Pictures of the Day CH310M/CH318M Fall 2007

9-21-07
Ethane Conformations
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)
Butane Conformations
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".

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-anti conformation, because that is lowest in energy.

Butane Movie

Longer Alkane Conformation - Octane

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.

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-anti conformation, because that is lowest in energy.

Octane Movie