Chair form

A monosubstituted cyclohexane, e.g. methylcyclohexane, exists theoretically in two isomeric forms with a chair-form ring, and the methyl substituent either axial or equatorial. Since these rapidly interconverl through a CH3... 

Figure 9-6 Deealiii (I CA IODEL. SeTena vvare). I he cis decaliri molecLile is two chair forms of cvdohcxanc fused at a common bond.

The most stable conformation of 1 3 dioxan 5 ol is the chair form that has its hydroxyl group in an axial orientation Suggest a reasonable explanation for this fact Building a molecular model IS helpful... 

The overall picture of the many results which have been obtained with hetero-substituted cyclohexane rings is a very consistent one. Cyclohexane itself in its lowest energy conformation adopts the so-called chair conformation, as depicted in Figure 3 by the two outer formulae (a, b). These are contained in energy wells ca. 42 kJ moP deep. Another conformation, of low abundance in cyclohexane at normal temperatures, but which is important in some substituted derivatives, is the twist form (c, d). This is ca. 22 kJ moP less stable than the chair forms, and it lies on the lowest-energy pathway between them. 

One interesting exception to the generalization that chair forms predominate in this series is shown by the tetrathian (44), which prefers the twist form in the solid phase, and also in CS2 solution at 0 C both chair and twist forms coexist in solution, with a free energy difference of ca. 3.5 kJ between them (67JA5978, 68JA2450). 

Substitution on a cyclohexane ring does not greatly affect the rate of conformational inversion but does change the equilibrium distribution between alternative chair forms. All substituents that are axial in one chair conformation become equatorial on ring inversion, and vice versa. For methylcyclohexane, AG for the equilibrium... 

In the absence of steric factors e.g. 5 ), the attack is antiparallel (A) (to the adjacent axial bond) and gives the axially substituted chair form (12). In the presence of steric hindrance to attack in the preferred fashion, approach is parallel (P), from the opposite side, and the true kinetic product is the axially substituted boat form (13). This normally undergoes an immediate conformational flip to the equatorial chair form (14) which is isolated as the kinetic product. The effect of such factors is exemplified in the behavior of 3-ketones. Thus, kinetically controlled bromination of 5a-cholestan-3-one (enol acetate) yields the 2a-epimer, (15), which is also the stable form. The presence of a 5a-substituent counteracts the steric effect of the 10-methyl group and results in the formation of the unstable 2l5-(axial)halo ketone... 

The isomer a-[NS(0)Cl]3 (8.15a) is a six-membered ring in the chair form with equal S-N bond lengths (1.57 A). The three chlorine atoms are... 

This reaction has been well studied by NMR. Another important exchange process is the inversion of cyclohexane between equivalent chair forms (Scheme XII), a process in which a proton is exchanged between equatorial and axial positions... 

Cycio-tetrametaphos-phoric acid (anions known in both boat and chair forms)... 

The factors influencing ring size and conformation have not yet become clear. Thus, the yellow (MeAs)5 has a puckered Asj ring with As-As 243 pm and angle As-As-As 102° there is also a more stable red form. (PhAs)6 has a puckered Asg (chair form) with As-As 246 pm and angle As-As-As 91°. Numerous polycyclic compounds As R have also been characterized, for example the bright-yellow crystalline tricyclo-... 

Identify each substituent in the following compound as axial or equatorial, and tell whether the amformation shown is the more stable or less stable chair form (yellow--green = Cl) ... 

The cyclitols are a group of carbocyclic sugar derivatives having the general formulation 1,2,3,4,5,6-cyclohexanehexol. How many stereoisomeric cyclitols are possible Draw them in their chair forms. 

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. 

Chloro-2,4-dimethyl-cyclohexane (less stable chair form)... 

The atomization energy, electron affinity and ionization potential have been calculated for 1//-azepine. and a difference in energy between the boat and chair forms of 64.8 kJ mol -1 deduced.98 The calculated dipole moment for l//-azepine is 4.67 D.98 Hiickel-London theory has been applied to calculate the ring-current octopole hypersusceptibilities of l//-azepine."... 

The same preferences have been calculated315 and observed319 in the 1,2-dithiane oxide system. Although the chair forms are also more stable than the twist or boat in 1,3-, 1,4-dithianes and 1,3,5-trithianes, the preference of the oxygen is highly variable, depending on steric and electronic interactions. 

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