways are called equatorial. Since they are commonly represented by letters a or e, this is why the substituents attached to these bonds are called axial or equatorial substituents respectively. 

Fig. 3 summarizes calculations 23) for the conversion of the chair form of cyclohexane to the boat. The calculated difference in energy between the two isomers (5.4 kcal/mole) agrees well with experiment (5.3 kcal/mole). We have not as yet calculated the complete potential surface for the interconversion however energies have been calculated for three possible transition states (a - c) in which four, five, and six carbon atoms respectively are coplanar. In each case the energy was minimized subject to this one constraint. It will be seen that the predicted intermediate is a, in agreement with calculations by... 

Fig. 39. Skeletal structures of the carbo cation (CgMegH) and the boat and chair forms of cyclohexane compared with the octadecahedron of (BuHu) -.

All six carbons in the chair form of cyclohexane are equivalent, but the hydrogens divide into two sets of six equivalent ""equatoriar hydrogens and six equivalent axiar hydrogens. 

Chemists routinely manipulate physical models in an attempt to ascertain what actually occurs during a conformational change. A successful example of this is in showing first-time students of organic chemistry that interconversion between anti and gauche conformers of w-butane involves a simple rotation about the central carbon-carbon bond (see discussion in Chapter 1). Much less satisfactory is the attempt to show the interconversion of chair forms of cyclohexane. Here, computer animations provide a better alternative. 

Figure 12-5 Chair form of cyclohexane showing equatorial and axial hydrogens. Top, space-filling model center, ball-and-stick model bottom, sawhorse representation. Notice that all the axial positions are equivalent and all the equatorial positions are equivalent. By this we mean that a substituent on any one of the six axial positions gives the same axial conformation, whereas a substituent on any one of the six equatorial positions gives the same equatorial conformation.

Even though Haworth formulas give a sound representation of the ring structures of sugars, the real structure conformation can be most accurately represented by the chair forms of cyclohexane as shown in... 

Concerning conformation , we prefer to adopt a broader definition than has generally been used heretofore. Excluding normal molecular vibrations, relative atomic motion that breaks no bonds within a molecular framework generates conformations. A usual definition which specifies that conformations are attained by rotations about bonds is far too restrictive, considering that the conversion of one chair form of cyclohexane to another involves several complicated coupled rotations and 11... 

For the 1,4-dithiane skeleton, two S-alkyl-l,4-dithianium salts 72 and 73 were studied by variable temperature NMR spectroscopy in the solid state (13C CP-MAS) (99JMS93) line shape variations were attributed to the conformational motion of the six-membered ring (cf. Scheme 26). Also, the vibrational frequencies of the 1,4-dithiane derivative were analysed in detail it was corroborated that the molecule exists in the C2h configuration similar to the chair form of cyclohexane (99SA(A)121). 

Time-dependent effects The NMR signals are sometimes influenced by time-dependent phenomena such as conformational or prototropic changes, which take place at a rate faster than the line width and comparable to (or faster than) the inverse of the differences between the frequencies of the transitions of the different sites. This means that kinetic phenomena may be studied by the NMR technique, especially if the temperature of the sample can be adapted. As an example we mention the two chair forms of cyclohexane, which are energetically stable there is a fast inversion from the one into the other via the boat form. At room temperature the PNMR spectrum exhibits one sharp peak, corresponding to the mean of the two chemical shifts. At -120 °C the dynamic equilibrium is frozen and the spectrum exhibits two sharp peaks, whereas at -60 °C the inversion is slow and the spectrum exhibits one broad peak. 

The chair form of cyclohexane is considered rigid since it must undergo cycle inversion of relatively high energy to yield other conformers. However, besides the rigid chair conformers, cyclohexane also exists as flexible forms which include the boat conformer LXXIXb and the twist form LXXIXc. In fact, the flexible form gives... 

The boat and the chair forms of cyclohexane show only equatorial and axial exocyclic positions. In contrast, the twist conformers display pseudoequatorial e pseudoaxial (a ) and isoclinal (i) positions [102]. 

Figure 15. Representation of the interaction of cyclohexane (chair form) with Ru(0001) as suggested by Madey and Yates (31) in their classic study of hydrocarbon chemisorption on this basal plane of ruthenium. Three Ru—H—C three-center bonds are formed with three of the axial hydrogen atoms on one side of the chair form of cyclohexane. Weaker three-center Ru—H—C bonds may also be extant with equatorial C—H hydrogen atoms.

In the second example the diazene possesses a four-carbon tether with two stereogenic centers. The major product can be viewed as arising via a pathway where the tether coils to resemble a chair form of cyclohexane with the methyl, silyl ether and tive-membered ring occupying pseudoequatorial positions. Iliat the observed stereospecificity is due to the preference for the substituents to orient themselves... 

A three-dimensional representation of the chair form is drawn in Figure 4.12. Before continuing, we must first learn how to draw the chair form of cyclohexane. 

A three-dimensional model of the chair form of cyclohexane with all H atoms drawn... 

The chair forms of cyclohexane are 7 kcal/mol more stable than the boat forms. The boat conformation is destabilized by torsional strain because the hydrogens on the four carbon atoms in the plane are eclipsed. Additionally, there is steric strain because two hydrogens at either end of the boat—the flagpole hydrogens—are forced close to each other, as shown in Figure 4.14. 

Thus, even though each cyclohexane carbon has two dilferent types of hydrogens—one axial and one equatorial— the two chair forms of cyclohexane rapidly interconvert them, and an NMR spectrum shows a single signal for the average environment that it sees. ... 

Diaxial interaction (Section 4.13A) A steric interaction between two axial substituents of the chair form of cyclohexane. Larger axial substituents create unfavorable 1,3-diaxial interactions, destabilizing a cyclohexane conformation. 

Name an Alkane Using the lUPAC System 122 Name a Cycloalkane Using the lUPAC System 126 Draw a Newman Projection 132 Draw the Chair Form of Cyclohexane 140 Draw the Two Conformations for a Substituted Cyclohexane 143 Draw Two Conformations for a Disubstimted Cyclohexane 146 Stereochemistry... 

Interesting QOOH chemistry would also be favourable and include not only the compounds analogous to those observed for cyclohexane, but also two oxabicyclooctanes which would have some associated stability arising from the boat and chair forms of cyclohexanes. 

In the solid state the molecular structures of (PhAs)g and (PhSb)g are similar. Both homocycles adopt the structure of highly puckered six-membered rings analogous to the chair form of cyclohexane in which phenyl groups occupy equatorial positions ... 

Cyclohexane is the most important of the carbocycles its structural unit is widely encountered in various natural products. It can adopt a chair conformation that is essentially strain free. The chair form of cyclohexane has two distinct types of hydrogens equatorial and axial. 

Although the chair form of cyclohexane is the preferred conformation, other conformations are known and in some systems are required. 

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