Conformations of alkanes [with free study guide]

When speaking about the conformations of alkanes, you see one of the cruel realities of organic chemistry: that it exists in three dimensions. However, you might have noticed that paper is only two dimensional.

This causes a real problem for visualizing organic chemistry molecules for many of us. It isn’t easy to look at something and visualize it in three dimensions when you’re limited to two on paper. How do we get around this? Well there is a number of simple and some complex ways to do this, and we call them visualizations. The first and most common way to visualize organic chemistry molecules in three dimensions is using wedges and dashes. A wedge signifies that the portion of the organic chemistry molecule on top of the wedge is coming out of the paper; the dash symbolizes that it is going into the paper. The best way to become familiar and comfortable with wedges and dashes are to practice them and compare it to a molecule in three dimensions. Here is an example of that below:

What you should notice with the wedges and dashes is that first they are simple to draw. Second, they are intuitive, in so much as dashes come out at you. Practice with these a little bit and it will be really easy to see and visualize that dash coming out at you.

Another way to visualize organic molecules is using the sawhorse model. In this we imagine that we are looking down the shaft of one particular bond, usually a carbon-carbon bond and everything else comes off of that carbon-carbon bond. In an undergraduate organic chemistry class, the most common place you will see a sawhorse is when trying to visualize butane. In the butane visualization, you are looking down the C2 – C3 axis, and can see where the rest of the molecule falls and where rotations need to occur to minimize strain and interaction. However, we are not limited to just butane. In fact, you can make your sawhorse visualization as complex as you’d like. In the one below we show you hexane.

Another visualization professors love is the Newman projection. This is similar to the sawhorse, except we are looking directly down a bond axis and have replaced that bond with a large ball. This symbolizes the atom that were looking down. Newman projections, as with sawhorse projections, are useful for looking at shorter organic molecules (usually alkanes) where you are trying to see the interaction between different parts of the molecule to see which conformation is the lowest energy. Below is the common Newman projection for butane, and then one that is a little more complex.

Finally, you should be familiar with Fischer projections. In Fischer projections, the backbone of the organic molecule is straight up and down on your page with the side chains coming off each end. We like to say that the side chains are actually bow ties, click here for an entire post on just how to look at Fisher’s using the bow tie method. Fisher projections are good because they allow us to look at larger organic molecules in a simple way. They are not so good because determining R-S configuration from a Fischer projection can be more difficult than in other ways. Let’s look at some examples of some Fischers below.

The toughest part of learning all of these projections is to be able to convert between them. If you can visualize this you have truly aced organic chemistry so far. We have two resources to help you with this. First below is a video that shows the interconversion between these four types of visualizations on the same molecule.

Second, we have found the most effective way to visualize organic molecules in three dimensions is using molecular model kits. Here is a link to one we created that we really liked. The kit comes with its own DVD showing you how to use the model kit to learn different methods and topics in organic chemistry.

Thanks for sticking with this till the end and as always happy reacting.