Conformational Analysis

CHEM 222                                                                                 FALL SEMESTER 2015

CONFORMATIONAL ANALYSIS

Students would understand the conformations of alkanes & cycloalkanes by the study of conformational analysis.  {WADE: Chapter 3-7→3-15}. There are two teaching videos uploaded onto the SGU Mediasite somewhere up in the “cloud” (address given @ the end of handout.) There are also videos on YouTube, Khan Academy etc. that you can access.  

Recall that a carbon-carbon single (σ) bond is formed by the overlap of cylindrically symmetrical sp3 hybrid orbitals.  Therefore, rotation about the bond can occur without changing the amount of orbital overlap.  The different spatial arrangements that result from rotation about a single bond are called Conformations.  Molecules with different conformations are called “conformational isomers” or “conformers”. [pic 1]

(We start our study of the topic with ethane)

The conformers produced by rotation about the carbon-carbon bond represent a “continuum” between the two extremes below: an eclipsed conformer (on the left) where all the hydrogens are “covered up” and a staggered conformer (on the right) where the hydrogens are as far apart from each other as possible.   

We use Newman projections to represent the three-dimensional spatial arrangements resulting from rotation about a σ bond.  A Newman projection assumes the viewer is looking along the longitudinal axis of a particular C –C bond.  The carbon in front is represented by a point (where three lines are seen to intersect), and the carbon at the back is represented by a circle.  The three lines emanating from each of the carbons represent its other three bonds.   (Above, Newman projection of the staggered conformer of ethane)[pic 2]

The staggered conformer of ethane is more stable than the eclipsed, and is therefore lower in energy than the eclipsed conformer.  Factors which contribute to the greater stability are:

• Better electron delocalization stabilizes the staggered conformation; whereas repulsions between bonds on adjacent atoms destabilize the eclipsed conformation.[pic 3]
• Staggered bonds have no torsional strain, while eclipsed bonds produce the most torsional strain. (Each H-C-C-H unit in ethane is characterized by a “torsion angle”, the angle between the H-C-C plane and the C-C-H plane).
• The staggered conformation is lower in energy (12 kJ/mol) than the eclipsed conformation.  Because of this energy difference, rotation about a carbon-carbon single bond is not completely free since an energy  barrier must be overcome when rotation about the C-C bond occurs.  However, the energy barrier in ethane is small enough (12kJ/mol) to allow continuous rotation.  At any one time approx. 99% of the ethane molecules will be in the staggered conformation because of its greater stability, and only 1% will be in the less stable eclipsed conformation. (Notice that in the diagram @ bottom of page 1, the staggered conformers are at energy minima, whereas the eclipsed conformers are at energy maxima.)

Butane (C4H10) has three carbon-carbon single bonds, and rotation can occur about each of them.  The Newman projections below show the staggered and eclipsed conformers that result from rotation about the C1-C2 bond.

 (Notice that the carbon with the lower number is placed in the foreground in a Newman projection).[pic 4]