Most organic compounds show an extremely regular set of structural features. These can serve as a starting point for understanding molecular structure and developing a sense of what kinds of behavior to expect. You will also be discussing the 3-dimensional features of molecules, and that requires a solid understanding of structure (and of our shorthand methods for expressing structure in a 2-D environment).

In this example, we will look at a compound with the extremely complex name Estren-17α-ethynyl-18-homo-17β-ol-3-one. This is a synthetic estrogen hormone maketed under the name Levonorgestrel. For our current purposes, it contains all of the common types of carbon that we will encounter, and allows us to see the regularities and similarities as well as to see some of the differences. You should begin to see these regularities and start to ask about the differences.

First, let's look at environment. Carbon can be connected to 2, 3, or 4 other atoms. (There are rare cases of carbon bound to only one other atom, but extremely few where it is connected to 5 or more. For this course, NEVER draw carbon connected to 5 or more other things!) | |

Now let's look at distances. Distances on an atomic level are best expressed in Ångstroms (Å); 1 Å = 10 Let's return to geometric arrangements as measured by the angles defined by the various things connected to each type of carbon. We'll start with the simple one: 2-coordinate carbon is linear, so the angle at each atom is 180°. The angles around 3-coordinate carbon are always close to 120°. We won't measure every one, but a couple examples prove the point that 4-coordinate carbon shows angles close to 109.5°, the expected angle in a perfect tetrahedron. |

- 4-coordinate carbon: distances between two of these will be 1.52-1.57 Å. Angles at the central carbon will be somewhere between 105° and 113°.
- 3-coordinate carbon: distances between two of these will usually be between 1.3-1.4 Å. Angles around the central carbon will be 116-124° and will add up to close to 360° (indicating the carbon and its three substituents are coplanar).
- 2-coordinate carbon: distances between any two of these will be 1.20-1.24Å, and the angles are always very close to 180°.