We have seen that a carbon–carbon single bond (a bond) is formed when an sp3or- bital of one carbon overlaps an sp3orbital of a second carbon. Figure 3 shows that ro- tation about a carbon–carbon single bond can occur without any change in the amount of orbital overlap. The different spatial arrangements of the atoms that result from ro- tation about a single bond are called conformations.
When rotation occurs about the carbon–carbon bond of ethane, two extreme con- formations result—a staggered conformation and an eclipsed conformation. An infi- nite number of conformations between these two extremes are also possible.
s
C C
왖Figure 3
Rotation about a carbon–carbon bond can occur without changing the amount of orbital overlap.
water soluble of the alkyl halides. The other alkyl halides are less soluble in water than ethers or alcohols with the same number of carbons (Table 7).
eclipsed conformers
0° 60° 120°
staggered conformations eclipsed conformations
180°
Degrees of rotation
240° 300° 360°
Potential energy
2.9 kcal/mol or 12 kJ/mol H
H
H
HH
H
H
HH
H
H H
HH H
H
H H
H
H
H H
H H
H
H H H
H H
H
H H H
H H
H
H H H
H H energy barrier
왖Figure 4
The potential energy of ethane as a function of the angle of rotation about the carbon–carbon bond.
Our drawings of molecules are two-dimensional attempts to communicate three- dimensional structures. Chemists commonly use Newman projections to represent the three-dimensional spatial arrangements resulting from rotation about a bond. A Newman projection assumes the viewer is looking along the longitudinal axis of a particular 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.
A staggered conformation is more stable, and therefore lower in energy, than an eclipsed conformation. Because of this energy difference, rotation about a carbon–carbon single bond is not completely free. The eclipsed conformer is higher in energy, so an energy barrier must be overcome when rotation about the bond occurs (Figure 4). However, the barrier in ethane is small enough ( or ) to allow continuous rotation. A molecule’s conformation changes from staggered to eclipsed millions of times per second at room temperature. Because of this continuous interconversion, the conformations cannot be separated from each other.
Figure 4 shows the potential energies of all the conformations of ethane obtained during one complete 360° rotation. Notice that the staggered conformations are at en- ergy minima, whereas the eclipsed conformations are at energy maxima. Conforma- tions at energy minima are often called conformers.
12 kJ/mol
2.9 kcal/mol CơC CơC
s
60°
H H H
H
H H
HH HH
H H Newman
projections
ethane
a staggered conformation for rotation about the C—C bond in ethane
an eclipsed conformation for rotation about the C—C bond in ethane
H3C CH3
A staggered conformation is more stable than an eclipsed conformation.
B I O G R A P H Y
Melvin S. Newman (1908–1993) was born in New York. He received a Ph.D. from Yale University in 1932 and was a professor of chem- istry at Ohio State University from 1936 to 1973. He first suggested his technique for drawing organic molecules in 1952.
3-D Molecules:
Staggered and eclipsed conformations of ethane
0°
A B C D E F A
CH3
H3C CH3 H3CCH3
CH3
CH3
CH3 CH3
CH3
CH3 H3C
H3C CH3
H
H H H
H
H
H
H
H H H H
H
H H
H H
H H
H H
H H H H H H H
Butane has three carbon–carbon single bonds, and rotation can occur about each of them.
The following Newman projections show staggered and eclipsed conformations that result from rotation about the C-2ơC-3bond.
ball-and-stick model of butane the C-2—C-3 bond
butane
the C-1—C-2 bond the C-3—C-4 bond CH3 CH2
1 2
CH2
3
CH3
4
The three staggered conformations do not have the same energy. Conformation D, in which the two methyl groups are as far apart as possible, is more stable than the other two staggered conformations (B and F) because of steric strain. Steric strain is the strain (that is, the extra energy) experienced by a molecule when atoms or groups are too close to one another, causing their electron clouds to repel each other.
In general, steric strain in molecules increases as the size of the interacting atoms or groups increases.
The eclipsed conformations resulting from rotation about the bond in butane also have different energies. The eclipsed conformation in which the two methyl groups are closest to each other (A) is less stable than the eclipsed conforma- tions in which they are farther apart (C and E).
Because there is continuous rotation about all the single bonds in a mole- cule, organic molecules with single bonds are not static balls and sticks—they have many interconvertible conformations.
The relative number of molecules in a particular conformations at any one time de- pends on the stability of the conformation: the more stable the conformation, the greater is the fraction of molecules that will be in that conformation. Most molecules, therefore, at any one time are in staggered conformations. The lower energy of a stag- gered conformation gives carbon chains the tendency to adopt zigzag arrangements, as seen in the ball-and-stick model of decane.
ball-and-stick model of decane
CơC
CơC
C-2ơC-3
3-D Molecule:
Decane Movie:
Potential energy of butane conformations
good overlap strong bond
poor overlap weak bond
a. b.
왗Figure 5
(a) The overlap of sp3orbitals in a normal bond.
(b) The overlap of sp3orbitals in cyclopropane.
s PROBLEM 28
a. Draw the three staggered conformations of butane for rotation about the bond.
(The carbon in the foreground in a Newman projection should have the lower number.) b. Do the three staggered conformations have the same energy?
c. Do the three eclipsed conformations have the same energy?
PROBLEM 29
a. Draw the most stable conformation of pentane for rotation about the bond.
b. Draw the least stable conformation of pentane for rotation about the bond.