Katherine B. Hoffman
Florida State University Tallahassee
I (
Pi and Sigma Bonding in Organic Compounds An experiment with models
Students often have opportunity to use commercially available wood stick-and-ball or Stuart Kits to assemble models of organic molecules. Although these structures show arrangement of atoms and bond angles, they do not give a concept of sigma and pi bonds. This exercise was designed to portray the approximate shape of s, p, sp, sp2, and spa orbitals and to give a picture of their overlap in bond formation. Instructions for performing this experiment, which requires about two hours, follow. If molecular model sets are available, the wires, wood sticks, and balls provided may be used t o construct models of ethane, H3C-CH3; ethylene, H2C = CHz; and acetylene, HC CH. To prepare one's own models, secure six spherical corks approximately 1 in. in diameter. Break clinical wooden swab sticks so that there are 13 pieces 1.5 in. long, 12 pieces 1.1 in. long, one piece 1.3 in. long, and one piece 1.2 in. long. Obtain a portion of cotton abont '/%in. by 6 in. by 6 in. in size. In this experiment a cork represents the kernel of a carbon atom. Properly shaped pieces of cotton are used as olbitals and wooden sticks are inserted into the cork abont I/, in. to support the orbitals. The pairing of electrons of opposite spins as an orbital of one atom overlaps an orbital of another atom constitutes the formation of a covalent bond between the atoms. The s orbitals are symmetrically located around the nucleus. The three p orbitals of each shell are dumbbell-shaped and are also symmetrically placed around the nucleus a t right angles to one another, as if their axes were the X, Y, Z axes of a Cartesian system. Hybridized spa, sp2, and sp orbitals are modiied by having one lobe of the p dumbbell reduced to a small node and by the angle between axes of hybridiied orbitals of a given type in an atom being greater than SO".' References below discuss and illustrate orbital hybridization in ethane, ethylene, and a ~ e t y l e n e . ~ - ~
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The Bonding in Ethane Make four small holes in each of two of the corks a t angles like those of the bonds in a carbon atom of a model of the ethane molecule. If no model is available to copy, prepare two corks as follows: (1) Tie a piece
of white thread around a straight pin by means of a double knot and call the knot X (Fig. la). (2) Use a pen to mark off a distance XY equal t o the radius of the cork multiplied by 1.90. For a cork 1 in. in diameter XY is 0.95 in. (Fig. lb). (3) Insert the pin into the cork as if it were a north-south pole and push the knot tightly against the cork's surface (Fig. lc). (4) Carefully hold the thread along the cork's circumference so that it passes through the north-south poles; mark the position of Y on the cork with a pencil (Fig. Id). (5) Rotate the cork about 1 cm, pivot the thread a t X, and repeat step (4). (6) Repeat step (5) until you can draw a circle of latitude, circle I, through t,he points obtained at Y (Fig. le). (7) Replace the pin with thread attached by another pin. (8) Insert the original pin and thread, again as if it were a north-south pole, into any point Z on circle I. Using the steps (4 and 5) above inscribe circle I1 around the cork (Fig. If). (9) 'Where circles I and I1 intersect, place pins as if they were north-south poles in the sphere. Note that no three pins lie in the same plane. Encircle the pin holes with pencil and enlarge the holes to accommodate the sticks you have prepared. These holes roughly represent the positions of the carbon atom's bonds in ethane.
Figure 1 . Illustrationr of method of determining pasitions of the carbon ot0rn.s bond. in ethone.
J. CHEM.ED.,27,504 (1950). Casoa, JAMES,"Essential Principles of Organic Chemistry," Prentice-Hall, Ino., Englewaod Cliffs, N.J., 1956, pp. 11-23; 5% 1 NOLLER, CARLR.,
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'CRAM,DONALD, J., AND HAMMOND, GEORGE S., "Organic Chemistry," McGraw-Hill Book Co., Inc., New York, 1959, pp. 98-107. T.A., principle^ of Organic Chemistry," W. H. GEISSMAN, Freeman and Ca., San Fmncisco, 1959, pp. 3843. 6 HERZBERG, GERHARD, "Atomio Spectra and Atomic Structore," Dover Publications, New York, 1944. 8 KELLEY, LOUISE,"Organic Chemistry," MoGmw-Hill Book Co., Inc., New York, 1957, pp. ZOO-11.
Connect the two corks by means of a 1.5-in.-long stick by putting the stick in one hole of each cork (Fig. 2a). Then insert 1.1-in.-long sticks in the six remaining holes of the corks (Fig. 2b). Wrap a small puff of cotton lightly in an elongated lobe (Fig. 2c), around each projecting stick (Fig. 2 4 . Each puff of cotton represents an spa orbital occupied by one electron of the carbon atom. Cover the stick connecting the two corks Volume 37, Number 12, December 1960
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with a cotton puff which is thicker in the middle than it is near the carbon kernels. This puff, which represents overlap of an spa orbital from each carbon directed along the internuclear axis constitutes a - C - C sigma bond. Now lightly roll a small bit of cotton int0.a ball about 1.0 in. in diameter. This sphere represents the hydrogen atom's non-directional 1s orbital which contains one electron (Fig. 2e). Gently push together the 1s orbital of hydrogen and an spa orbital of a carbon atom. Overlap of these orbitals along the internuclear line constitutes a covalent -C-H sigma bond. Complete the model of ethane by attaching 1s orbitals of hydrogen to the remaining spaorbitals of the carbons (Fig. 20.
dicular to the stick connecting the two carbons (Fig. 4c). Wrap cotton lightly on all four sticks as before. On each carbon the cotton dumbbell, whose axis lies in plane 11, represents an unhybridized 2a, orbital, which contains one electron. Overlap of the two p orbitals below and above plane I constitutes the pi (a) bond between C-1 and C-2. (The electrons forming the bond are called pi electrons.) Overlap of the two spz orbitals directed along the internuclear axis between C-1 and C-2 constitutes a sigma bond between the two carbons. Complete the model of ethylene by attaching the 1s orbital of hydrogens to the remaining sp2 orbitals of the two carbons. Overlap of an sp2orbital of carbon with the s orbital of H forms -C-H sigma bonding.
sticks
-,-r -A\, --7'
C _ _ - - l
4c
2c
2d
2e
Figure 2. Angles ond orbitals, both hybridized m d unhybridired, used to make the model of ethane. Note the projecting stick indicated in 2d. In 2f the r orbitals shown ore thole of the hydrogen otomr combining
The Bonding in Ethylene
Axes of the three sp2 orbitals of each carbon atom in ethylene lie in a plane at 120' angles t o one another. To trisect the cork's equator, calculate its length: circumference =ad; for a cork 1 in. in diameter this distance = 3.14 in. Tie a thread a t M around a straight pin as before and mark a point ( X ) on the thread 3.14 in. away from the pin. Use a ruler t o divide the circumferential distance into thirds: MZ, ZY, YX (Fig. 3a). Mark points Z and Yon the thread. Insert the pin into the cork (Fig. 3b) and as best you can, lay the thread as an equator around the cork and mark points Z and Y on the cork. Insert pins at Z and Y to locate the bond angles in ethylene. Connect the F
y
Figure 3.
n
3.4416 in.
Laying off the trisected circumference.
two corks by placing a 1.3-in.-long stick in a hole of each cork; insert 1.1-in.-long sticks in the other four holes. To represent sp2orbitals wrap cotton lightly on the projecting sticks and on the connecting stick. Now imagine two planes, one running through axes of all the spZ orbitals (Fig. 4a) and a second plane passing through the center of each carbon perpendicular to the first plane (Fig. 4b). In the second imaginary plane place two 1.5-in.-long sticks in each carbon perpa638
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4d
Figure 4. Plane 11401 contdnr d l atoms composing the ethylene moleculePlane I1 (4b) parses through the C-C bond perpendicvlor to Plone I. The sticks 144 show direction of unhybridired p, orbitals in plane I\. The overlop of p orbitals in Plone I1 shows farmationof pi bond at area 1 or area 2 14dl.
The Bonding in Acetylene
Axes of the two sp orbitals of each carbon atom in acetylene lie in a straight line, that is, a t angles of 180" to each other. Make two small holes 180" apart in the "equator" of each of two corks. Connect the corks by placing a 1.2-in.-long stick in a hole of each cork: insert 1.5-in.-long sticks in the other two holes and as before, wrap these sticks and the stick between the carbons with cotton, in this case to represent sp orbitals. Now imagine two bisecting planes placed perpendicularly to each other whose intersection passes through the axes of the sp orbitals. These are like plane I and plane I1 of ethylene. In each carbon insert two I!/%in.-long sticks in each plane, perpendicular (at 90') to the-C-Cbond axis. Wrap these sticks lightly with cotton as in making the unhybridized p orbital dumbbells in ethylene. Xote that there are two sets of overlapping p orbitals, so that acetylene has two "pi" electron pairs (arranged as a sort of cylinder around the -CC axis) instead of the one pair characteristic of ethylene. Overlap of the two sp orbitals directed along the internuclear - C - C axis constitutes the sigma bond between carbons. Complete the model of acetylene by attaching the 1s orbital of hydrogens to the remaining sp orbitals of the two carbons. Overlap of an sp orbital of carbon with the 1s orbital of hydrogen constitutes -C-H sigma bonding. Examine the differences among carbon t o carbon single, double, and triple bonds. Consider the stability of pi versus sigma bonds, the decrease in carbon to carbon bond distance when pi bonding occurs, and the lack of free rotation about a double bond.
