Molecular Representations

Organic chemistry, as a science, has its own “written language” that we use to communicate the ideas in OChem. You have already seen molecules before in general chemistry and are familiar with some types of molecular representations. Let’s go over the typical ways we draw molecules in organic chemistry and discuss their pros and cons.

Molecular Formulas

Molecular formulas show us the number and ratio of atoms in a molecule. So, by looking at the molecular formula you can easily determine how many atoms of each type you have in the molecule’s composition.

Examples of molecular formulas.

Some small molecules, like HCl or H2SO4 are simple enough, so we know that those are the hydrogen chloride (or hydrochloric acid when in solution) and sulfuric acid. Those can’t really represent anything else, so it’s easy to guess what the molecule is. THe other two examples, however, are much trickier. Take, for instance, the first molecule I have in this set: C3H8O, this seemingly simple molecular formula can actually represent multiple different molecules. In fact, C3H8O can be one of 3 completely different molecules with different names, structures, physical and chemical properties!

Molecular formulas are, however, the least useful type of molecular representation in organic chemistry. Organic chemistry is all about structure. So, by not showing the molecular structure, you can’t actually tell what the molecule is and what properties it may have.

Lewis Structures

There are two types of Lewis structures that we use for the molecular representations.

Complete Lewis Structures

There are full (complete) and condensed Lewis structures. While the full structures have their uses, they are rarely used for molecular representations in organic chemistry. We do however use condensed structures from time to time.

complete lewis structures

The most valuable aspect of the complete Lewis structures as they show all bonds and non-bonding (spare) electrons in the molecule. The big issue with the Lewis structures is that they take a very long time to draw. It’s ok for one or two molecule, but when you have to draw a few dozens, it becomes very tedious. And, as you can imagine, you’ll be drawing a bunch of molecules in organic chemistry! Here’s an example:

Try copying the molecule above a few times and see how much time it’ll take you. 😹 It’s also easy to lose some atoms especially in big molecules. And lost atoms will lose you points on the test. 😭

Condensed Lewis Structures

Condensed structures, however, are more common in organic chemistry as they are much easier and faster to draw. Here are a couple of example of the molecules from above:

examples of condensed lewis structures

In condensed Lewis structures we only show the bonds between the groups of atoms connected to each other. In the example above, the first molecule has a methyl group (CH3) shown as a group. It’s the implicit assumption that hydrogens are attached to carbon, which in turn is connected to the rest of the molecule through the bond that we show. We also often skip the implicit electron pairs on heteroatoms like O or N. We’ll only show those when they are important for a reaction or a mechanism we’re discussing.

We also tend to skip the bonds between the groups of atoms in long chains. For instance, look at the molecule of 2-methylhexane, which can be shown with or without bonds between the groups.

In the left example, you show all bonds between the groups. In the right example, we don’t show the bonds, so you gotta keep those in mind when thinking about your molecule.

Skeletal (Line) Structures

Skeletal structures is the most common type of the molecular representation in organic chemistry. In the skeletal structures we show the absolute minimum that we need to represent a molecule in a non-ambiguous way. Thus, skeletal drawings have certain rules we use to draw them:

  1. Carbons and Hydrogens are implicit. We dont show hydrogens attached to carbons and we only show bonds connecting carbons as lines.
  2. If you have heteroatoms like O or N, you must show them and you must show hydrogent on those atoms too (if any).
  3. Electron pairs are optional (often implicit), however, charges are not. So, it’s ok not to show the electrons but you absolutely must show the charges like a ⊕ or a ⊖ on the corresponding atoms in your molecule.

Thus, the Lewis structures that we’ve seen before:

Can be drawn using the skeletal structures as following:

examples of skeletal structures

Skeletal structures are much faster and easier to draw, so those are the preferred method of molecular representation in organic chemistry. You definitely wanna make sure you know how to interpret those if you wanna ace your tests!

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