Cycloalkanes
Cycloalkanes
If an alkane is in a ring, a cycloalkane, we name it by using the prefix cyclo-.
Cycloalkane examples
If there are two groups on a ring, we need to say whether the groups point towards the same face of the ring (cis) or opposite faces of the ring (trans).
Examples of cis/trans cycloalkanes
13. Name the following compounds (use cis/trans when appropriate).
14. Draw the following compounds.
a) trans-1-ethyl-3-propylcyclohexane
b) cis-1-propyl-3-(2,2-dimethylpropyl)cycloheptane
c) cyclopropylcyclobutane
d) cis-1,3-dichlorocyclopentane
Bicycloalkanes
When two cycloalkanes are fused together, they are called bicycloalkanes or a bicyclic compound. These two rings always share two carbon atoms. These two shared carbon atoms are the bridgehead carbons. In the following bicyclic compounds, the bridgehead carbons are circled.
Bicycloalkane examples
Three bridges connect these bridgehead carbon atoms. The number of carbon atoms in each of these three bridges need to be counted, but do not include the bridgehead carbon atoms in this count!
Bicycloalkanes are named by writing bicyclo, followed by brackets that contain three numbers separated by periods, followed by the alkane name for the total number of carbon atoms in the entire molecule. The three numbers are the number of carbon atoms in the three bridges arranged from highest to lowest. It takes the form bicyclo[#.#.#]alkane.
Names of some bicycloalkenes
15. Name the following compounds.
16. Draw the following compounds.
a) bicyclo[2.1.1]hexane
b) bicyclo[2.1.0]pentane
c) bicyclo[2.2.2]octane
Stability of cycloalkanes
Alkane carbon atoms are sp3 hybridized which naturally produces tetrahedral bond angles of 109.5°. In order to form cycloalkanes, the bond angle must deviate from this desired 109.5. For instance, in cyclopropane, the bond angle inside the ring is 60°. This is 49.5° away from the ideal bond angle. Angle strain is the strain that comes from pulling the bonds away from the idea bond angle into this strained conformation. Also, cyclopropane has some torsional strain because it is locked into a conformation that caused the C-H bonds to overlap. The strain caused by angle and torsional strains combined is called ring strain.
Ring strain examination of cyclopropane
Cyclobutane also has quite a bit of ring strain. If cyclobutane was a planar, regular square, it would have high angle strain with the deviation of a 90° ring being 29.5° away from the idealized 109.5°. But, if it is planar, it has terrible torsional strain. So cyclobutane twists or folds a bit so it is not planar. It hurts the angle strain because by folding, it becomes an 88° angle in the ring. But, the torsional strain is much better. We do see cyclobutane rings at times, but they are still fairly rare because of the high ring strain.
Ring strain examination of cyclobutane
If cyclopentane was a planer, regular pentagon, it would have bond angles of 108°. This does not deviate much from the ideal 109.5°. In order to reduce the torsional strain cyclopentane would have if it was planer, it folds a bit. Cyclopentane is quite stable with low ring strain. Cyclopentane is a fairly common cycloalkane.
Cyclopentane
Cyclohexane Chair
Cyclohexane is the most stable of the cycloalkanes. It is a very, very common ring structure in organic chemistry. Many organic molecules have this ring structure. Therefore, it is incumbent upon us to investigate and have a very good understanding it. Cyclohexane is not flat. It has what is called a chair conformation. This chair conformation has no angle strain. Each carbon atom is a perfect 109.5°. The C-H bonds are also all in a staggered conformation. In fact, the chair form of cyclohexane rings is so stable, they are found as repeating units in the carbon structure of diamond. Look carefully at the piece of diamond structure. Can you see some chairs in it?
Let’s practice drawing this chair conformation. First, draw two parallel lines at an angle.
At the top of the two lines, draw a V down connecting the two lines. At the bottom of the two lines, draw a v up connecting the two lines. This makes the base form of the cyclohexane chair.
At the top of the two lines, draw a V down connecting the two lines. At the bottom of the two lines, draw a v up connecting the two lines. This makes the base form of the cyclohexane chair.
Axial and equatorial
First, on each corner, draw a line straight up on carbon atoms that point up, and draw a straight line down on carbon atoms that point down. These bonding positions that are straight up or straight down are called axial positions. Axial bonds point straight up on up carbons and point straight down on down carbons.
Axial bonds
Next, let’s look at the equatorial bonds. For every up carbon, an equatorial bond is drawn down and for every down carbon, an equatorial bond is drawn up. The four equatorial bonds on the ends are easiest to draw, so draw them first.
Equatorial bonds
The last two equatorial bonds are sometimes confusing for students at first. For the up carbon atom, we know the equatorial bonds needs to point down, but do we angle it down to the left or the right? As a beginner, both may look OK to you.
This is the correct way.
This is the wrong way.
The way to remember which way to point these last two equatorial bonds is to look for the “M” and “W”. Below is the correct way to draw in the equatorial bonds.
17. Practice drawing a chair. Draw the chair five times without the axial and equatorial bonds on it.
18. On this chair, label the axial position with an “A” and the equatorial positions with an “E”.
19. Draw a chair and put the six axial bonds on it.
20. Draw a chair and put the six equatorial bonds on it.
21. Draw a chair and put all axial and equatorial bonds on it.
There is enough energy for a cyclohexane chair to convert to another cyclohexane chair conformation. This chair interconversion, or chair flip, goes through a boat conformation. It is called the boat conformation for its obvious similarities to a boat. This boat conformation is less stable than the chair because of steric hindrance. Two groups end up getting close to each other. But, overall, this problem is easily overcome with room temperature thermal energy.
Example of a cyclohexane chair flip
Monosubstituted chair
When a chair flip occurs, all of the axial groups become equatorial groups, and all of the equatorial groups become axial groups. This can be seen in the example of methylcyclohexane.
Axial group to equatorial group with chair flip
Larger groups prefer to be in the equatorial positions on a chair. Steric hindrance is a problem when a large group is in an axial position because the large axial group ends up too close to other axial groups. This particular type of steric hindrance is called 1,3-diaxial interaction because the two groups are both axial on carbon atoms 1 and 3 of a cyclohexane chair.
1,3-diaxial interaction
If the group is larger than a methyl, it is even more likely to be found in an equatorial position. In fact, a t-butyl group is so large, that you may never see it in an axial position.
Important point: On a chair, a large group in an equatorial position is more stable than in an axial position.
22.
a) Use the following two lines to begin to draw a chair. Finish drawing an ethyl group on the chair to make the lowest energy form of ethylcyclohexane. Be very careful and particular about the direction your draw your bonds.
b) Draw the chair conformation after your ethylcyclohexane drawing from part a undergoes a chair flip. You may want to use these parallel lines to begin to draw your chair.
23. Identify the following groups on cyclohexane chairs as axial or equatorial.
a)
b)
c)
d)
Disubstituted chairs
When two groups are attached to a cyclic compound, cis-trans isomers exist. This is because there are two sides to the ring. Because cyclohexane is a ring, cis-trans isomers exist when two groups are attached to it. It is easy to see if the two groups point in the same direction when the ring is drawn planar.
cis vs. trans disubstituted cycloalkanes
But, sometimes in a chair conformation, students have more difficulty at first when trying to determine cis or trans. It is really still pretty simple. You need to remember that if two groups both point up or both point down, it is cis. If the two groups point in opposite directions, one up and one down, then it is trans. On the following chair, all of the bonds are labeled either up (u) or down (d) based on the direction the bond points.
So, let’s look at the various possible forms of 1,2-dimethylcyclohexane.
cis vs. trans on disubstituted cyclohexane chairs
Notice that the cis and trans determination of a chair is solely based on the direction (up or down) of the groups and not by axial or equatorial designations.
24. Name the following cyclohexanes. Remember to use cis or trans in the name.
a)
b)
c)
Let’s draw a structure from a name. Let’s draw trans-1,4-dimethylcyclohexane in its lowest-energy chair conformation.
First, let’s draw a chair. You can draw either chair-flipped form of the chair.
Draw a methyl group on any carbon atom. We’ll call this carbon atom #1. This methyl is in the “up” position.
Draw in the axial and equatorial bonds on carbon atom #4. Since carbon #4 is a down carbon atom, the axial bond goes straight down and the equatorial bond must be up.
Since the methyl we have already drawn is “up” and we need trans, the second methyl group must point in the opposite direction. Therefore, we draw the methyl group down, which is the axial position on carbon #4. We can draw a hydrogen atom in the equatorial position, or we can simply delete it.
We must also consider the chair flipped version of this compound. Every axial group then moves to an equatorial position.
We then compare the two forms to see which one is the lowest energy form. We like the one on the right with the larger methyl groups equatorial instead of both axial like on the left. Our final answer is:
What if we have two different groups? Let’s look at the lowest energy conformation of trans-1-isopropyl-3-methylcyclohexane.
In each of these, there is one axial and one equatorial group. Which do we choose? Since larger groups prefer to be equatorial, we prefer the one with the isopropyl group in the equatorial position.
25. Draw the following cyclohexane compounds as chairs in the lowest energy, most stable configuration. Pay careful attention how you draw your axial and equatorial groups!
a) trans-1,2-diethylcyclohexane
b) cis-1-(1,1-dimethylethyl)-4-propylcyclohexane
Answers
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22. Your answer may look slightly different depending on where you chose to draw your ethyl group. Make sure it is equatorial in answer a and axial in answer b.
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