The vast majority of structures that you’re going to see in organic chemistry are the skeletal or line structures. Skeletal structures use a lot of simplifications to make the structure drawing fast and efficient. Here are some examples below:
Skeletal structures “rules”
When we draw the skeletal structures we make the following assumptions:
- We focus on showing the bonds. Since the majority of atoms in organic molecules are typically carbons, we don’t show them. So, every angle in a line structure represents a carbon atom. The end of a bond is a carbon as well (unless a heteroatom is specified).
- Hydrogens are implicit. This means that we don’t show the hydrogens unless it is absolutely essential. The only time when hydrogens must be shown is when they are on the heteroatoms (aka, anything that is not a carbon, like a nitrogen or an oxygen).
- Electron pairs are implicit as well unless they are needed for some purposes. For instance, you generally won’t show an electron pair unless you’re using in in the mechanism.
Thus, it’s important to remember how many bonds and spare electron pairs atoms typically have. This way you’ll always be able to spot the “hidden” or implicit electron pairs or hydrogens. Here’s a quick reminder reference table:
Let’s look at the same molecule written out as a skeletal structure, a skeletal structure with the implicit hydrogens shown, and the condensed structure:
Breaking skeletal structures one line at a time
Reading the skeletal structures takes some practice. So, let’s look at the steps that you want to be doing in your head every time you see a skeletal structure to help you understand what exactly the structure entails.
Step 1: Identify the carbons in the molecule
Every angle is a carbon. Every end of the line is a carbon. However, if you have another atom shown at the end of the line, it’s not a carbon, it’s the atom that is shown! Thus, the molecule above has 6 carbons highlighted in red, not 7!
Step 2: Add implicit hydrogens and electron pairs to satisfy the octets (see the reference table above)
How to deal with charges in the skeletal structures?
We’re going to use the same reference table from above. The two main things to remember:
- A positive charge on carbon means an empty p orbital. Other elements like nitrogen or oxygen are too unstable with only 6 electrons in the outer shell, so other elements rarely have a positive charge and not have a full octet.
- A negative charge on carbon or any other atom means an extra electron pair.
Here’s an example:
So, in a nutshell, your atoms should always have 4 “things” attached to them, whether those are bonds, electron pairs, or empty orbitals.
Dos and Don’ts of the skeletal structures
There are certain guidelines for drawing your skeletal structures. They are not strict rules, but rather a recommendations how to make your molecules look better and convey more accurate information about the molecule.
Guideline 1: Spread your groups
Don’t cluster your groups into a “broom” all hanging in one direction! The VSEPR theory clearly states that the groups are distributed evenly around the central atom, so follow this general principle when drawing your structures too. The general rule of thumb is to add your groups in the same direction of the zigzag angle:
Guideline 2: Spread your molecule in space.
While you can twist your molecule in any direction, molecules tend to be more stable when they are spread out in space.
Plus, the spread out molecule looks more aesthetically appealing. Well, at least to the majority of organic chemistry instructors.