Cyclohexane, axial bonds bond angles

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. 

This discussion may still be oversimplified for we have not made allowance for changes in Bayer strain relative to cyclohexane. This could be a result of interaction of axial substituents which leads to bond angle corn-... 

Miscellaneous Photochemical Reactions.—14a-Steroids are converted into an equilibrium mixture of 14oc- and 14j8-isomers ca. 1 19) on irradiation at 254 nm, in cyclohexane containing mercuric chloride or bromide/ The reaction comprises abstraction of 14-H and recombination, probably involving bromine atoms. Other isomerisations at tertiary hydrogen permit equilibrations, e.g. of cis- and trans-decalins, but the 5)3 — 5a-steroid conversion is very slow compared with reaction at C-14. Distortion from ideal bond-angles at C-14 probably loosens the hydrogen atom here. Axial secondary methyl substituents can also be epimerised into the more stable equatorial conformation. 

The effect of introducing /j -hybridized atoms into acyclic molecules was discussed in Section 2.2.1, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are seen when sp centers are incorporated into six-membered rings. Whereas the energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecy-clohexane ° and to 4.9 kcal/mol in cyclohexanone. The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffraction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. The C(l)—C(2) bond length is 1.335 A, and the C(l)-C(2)-C(3) bond angle is 123°. The substituents at C(3) and C(6) are tilted from the usual axial and equatorial directions and are referred to as pseudoaxial and pseudoequatorial. 

The lower-energy, chair conformation of cyclohexane has a sixfold axis of alternating S3mimetry, which divides the carbon-hydro-gen bonds into two types, shown in Mg.3. Six bonds are parallel to the axis the axial bonds, and six extend radially outward at angles of 109 5° to the axis the so-called equatorial bonds. This fact, that the carbon-hydrogen bonds divide into two types is important in considering energy differences between substituted cyclohexanes, referred to later. 

Cycloalkanes C H2 hydrocarbons n = 3, 4 Cyclopropane, cyclobutane (strained bond angles) n = 6 Cyclohexane (most stable, chair conformation) axial (less stable) equatorial (more stable) substituent positions (4-2, 3)... 

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°. ... 

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