Experiment 5 CHEM 216 Fall 2024.pdf

CHEM 216 Expt. 5 Conformation of Butane Text book:Introduction to Organic Laboratory Techniques, 4th edition, by D. Pavia, G. Lampman, G. Kriz and R. Engel, 2007, Thomson Brooks/Cole, ISBN-13:9780495388876 Conformations of Acyclic Alkanes Conformations are different arrangeme...

CHEM 216 Expt. 5 Conformation of Butane Text book:Introduction to Organic Laboratory Techniques, 4th edition, by D. Pavia, G. Lampman, G. Kriz and R. Engel, 2007, Thomson Brooks/Cole, ISBN-13:9780495388876 Conformations of Acyclic Alkanes Conformations are different arrangements of atoms that are interconverted by rotation about single bonds.  Names are given to two different conformations. In the eclipsed conformation, the C—H bonds on one carbon are directly aligned with the C—H bonds on the adjacent carbon. In the staggered conformation, the C—H bonds on one carbon bisect the H—C—H bond angle on the adjacent carbon.. Rotating the atoms on one carbon by 60° converts an eclipsed conformation into a staggered conformation, and vice versa. The angle that separates a bond on one atom from a bond on an adjacent atom is called a dihedral angle. For ethane in the staggered conformation, the dihedral angle for the C—H bonds is 60°. For eclipsed ethane, it is 0°..  End-on representations for conformations are commonly drawn using a convention called a Newman projection. How to Draw a Newman Projection: Step 1. Look directly down the C—C bond (end-on), and draw a circle with a dot in the center to represent the carbons of the C—C bond.. Step 2. Draw in the bonds. Draw the bonds on the front C as three lines meeting at the center of the circle. Draw the bonds on the back C as three lines coming out of the edge of the circle.. Step 3. Add the atoms on each bond.. Figure 4.6 Newman projections for the staggered and eclipsed conformations of ethane.  The staggered and eclipsed conformations of ethane interconvert at room temperature, but each conformer is not equally stable. The staggered conformations are more stable (lower in energy) than the eclipsed conformations. Electron-electron repulsion between bonds in the eclipsed conformation increases its energy compared with the staggered conformation, where the bonding electrons are farther apart. 29  The difference in energy between staggered and eclipsed conformers is ~3 kcal/mol, with each eclipsed C—H bond contributing 1 kcal/mol. The energy difference between staggered and eclipsed conformers is called torsional energy. Torsional strain is an increase in energy caused by eclipsing interactions. Figure 4.8 Graph: Energy versus dihedral angle for ethane 30 An energy minimum and maximum occur every 60° as the conformation changes from staggered to eclipsed. Conformations that are neither staggered nor eclipsed are intermediate in energy. Butane and higher molecular weight alkanes have several C—C bonds, all capable of rotation. It takes six 60° rotations to return to the original conformation. Figure 4.9 Six different conformations of butane.  A staggered conformation with two larger groups 180° from each other is called anti. A staggered conformation with two larger groups 60° from each other is called gauche. The staggered conformations are lower in energy than the eclipsed conformations. The relative energies of the individual staggered conformations depend on their steric strain. Steric strain is an increase in energy resulting when atoms are forced too close to one another. Gauche conformations are generally higher in energy than anti conformations because of steric strain...  Figure 4.10 Graph: Energy versus dihedral angle for butane. The energy difference between the lowest and highest energy conformations is called a barrier to rotation. Since the lowest energy conformation has all bonds staggered and all large groups anti, alkanes are often drawn in zigzag skeletal structures to indicate this. 35