Tuesday, October 9, 2012

Drawing cyclohexane rings

Drawing Cyclohexane rings.

Students often complain they are terrible at drawing organic chemistry structures especially chair conformations of the cyclohexane rings. Since getting the questions right often depends on the person marking your papers being able to interpret your ring correctly I feel its worth spending extra time getting your ring right.
I will walk you through two methods for getting a perfect ring. Firstly it is unlikely you will be able to draw a perfect ring without picking your pen up off the paper. Please refer to my diagrams when reading the methods

Method 1.

Step 1. Start with a pair of lines drawn in pencil as guides. Draw a triangle as shown in the picture.
Step 2. Draw a pair of parallel lines coming off this triangle. Note the lines should not be horizontal but pointing upwards towards the pencil line. The two top points should both be on the upper pencil guide line.
Step 3. Close the ring by drawing two more lines to make another triangle.

Method 2.

The second method makes use of the fact it is made up of three sets of parallel lines. In my example I have chosen the middle two lines to draw first, you may prefer the end two. As you can see in both my examples of have used the same color for the lines that are parallel.
Step a. I have again started with a pair of lines drawn in pencil as guides. Although not essential to draw the lines it is important that the two top points should be on the upper pencil line.
Step. 2 more parallel lines.
Step c. Close both sides of the ring with another pair of parallel lines.

Now that you have the perfect ring we need to place the axial and equatorial bonds. Once the perfect ring has been drawn its easy to place the axial bonds. They should all be drawn vertical. I find it easiest to start on a bottom point and draw a vertical line down, any up points should have a vertical line up. This is an alternating pattern of up and down. When ever I answer a problem involving energy conformations I will always start with a quick diagram of a cyclohexane ring with the axial positions drawn in.

One common mistake is to start with two lines horizontal. This makes it difficult to put the axial positions on correctly as they would no longer be vertical. Another common mistake is to have the axial position going in the wrong direction i.e going up when it should be going down. This makes it impossible for the carbons to be tetrahedral. Do not draw your rings like the examples in the next two pictures.

Next drawing the equatorial positions. Notice if the axial position goes down the equatorial position goes up. This is why I recommend putting the axial positions in first. The equatorial positions should be drawn parallel to the lines in the ring. Again in my example I have color coded which line is parallel with each side. Its worth noticing that there should be 3 sets of 4 lines now all parallel.

No comments:

Post a Comment