Cyclohexane puckered ring

Therefore the concept of puckered rings was introduced and the existence of boat and chair forms in cyclohexane. 

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

On the other hand, cyclohexane, unlike benzene, is saturated, and its structure can readily be explained on the basis of a tetrahedral carbon atom. Hermann Sachse in 1890 deduced that cyclohexane is a cyclic hexamer. It is, however, nonplanar with several forms for the puckered rings existing in solution in an equilibrium. These ideas, surprisingly, lay dormant for many years until Ernst Mohr ° reintroduced them in 1915. 

Let us look more closely at the matter of puckered rings, starting with cyclohexane, the most important of the cycloalkanes. Let us make a model of the cule, and examine the conformations that are free of angle strain. 

Puckered ring When a ring system adopts a conformation that is not planar, it is said to be puckered, e.g. the boat or chair conformations of cyclohexane. 

The oxidation of inositols to inososes (cycloses) by Acetobacter suboxydans is of importance in the determination of configuration (e.g., Posternak s work on mi/o-inositol, p. 275) and in the interconversion of inositols by reduction of the inosose. Bertrand s rule (p. 133) accurately predicts the point of attack in the acyclic series, but the situation is more complex in the inositol series. The specificity of A, suboxydans appears to be related to the conformation of the cyclohexane ring of the inositols. Inositols, like other substituted cyclohexanes, may exist in boat or chair forms (86), (See Chapter I.) The chair form in which the distances between the hydroxyl groups is at a maximum appears to be the preferred conformation. Substituents that are oriented nearly parallel to the average plane of the puckered ring are called equatorial. They lie alternately above and below the plane. Substituents that are perpendicular are called axial (formerly, polar) (87), Conversion to the second chair form causes interchange of... 

Fig. 11.34 A fragment of the crystal structure of ice. Each O atom is at the center of a tetrahedron of four O atoms at a distance of 276 pm. The central O atom is attached by two short O-H bonds to two H atoms and by two relatively long 0—H bonds to two neighboring HjO molecules. Overall, the structure consists of planes of hexagonal puckered rings of H2O molecules (like the chair form of cyclohexane).

Cyclohexane is strain-free because it adopts a puckered chair conformation, in which all bond angles are near 109° and all neighboring C—H bonds are staggered. Chair cyclohexane has two kinds of positions axial and equatorial. Axial positions are oriented up and down, parallel to the ring axis, whereas equatorial positions lie in a belt around the equator of the ring. Each carbon atom has one axial and one equatorial position. 

For cyclohexane, there are two extreme conformations in which all the angles are tetrahedral. These are called the boat and the chair conformations and in each the ring is said to be puckered. The chair conformation is a rigid structure, but the boat form is flexible and can easily pass over to a somewhat more stable form known as the twist conformation. The twist form is 1.5 kcal mol (6.3 kJ mol )... 

Generally six-membered rings are more stable than five-membered ones and on the basis of the tetrahedral theory Sachse in 1890 pointed out that cyclohexane could exist in two forms known as boat and chair conformations and neither of which is planar. They are actually puckered i.e., non-planar and are free of angle strain and so are called strainless rings. 

Figure 3. The conformational sphere for pyranoid rings. The perfect chairs are at the north and south poles (0=0 and 180 , respectively). The boat and skew (B and S designations) at the equator permit pseudorotation that is slightly hindered, at least for cyclohexane. The envelopes, E (also called sofas and half-boats), and half-chairs, H, are not observed for rings coiqposed of saturated carbon and oxygen atoms, but are iiqportant forms for rings with unsaturated carbon atoms. The aiqplitude of puckering corresponds to the radius of the sphere.

Cyclohexane minimizes its ring strain by being puckered rather than flat. The two extreme conformations are the more stable chair and the less stable boat. The twist-boat conformer is less stable than the chair by about 23 kJ/mol, but is more stable than the boat. It is formed from the boat by moving one flagpole" to the left and the other to the right. See Fig. 9-5. 

X-Ray crystallographic analyses show that tetrahydro-l,2-oxazines adopt a chair conformation, although the ring is considerably more puckered than that of cyclohexane. The lone pair electrons of IV-methyl derivatives assume an axial orientation and in the case of 2,5-dimethyl compounds the trans conformer is strongly preferred (75T523). 

Explain why cyclohexane, a substance that contains a six-membered ring of carbon atoms, is not flat but instead has a puckered, nonplanar shape. Predict the values of the C-C-C bond angles. 

Saturated six-membered rings are the most common ring systems in nature because they present an optimal conformational situation. As seen in cyclohexane, die molecule adopts a puckered shape called a chair conformation in order to avoid angle strain in the ring bonds. In the chair form, all bond angles are 109° and all the bonds are staggered. 

The prefix cyclo- is used to name cycloalkanes. Cyclopropane is planar, but larger carbon rings are puckered. Cyclohexane exists mainly in a chair conformation with all bonds on adjacent carbons staggered. One bond on each carbon is axial (perpendicular to the mean carbon plane) the other is equatorial (roughly in that plane). The conformations can be interconverted by flipping the ring, which requires only bond rotation and occurs rapidly at room temperature for cyclohexane. Ring substituents usually prefer the less crowded, equatorial position. 

In cyclobutane and cyclope- ntane the angle strain is minimised by ring puckering, while for cyclohexane there is no angle strain., ... 

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