Structures A and B represent cyclohexane in its most stable conformation--the chair. Contrary to Baeyer's Strain Theory (1885), which postulated that cyclopentane is the least strained of the C3, C4, C5 and C6 cycloalkanes, it is cyclohexane that is the least strained. Baeyer assumed that the carbons of the cycloalkanes lie in a plane. While this assumption is, of geometric necessity, true for cyclopropane (C3), it is not true for other cycloalkanes. The recognition of the tetrahedral nature of carbon in alkanes by van't Hoff and LeBel (1876) led Sachse (1890), as refined by Mohr (1918), to conclude that cyclohexane is non-planar in its most stable conformation.
Consider structure A. The gray atoms are carbons; the red and blue atoms are hydrogens.Structures A and B are 3-dimensional objects illustrated in two dimensions. Imagine the plane of your terminal screen passing through C1 and C4 and their attached hydrogen atoms. Then C2 and C3 (and their hydrogens) lie in front of the screen while C5 and C6 not numbered) and their hydrogens lie behind the plane of the screen. This view also shows that bond C2-C3 is parallel to bond C5-C6. Thus, C2, C3, C5, and C6 lie in a plane with C4 above the plane and C1 below it. The observant eye will also recognize that bond C1-C2 is parallel to C4-C5 and C1-C6 is parallel to C3-C4. Each pair of parallel bonds form a plane. Which atoms are above and below these planes?
The C-H bonds in cyclohexane are of two types: axial and equatorial (not equitorial! Think equator!) In structure A, the axial bonds have red hydrogens; the equatorial bonds have blue hydrogens. The equatorial bonds, or hydrogens, lie about the "equator" of the cyclohexane ring, or in the "plane" of the ring. The axial bonds project perpendicular to the "plane" of the ring, or to continue with the earthly metaphor, toward the north (above the plane of the ring) and the south (below the plane of the ring) poles. In structure A, the axial bonds alternate pointing "north" (C2, C4, C6) and south (C1, C3, C5). A similar pattern also exists for the equatorial bonds. The blue equatorial hydrogen at C1 is above its axial red hydrogen. This situation is also true at C3 and C5. At C2, C4 and C6, the equatorial blue hydrogens are below the axial red hydrogens.
Cyclohexane is not a static structure. It is capable of undergoing conformational inversion, i. e., able to go from one chair to another. A simple way to understand this process (not the actual way it occurs) is to once again imagine C2, C3, C5, and C6 lying in a rigid plane with the proviso that rotation can occur about C-C bonds. If C1 in structure A is pushed from below the plane to above the plane, and C4 is pushed below the plane, structure B is formed, a cyclohexane equal in energy to A. In this conformational inversion, the blue equatorial hydrogens of A become axial in B, and the red axial hydrogens of A become equatorial in B.