The stick, or Deriding, model shows the carbon at the center of the tetrahedron (dark gray) with the hydrogens at each vertex (light gray); the covalent radius of each atom is approximated by the size of the color band. The ball-and-stick model provides similar information and is sometimes easier to visualize, and the true Van derWalls radius of the atoms is best shown by the space-filling model (shown with a ball-and-stick overlay). More complex hydrocarbons containing carbon chains can be formed by creating additional carbon-carbon bonds, as shown below for chains containing two, four and six carbon atoms; you should note that in these structures, each carbon remains bonded to four other atoms (a valence of four).
These molecules are also shown below in space-filling format:
When viewing organic molecules it is important to note that the rotation around carbon-carbon single bonds is generally very rapid (greater than 106 rotations per second) and the chain can assume a large number of conformations (termed conformational isomers) which are rapidly interconverted and cannot be separated under normal circumstances. A sample of conformational isomers for four- and six-carbon chains is shown below:
While drawings like those shown above most clearly show the structural features of organic molecules, it is clear that simpler methods are needed for routine representation of organic structures. The simplest, and least informative, is the simple representation of the molecular formula, showing ratios of atoms. An alternative format in which all atoms and all covalent bonds are shown is the "line-bond" or Kekuleé structure. While this does provide information regarding bonding in the molecule, structures of this type are tedious to draw, and organic molecules are more commonly represented using some form of abbreviated "condensed structure".
A further condensation of structural information is accomplished in "line" or "structural" drawings. In this format, each vertex in the drawing corresponds to a -CH2- group and each terminal line to a -CH3 group. It is assumed that all carbons have the appropriate number of hydrogens, and these are typically not shown. Multiple bonds are shown in structural drawings as double or triple lines, and it is again understood that the appropriate number of hydrogen atoms are attached.
In actual practice, you will find organic structures represented by hybrids of all of these methods. As shown above, line structures are often utilized to represent the backbone of a molecule, and some variation of condensed structures is utilized to show side-chains, or three-dimensional information. As a student of organic chemistry, it is essential to learn to recognize these structural representations and to be able to interchange formats and to use these to visualize the full three-dimensional molecule in question.
