Alkanes three-dimensional representation

Propane is the three-carbon alkane, with formula C3H8. Figure 3-8 shows a three-dimensional representation of propane and a Newman projection looking down one of the carbon-carbon bonds. 

The three-dimensional representations and the ball-and-stick models for these alkanes indicate the tetrahedral geometry around each carbon atom. In contrast, the Lewis structures are not meant to imply any three-dimensional arrangement. Moreover, in propane and higher molecular weight alkanes, the carbon skeleton can be drawn in a variety of different ways and still represent the same molecule. 

Every branched or unbranched alkane chain is built as a zigzag chain. When a chain contains three or more C atoms a part of the molecule can turn freely around a single bond in relation to the rest of the molecule. In a spontaneous process the molecule converts from one three-dimensional structure to another. According to a chemists the molecule changes from one conformation into another . Figure 3.15 is a representation of an alkane chain and two conformations of the same molecule. 

Figure 26 Schematic representation of the phase behaviour of binary COg + alkane systems in the three-dimensional p-T-x space x = mole fraction, HC = hydrocarbon). a, b, Type found for COg + octane, c, d, type found for COg + hexadecane and for COg + l,b, Q, 5, 9,2 i-hexamethyltetracosane...

Three-dimensional (3D) Shapes and Dynamics of Some Alkanes Before proceeding, it is worthwhile to appreciate the pretty 3D shapes of some alkanes. Figures 4.1a-c show ethane, propane, and butane using a ball-and-stick representation. 

Configuration of cycloalkanes is based on the combinations of tetrahedrons, similarly to the case of alkanes. However, in some cases the ring structure requires that the angles between C-C bonds deviate from the normal tetrahedral values (109°28 ). Almost ideal tetrahedral angles are present in the molecule of cyclohexane. For the pictorial representation of three dimensional molecular structures of cyclic molecules special descriptive projection is used. Let us analyse the structure of cyclohexane molecule in more detail. 

As is the case with the acyclic alkanes, it will be necessary to keep in mind that the two-dimensional representations of Figure 3.6 have a three-dimensional reahty, which can be represented, among other ways, as shown in Figure 3.7. 

---