Alkanes and Cycloalkanes

Structure of Alkanes

To visualize a molecule in three dimensions, a wedge and dash can be used on a bond line formula to show orientation of the groups. Sawhorse structures and Newman projections can also be used to show the three-dimensional view of molecules.

Molecules exist in a variety of conformations, or different arrangements of a molecule in space, formed by rotation around single bonds. Analyzing a molecule in three dimensions is known as conformational analysis. Conformational analysis is the three-dimensional analysis of molecules to find the least strained conformation or arrangement in three-dimensional space. Conformational analysis is helpful when considering its properties.

Three methods for drawing molecules in three dimensions are the wedge-and-dash projection, the sawhorse projection, and the Newman projection. A wedge-and-dash projection is a way to represent molecular conformation with solid and dashed wedges to indicate the position of substituents. A sawhorse projection is a representation of a molecule from an angle that resembles a carpenter's sawhorse. A Newman projection is a projection that depicts the molecular conformation and is particularly useful for determining the most stable conformation of the molecule around a bond. Alkanes can be drawn in any of these three-dimensional structures. The wedge-and-dash projection is the most common way of drawing alkanes. The Newman projection is used mostly to determine the best arrangement of groups to avoid bumping into each other in three-dimensional space, i.e., conformational analysis.

Different Ways of Drawing Structures

Wedge-and-dash, sawhorse, and Newman projections are ways to visualize 4-bromo-3-hexanol. The three projections represent different ways of showing 4-bromo-3-hexanol.
A bond line formula can be converted into a Newman projection to help understand and evaluate the conformational analysis of a structure. There are 360 degrees of rotation around a single bond, but important conformers (different views of a molecule by rotation around covalent bonds) can be found every 60 degrees. Ethane (C2H6) is a simple alkane that exhibits two characteristic conformations: staggered and eclipsed. The staggered conformation is the conformation of an alkane where the dihedral angle is 60°. The eclipsed conformation is the conformation of an alkane where the dihedral angle is 0°. The dihedral angle is the angle between two atoms on adjacent carbons in a Newman conformation. In the staggered conformation of ethane, the CH{\rm {C{-}H}} bond of one carbon bisects the HCH{\rm {H{-}C{-}H}} bonds of the other carbon. In the eclipsed conformation, the CC{\rm {C{-}C}} bonds of the two carbons are aligned.

Staggered and Eclipsed Conformations of Ethane

Ball-and-stick models help show the staggered and the eclipsed conformations of ethane.
The two molecular conformations interconvert (change between conformations) rapidly by rotating around the CC{\rm {C{-}C}} bond. The Newman projection represents the carbon-carbon bond from a head-on perspective, making it easy to determine whether the molecule is in the staggered or eclipsed conformation. In a Newman projection, the front carbon is a point, and the back carbon is a circle. The three bonds radiate off the carbons.

Representing Staggered and Eclipsed Conformations of Ethane

Wedge-and-dash, sawhorse, and Newman projection models are three ways to represent the staggered and eclipsed conformations of ethane. Staggered conformations have a dihedral angle of 60°, while eclipsed have a dihedral angle of 0°.
The Newman projection shows the dihedral angle, or the angle between the CCH {\rm {C{-}C{-}H}} plane and the HCC {\rm {H{-}C{-}C}} plane in ethane. Another way of understanding dihedral angles is to observe two substituent groups (or hydrogen atoms) in a Newman projection. If the two groups (or atoms) are right on top of each other, then the dihedral angle is 0°, and the structure is classified as an eclipsed conformation. When the two groups (or atoms) are separated from each other by a dihedral angle of 60°, the conformation is classified as staggered. Steric strain is a strain resulting from nonbonded atoms being forced close to each other. The staggered conformation is classified as gauche or anti. The gauche conformation is a staggered conformation that exhibits steric strain due to two groups on adjacent atoms having a dihedral angle of 60°. The anti-conformation (or anti) is a staggered conformation that exhibits no steric strain due to two groups on adjacent atoms having a dihedral angle of 180°. Bonds in staggered conformations have only gauche or anti relationships.

Spatial Relationships in Ethane

The Newman projection is particularly useful to visualize the eclipsed, gauche, or anti states of a molecule.
Analysis of each of the six positions around the Newman conformations is used to determine the more stable orientation (lower energy conformer) of a molecule. The staggered conformation is always more stable than the eclipsed. Eclipsed conformations always exhibit torsional strain. Torsional strain is a resistance to bond rotation due to electronic repulsion that results from nonbonded atoms being forced close to each other. In eclipsed conformation, one carbon of the molecule can rotate to relieve the strain of the eclipsing groups. During this rotation, the dihedral angle changes from 0° to 60°. By changing the dihedral angle, the molecule has gone from an eclipsed conformation (with torsional strain) to a staggered conformation (without torsional strain).

Potential Energy of Ethane in Different Conformations

The variable potential energy of ethane changes as the substituents rotate around the carbon-carbon bond. The eclipsed are higher energy and less stable. The staggered are lower energy and more stable.
Butane has two different staggered conformations, one in which the methyl groups are gauche and one in which they are anti. The gauche conformation brings the two methyl groups in close proximity to each other, and the molecule is destabilized by the resulting repulsive force. Therefore, in butane, the anti conformation is more stable than the gauche conformation.

Alkanes with longer, unbranched carbon chains are most stable in their anti conformations. When looking at these molecules from the side perspective, the carbon chain will appear zigzagged.