Cyclohexane bond angles

Cyclohexane adopts a strain-free, three-dimensional shape, called a chair conformation because of its similarity to a lounge chair, with a back, a seat, and a footrest (Figure 4.7). Chair cyclohexane has neither angle strain nor torsional strain—all C—C-C bond angles are near 109°, and all neighboring C-H bonds are staggered. 

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. 

Petroleum contains hydrocarbons other than the open-chain alkanes considered to this point. These include cycloalkanes in which 3 to 30 CH2 groups are bonded into closed rings. The structures of the two most common hydrocarbons of this type are shown in Figure 22.5 (p. 585). Cyclopentane and cyclohexane, where the bond angles are close to the ideal tetrahedral angle of 109.5°, are stable liquids with boiling points of 49°C and 81°C, respectively. 

O-M-O bond angles required. One can relate bite sizes (distances between the donor atoms) of strain-free chelate rings of different sizes to the dimensions of the chair form of the cyclohexane ring (39), which has the minimum steric strain possible for a cycloalkane. The bite size of strain-free four- and six-membered chelate rings both correspond to... 

Cyclopentane has a smaller amount of strain than does cyclohexane when forced to have all its carbons in a plane (flat). Cyclopentane is nearly flat since the bond angles in a regular pentagon (108°) are near the tetrahedral angle (109.5°). The bond angles in a flat cyclohexane are 120°, which are not near the 109.5° bond angle. To avoid this strain, the cyclohexane ring buckles and becomes nonplanar. 

An instructive illustration of the effect of molecular motion in solids is the proton resonance from solid cyclohexane, studied by Andrew and Eades 101). Figure 10 illustrates their results on the variation of the second moment of the resonance with temperature. The second moment below 150°K is consistent with a Dsi molecular symmetry, tetrahedral bond angles, a C—C bond distance of 1.54 A and C—H bond distance of 1.10 A. This is ascertained by application of Van Vleck s formula, Equation (17), to calculate the inter- and intramolecular contribution to the second moment. Calculation of the intermolecular contribution was made on the basis of the x-ray determined structure of the solid. 

Cyclohexane as a planar molecule Most stable chair conformation Bond angle 120 of cyclohexane... 

The fully saturated rings share with cyclohexane the property of being able to adopt one or more conformations which are virtually free of torsion or bond angle strain. 

Since there is a difference in reactivity between the different cyclopar-affins, we will begin with the least reactive—cyclohexane. If cyclohexane existed as a flat hexagon, the carbon-carbon bond angle would be 120°. However, cyclohexane takes on a puckered structure as shown in the two conformations in Figure 1-10. The bonds in the conformations of cyclohexane have bond angles of 109.5°. Thus, the stability of the bonds is the same as in the straight-chain alkanes. 

The five carbon atoms of cyclopentane form a flat regular pentagon with internal angles of 108°, which are very close to normal carbon bond angles. Actually, cyclopentane molecules are not exactly flat however, the bond angles are close enough to the normal bond angle that these bonds are substantially as stable as those of cyclohexane. 

Fig. 1-10. two configurations of cyclohexane with 109.5° bond angles (hydrogen omitted). 

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