Pictures of the Day CH320M328M

The ethane molecule depicted here is in the eclipsed conformation. This is the least stable form of ethane. It is least stable because there is a minimum distance between electron densities of C-H bonds, and therefore a maximum amount of electron-electron repulsion. This interaction is called torsional strain. Eclipsed ethane is NOT least stable because hydrogen atoms are crashing into one another (non-bonded interaction strain), since as can be seen, they are not close enough together to crunch into each other even when vibrating! The exact reasons for torsional strain are still being debated, but all agree it is an effect of the sigma bonding electron density that favors the staggered conformation.

Here is ethane in the staggered conformation. Shown also is the Newman projection of the same ethane molecule. Remember, the circle represents the front carbon atom that is seen when the molecule is viewed down the carbon-carbon sigma bond. Ethane spends most of its time in a staggered conformation, because this is the most stable conformation.

You can see some movies we have made that show how bond rotation in alkanes is really the result of bond vibrations. At any given instant, the molecule is vibrating, and since there is very little rotation barrier around the C-C sigma bond, random rotation occurs largely as a consequence of the vibrations. Note that although very dynamic, the molecule spends the majority of its time in the staggered conformation, because that is lower in energy.

Movie of Ethane Conformation (Side view)

Movie of Ethane Conformation (Newman Projection)

Here, the butane molecule is in one of two possible gauche conformations. It is slightly less stable in this conformation than when it is in the anti conformation (below). The methyl groups are in closer proximity and can actually crunch into each other when they are vibrating in the gauche conformation. Thus, gauche conformations are less stable than the anti conformation due to STERIC STRAIN, also called NON-BONDED INTERACTION STRAIN. The Newman projection is drawn from the perspective indicated by the "eye".

Here the butane molecule is in the anti conformation. The Newman Projection at the bottom clearly shows that this conformation maximizes the distance between the methyl groups. This, in turn, minimizes steric strain and makes the butane molecule most stable in the anti-conformation. The Newman projection is drawn from the perspective indicated by the "eye".

Butane Movie

Here is a picture of octane in the staggered, anti conformation. Staggered, anti is the preferred conformation of alkanes because both torsional and steric strain are minimized. It is important to remember that at any given instant at room temperature, the molecule is likely to be bent while vibrating and rotating. It is only the TIME-AVERAGED view that is being represented by the entirely staggered, anti conformation. Also, the molecule will be in the entirely staggered, anti conformation near absolute zero temperature, where there is not enough energy to vibrate and thus rotate bonds. The preference for the staggered, anti conformation is why we use the zig-zag represention for alkanes.

Octane Movie

Cyclopropane is a 3-carbon ring structure. The bond angles form an equilateral triangle with bond angles of 60 degrees. The bond angles between the carbon atoms would prefer to be 109.5 degrees but because of geometrical constraints of the 3 carbon ring, this is not possible. Thus, there is considerable angle strain in cyclopropane. There is no non-bonded interaction strain in cyclopropane because no atoms crash into one another. but there is significant torsional strain in cyclopropane because the bonds are eclipsed, causing electron repulsion. You can see the torsional strain if you draw Newman projections for any of the C-C bond in cyclopropane.

Cyclobutane is more stable than cyclopropane. Cyclobutane has considerable angle strain, but not as much as in cyclopropane. Unlike cyclopropane, which is flat, cyclobutane puckers to lessen somewhat (not eliminate, however) torsional strain. Puckering allows the bonds to remain only partially eclipsed. There is no evidence of non-bonded interaction strain for cyclobutane, as can be seen by inspecting the space-filling model.

Cyclopentane has little angle strain because the interior angles of a pentagon are 108 °. Like cyclobutane, cyclopentane is able to pucker, making the bonds only partially eclipsed, and thereby relieving some, but not all, of its torsional strain. There is no evidence of non-bonded interaction strain for cyclopentane, as can be seen by inspecting the space-filling model.

This is cyclohexane in the chair conformation. This is the most stable conformation for cyclohexane. Chair cyclohexane has no angle strain, as the bond angles around carbon are very close to the ideal 109.5°. Notice also how all the bonds are perfectly staggered, thereby eliminating torsional strain. As can be seen in the space filling model, there is no appreciable non-bonded interaction strain in unsubstituted chair cyclohexane. Non-bonded interactions strain is only introduced when an axial hydrogen atom is replaced by a larger atom or group.

Side (Above) and Top (Below) views of chair cyclohexane. Chair cyclohexane has two distinct types of hydrogen atoms, axial (red) and equitorial (purple). The axial positions point in a direction that is normal to the mean plane of the cyclohexane ring, while the equatorial positions are splayed out around the periphery of the ring. You need to be able to recognize the difference between axial and equatorial positions, and you need to be able to draw chair cyclohexane on a piece of paper.