Bond Angles and Ball-and-Stick Models

It is well established that the normal carbon atom forms its four single bonds in compounds of the type CX4 so that the four attached atoms lie at the corners of a regular tetrahedron. The bond angles X−C−X are 109.5^o and this value is the "normal" valence angle of carbon. For many purposes, ball-and-stick models of organic compounds give useful information about the spatial relationships of the atoms, and for CX4 the angles between sticks are set at 109.5^o (Figure 2-1). Organic molecules strongly resist deformation forces that alter their valence angles from normal values. Therefore ball-and-stick models correspond better to the behavior of actual molecules if the connectors representing single bonds are made to be rather stiff.

Figure 2-1: Ball-and-stick model of CX4

Figure 2-2: Ball-and-stick models of some simple organic molecules

Whereas methane, CH4, is tetrahedral, ethene, C2H4, is not. According to the best available physical measurements, all six atoms of ethene lie in a single plane and the H−C−H bond angles are 117.3^o. Methanal (formaldehyde) also is a planar molecule with an H−C−H bond angle of 118^o.

Figure 2-2), but the H−C−H angles are inaccurate because they are 109.5^o rather than the observed 117o to 118^o.

Ethyne, C2H2, has been established experimentally to be a linear molecule; that is, the H−C−CH bond angle is 180^o:

Structural units that have C−C−C valence angles substantially less than the tetrahedral value include double and triple bonds, and small rings such as cyclopropane. Several bent bonds are required to construct models of compounds containing these units. Interestingly, such compounds are much less stable and more reactive than otherwise similar molecules for which models can be constructed with straight sticks at tetrahedral angles.