Interpret the Conformation of Straight-Chain Alkanes Using Newman Projections
One of the best ways to look at different conformations of straight-chain alkanes (individual conformations are called conformers) is to use Newman projections. A Newman projection is a convenient way of sighting down a particular carbon-carbon bond.
Straight-chain alkanes can exist in different conformations — different spatial arrangements of atoms that can be interconverted by rotation about a single bond — where not all the conformations are of equal energy. And molecules prefer to be in low-energy conformations rather than high-energy conformations.
This figure shows a perspective drawing of ethane and the Newman projection of the same molecule.
The solid wedges in Lewis structures indicate a bond that’s coming out of the paper, while dashed wedges indicate a bond that’s going back into the paper.
In a Newman projection, the three lines in the shape of a Y represent the three bonds of the first carbon that you’re sighting down; where the three lines connect is where the front carbon is. A circle (drawn “behind” the front carbon) represents the back carbon; the three lines coming out of the circle represent the three bonds that come off that carbon. (Note that the fourth bond for each of these carbons is the carbon-carbon bond that you’re looking down.) A Newman projection can help you analyze the rotations around a particular carbon-carbon bond.
Using Newman projections, rotating around a specific bond to reach other conformers is a fairly straightforward task. The best way to reach other conformers is to rotate just one carbon at a time — either the front carbon or the back carbon.
Although an infinite number of conformations exist (just rotate one of the carbons by a fraction of a degree and — voilà! — you have a different conformation), the two most important ones are the eclipsed and staggered conformations, as shown here.
An eclipsed conformation results when the bonds from the front carbon and the bonds from the back carbon align with each other, and the angle between the bonds (called the dihedral angle) is 0 degrees. On the Newman projection, the eclipsed bonds are drawn a little ways apart so that the substituents on the back carbon can be seen. A staggered conformation results when the bonds from the front carbon and the bonds from the back carbon have a dihedral angle of 60 degrees. In the staggered conformation, the bonds coming off the front and back carbons are as far apart from each other as possible.
When the bonds from the front carbon and back carbon are aligned in an eclipsed conformation, the electron repulsion between the bonds is higher than when the bonds are staggered and farther apart. This electron-electron repulsion between the bonds is called torsional strain. Because staggered conformations have less torsional strain than eclipsed conformations, staggered conformations are, as a general rule, more stable (that is, lower in energy) than eclipsed conformations.