Axial bonds, of cyclohexane

The axial bonds of cyclohexane are those that are perpendicular to the average plane of the ring. There are three axial bonds on each face of the cyclohexane ring, and their orientation (up or down) alternates from one carbon to the next. 

Larson and Cremer have explored another approach to dissecting BDE into inherent and RSE effects4 There is a relationship between C-H BDE and the vibrational frequencies of the bonds Furthermore, the vibrations can be determined for C—H bonds in speeific conformations, for example, the equatorial and axial bonds in cyclohexanes or the anti and gauche bonds in amines. 

A similar situation prevails when we compare conformations of cyclic ethers with those of the corresponding cycloalkanes. For example, tetrahydropyran, the ether analog of cyclohexane, exists in a chair conformation. Following predictions of VSEPR theory, the two oxygen lone pair electrons are shown in positions corresponding to the axial and equatorial C —H bonds of cyclohexane. The conformation of tetrahydropyran is particularly important because many carbohydrates, such as glucose, exist as six-membered tetrahydropyran rings. 

The C—H bonds in the chair conformation of cyclohexane are not all equivalent but are divided into two sets of six each called axial and equatorial... 

Conformational inversion (ring flipping) is rapid in cyclohexane and causes all axial bonds to become equatorial and vice versa As a result a monosubstituted derivative of cyclohexane adopts the chair conforma tion in which the substituent is equatorial (see next section) No bonds are made or broken in this process... 

Axial bond (Section 3 8) A bond to a carbon in the chair conformation of cyclohexane oriented like the six up and down bonds in the following... 

The 12 hydrogen atoms of cyclohexane do not occupy equivalent positions. In the chair conformation six hydrogen atoms are perpendicular to the average plane of the molecule and six are directed outward from the ring, slightly above or below the molecular plane (see Fig. 1.6). Bonds which are perpendicular to the molecular plane are known as axial bonds, and those which extend outward... 

The chemical shift of a nucleus depends in part on its spatial position in relation to a bond or a bonding system. The knowledge of such anisotropic effects is useful in structure elucidation. An example of the anisotropic effect would be the fact that axial nuclei in cyclohexane almost always show smaller H shifts than equatorial nuclei on the same C atom (illustrated in the solutions to problems 37, 47, 48, 50 and 51). The y-effect also contributes to the corresponding behaviour of C nuclei (see Section 2.3.4). 

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°. ... 

Axial bond (Section 4.6) A bond to chair cyclohexane that lies along the ring axis perpendicular to the rough plane of the ring. 

Atorvastatin, structure of, 105. 516 ATP (see Adenosine triphosphate) ATZ, see Anilinothiazolinone, 1031-1032 Aufbau principle. 6 Axial bonds (cyclohexane), 119 drawing, 120 Azide, amines from, 929 reduction of, 929 Azide synthesis, 929 Azo compound, 944 synthesis of, 944-945 uses of. 945... 

On each carbon, one bond is directed up or down and the other more or less in the plane of the ring. The up or down bonds are called axial (a) and the others equatorial (e). The axial bonds point alternately up and down. If a molecule were frozen into a chair form, there would be isomerism in monosubstituted cyclohexanes. For example, there would be an equatorial methylcyclohexane and an axial... 

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. 

X-ray analysis has shown that cyclodecane in its most preferred conformation exists as shown below in which the two chain forms of cyclohexane are joined by 1,3 axial bonds and six hydrogen atoms are intraannular and 14 peripheral. 

On each carbon, one bond is directed up or down and the other more or less in the plane of the ring. The up or down bonds are called axial and the others equatorial. The axial bonds point alternately up and down. If a molecule were frozen into a chair form, there would be isomerism in monosubstituted cyclohexanes. For example, there would be an equatorial methylcyclohexane and an axial isomer. However, it has never been possible to isolate isomers of this type at room temperature.219 This proves the transient existence of the boat or twist form, since in order for the two types of methylcyclohexane to be non-separable, there must be rapid interconversion of one chair form to another (in which all axial bonds become equatorial and vice versa) and this is possible only through a boat or twist conformation. Conversion of one chair form to another requires an activation energy of about 10 kcal/mol (42 kJ/mol)220 and is very rapid at room temperature.221 However, by... 

The six axial bonds are directed upward or downward from the plane of the ring, while the other six equatorial bonds are more within the plane. Conversion of one chair form into another converts all axial bonds into equatorial bonds and vice versa. In monosubstituted cyclohexanes, for electronic reasons, the more stable form is usually the one with the substituent in the equatorial position. If there is more than one substituent, the situation is more complicated since we have to consider more combinations of substituents which may interact. Often the more stable form is the one with more substituents in the equatorial positions. For example, in ct-1,2,3,4,5,6-hexachlorocyclohexane (see above) four chlorines are equatorial (aaeeee), and in the /Tisomer all substituents are equatorial. The structural arrangement of the /3-isomer also greatly inhibits degradation reactions [the steric arrangement of the chlorine atoms is unfavorable for dehydrochlorination (see Chapter 13) or reductive dechlorination see Bachmann et al. 1988]. 

---