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!)
Show all the 4-coordinate carbons
Show all the 3-coordinate carbons
Show the 2-coordinate carbons
Observe that the 4 atoms connected to 4-coordinate carbon are always splayed out, pointing to the 4 points in a tetrahedron. We never see all 4 connected atoms in a single plane. When we have 3 things bound to carbon, the plane they define always contains the central carbon. When there are 2 things connected, they lie on a line containing that carbon.

Now let's look at distances. Distances on an atomic level are best expressed in Ångstroms (Å); 1 Å = 10-10 m or 0.1 nm. If we measure the C-C distances between any two 4-coordinate carbons, we find they all wind up as about 1.54 Å. Some are a little longer. However, if we look at distances between any two 3-coordinate carbons, they are shorter (1.34 Å is typical, but as you will see next term, some effects can lengthen this) Rarely--if ever--will such a distance be greater than 1.4 Å.. When we measure the distance between 2-coordinate carbons we see a shorter value still--1.21 Å. There are intermediate distances between carbons of different coordination number.

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.

When we look across the tens of millions of organic compounds that exist, we find most of the carbons conform to these several regular structural observations. We can connect several geometric parameters with the coordination number of carbon:
  1. 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°.
  2. 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).
  3. 2-coordinate carbon: distances between any two of these will be 1.20-1.24Å, and the angles are always very close to 180°.
There are, of course, exceptions. Small rings constrain bond angles, and the molecule often responds by showing longer-than-normal distances. We will be exploring how an understanding of bonding can get us to a predictive model of what a bond length or angle ought to be.