D. W. Rogers and F. J. McLafferty The Brooklyn Center Long Island University Brooklyn, N e w York 11201
Heats of Hydrogenation A physical-organic laboratory experiment
The heat of hydrogenation of an unsaturated compound is the most direct way of measuring its "resonance energy" and is superior for this purpose to the heat of combustion (1). Moreover, the heat of hydrogenation is the most sensitive thermochemical way of detecting and evaluating the extent of strain in molecules, "hyperconjugation" energies and other abnormalities or departures from the rule of simple additivity of bond energies. Despite the controversy over the proper theoretical rationalization of nonadditive bond energies (2), the thermochemical effect is real and must be explained by whatever theory organic chemists may ultimately elect. Beginning with Kistiakowsky's (S), hydrogen calorimeters (4-6) have been rather formidable and possibly dangerous machines not a t all suited to the undergraduate laboratory. This paper describes a simple apparatus which allows rapid determination of heats of hydrogenation. While the accuracy is not comparable to the best in the literature (3, 6), it enables the student to determine the number of double bonds in a compound, to detect the diierence between cyclio double bonds and straight-chain double bonds and to observe the effect of structural strain.
alkene is lowered, decreasing the heat given off during the reaction. Conversely, if t,he alkene is strained, its internal energy and hence its enthalpy is increased by the potential energy of the strain causing AH to be abnormally high. Finally, if an unstrained alkene produces a strained alkane, the heat of hydrogenation is abnormally low. I n some cases these structural features may operate simultaneously and competitively making unequivocal theoretical interpretation of thermochemical data difficult or impossible. Experimental
Apparatus. The reaction chamber is a 100-ml cylindrical glass vessel fitted with a vacuum seal cap. (Small baby food jars work well.) The cap is drilled and reamed off center to receive a No. 1, one-hole ruhher stopper as shown in Figure 2. The rubber
H2 SOURCE
nur3oERCAP METAL l F
MlCROSIElME
NMR TUBE
Theory
The heat of hydrogenation of a normal terminal double bond such as that in l-hexene is about 30 kcal/ mole. On hydrogenation of an alkene to an alkane at constant pressure, heat is given off indicating a decrease in enthalpy as shown in Figure 1. The alkene may have multiple double bonds which, if they are isolated, simply produce a multiple of the normal heat of hydrogenation. If conjugation or "hyperoonjugation" exists, the enthalpy level of the s t r a i n [reactant1
ALKENE resonance or hyperconjugation
J strain (product1
ALKANE
Figure 1.
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Enthdpy levels in strained and coniugated molecules.
Journal of Chemical Educafion
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Figure 2.
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The reaction chamber.
stopper contains an nmr tube which serves as: a thermistor well. The stopper is pierced by two hypodermic needles, one to admit hydrogen and one to admit sample. The hydrogen inlet is connected through a Luer joint to a hydrogen tank, manometer, and T joint fitted with a clamp which serves as an outlet. The hydrogen needle is left in the stopper, and the sample needle is withdrawn leaving a self-sealing hole much as piercing the ruhher septum of a glc does. The stopper fits into the metal cap upside down so as not to be blown out by hydrogen pressure in t,he chamber. A pin (not shown in Fig. 2 ) is driven through the stopper horizontally and flush with the top surface of the cap to prevent the stopper from going into the reaction vessel. The vessel is enclosed by thick styrofoam walls on the bottom and d l four sides. The bottom of the styrofoam container is fitted to receive stirring magnet powered by an external motor. The top is left open to facilitate sample injection. The thermistor constitutes the unknown arm of a Lee& & Northrup Wheatstone bridge. Galvrtnometer leads from the bridge are connected to a Leeds & Northrup lamp and scale galvanometer. Potential across the bridge is maintained constant by an automobile storage battery. Potential across the bridge and hence sensitivity can he varied by means of a variable series resistor (seeFig. 3) whiehis usually set at 2,000-3,000 ohms.
Heats of Hydrogenation of Several Representative Unsoturoted Com~ounds(Student Rerultsl
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REACTION CHAMBER thermirlor
whratst~nr bridge Figure 3.
Block diagram for the temperature sensing circuit.
Reagents. All alkenes, 5% palladium black catdyst on charcoal, glacial acetic acid and electrolytic hydrogen were used as received, without purification. Procedure. Fifty ml of glacial acetic acid and 0.6 g of Pd catalyst on charcoal are placed in the reaction vessel and stirred vigorously by the magnetic stirring arrangement. The Wheatstone bridge is balanced so that the galvanometer reads on scale. If the galvanometer cannot he made to read on scale, the sensitivity is too high. Increase the series resistance to the wet cell. Galvanometer drift should be slight. If there is a rapid drift indicating temperature increase, heat is leaking into the system which should he warmed by holding a hot object over the resction vessel and until it is at ambient temperature. Heat leak out of the system can he prevented by cooling the calorimeter to ambient by holding a piece of Dry Ice over the calorimeter for 10-15 sec allowing the cool CO? to flow over it. Hydrogen is admitted, the system is flushed by filling the system to 80 cm Hg over ambient pressure, relessing the excess pressure through an outlet value and repeating the process two or three times. Hydrogen pressure is maintained a t 80 cm over atmospheric pressure for the remainder of the experiment by having pre-set the value on the hydrogen tank. At this point, the galvanometer spot should be swinging rapidly doe to the heat given offas palladium oxide is reduced to the active metal catalyst. Changes in the setting of the Wheatstone bridge are neeessary to keep the galvanometer on scale. After four or five minutes, the catalyst should be completely reduced and the cell can be returned to ambient temperature by the dry ice technique just mentioned. The galvanometer spot is bmught to the low temis ready perature end of its scale with Dry Ice and the aooaratus -. for use. After a base line is established, 40 pl of 1-hexene are injected by means of a microsyringe inserted through the hole previously made in the rubber stopper. The galvanometer swing due to hydrogenation should he rapid indicating s resction time of s. minute or 80. If slower, the stirring rate should he increased. Readings are taken for 3 or 4 min before and after the reaction is complete; they are graphed and extrapolated in the usual way (7). The result is a typical temperature versus time plot with a. shape similar to those obtained in oxygen-bomb calorimetry. The galvanometer reading is returned to the low temperature end of the scale with Dry Ice, and the apparatus is ready to hydrogenate again. Repetitive samples should show a relative average deviation fmm the mean of e. percent or so. The process is repeated with 40 p1 samples of unknown alefins. Polyolefins may give more than full s a l e deflection and require increased series resistance. Computalk. 1-hexene is used as a calorimetric standard much as benzoic acid is used in oxygen calorimetry. Hexene is assumed to have a heat of hydrogenation of 30.3 kcal/mole and the galvanometer scale deflection of the unknovn relative to that of 1-hexene is used to determine its heat of hydrogenation. One obtains the galvanometer scale deflection per millimole (SD/mM) for each substance and solves the ratio
where AH, is the desired heat of hydrogenation. In eqn. (I), SD is read directly from the temperature-time plot treated exactly as in oxygen-bomb calorimetry (7). Computation of the millimoles, mM of sample taken requires the knowledge of densities, p, and gram molecular weights, gmw, of both 1-hexene and the unknown
Results and Discussion
Typical results are shown in the table. Material for interpreting results may be found in references (5-6) and in many textbooks. This experiment may he extended or amplified into a research project by hydrogenating some of the many interesting unsaturated substances which are n o t reuorted in the literature. Heat of Mizing Correction. A correction which is usually small in magnitude but which illustrates an im~ortant thermochemical ~ r i n c i ~ lise the heat, of -mixing correction (4). If i t is desired to make this correction, inject Samples of hexane and the saturated reaction product of the unknown olefin in amounts equal to the amounts of sample taken. The scale deflection for hydrogenation is then corrected for the small deflection due to the heat of mixing of the saturated compound and acetic acid. (Add if mixing is endothermic and subtract if it is exothermic.) The thermochemical principle illustrated is that, although an alkene is added in the hydrogenation procedure, it a d s up as an alkane. Thermochemically, the final state of the system is all that matters, being independent of its history. ~
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Literature Cited
S*I.EM, L.. "The
Molecular Orbital Theory of Conjugated Systema." W. A. Benjamin, New York, 1966, p. 103. (2) Dr;w*a. M. J. S.. AND SCHMEIBINO, A.. "Conference on Hyperconjuga(1)
..-"
tion," Indiana University, Bloornington. Indiana. Pergamon Press, W"."
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(3) K~srr*aowsau,G.. ct el., J . Aner. Chem. Soe.. 57,65 (1935); 58,137, 146 (1963): 59, 831 (1937): 61, 1868 (1939). R. B.. J . Amsr. Cham. 900.. 64. 1395 (4) WIILIALIB. ~ (10421. ~ , SKINNER, k. A.,'AND SNELBON, A,, T V . ~ S . F.I. SOC.. 55, 404 (1959). (6) Tamsn, R. B., et al., J . Amer. Chern. Soo.. 79, 4116 (1957); 86, 3585 (1964): 90,4315 (1968). (7) DANIEL*. F.. et al.. ''Experimental Physical Chemistry," MoGrau-Hill Book Co., New York, 1970, pp. 39, 40. (8) BBETBCRNEIDER, E.,A N D RORERB, D. W., Microehim. Acto, in preaa.
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Volume 48, Number 8, August 1971
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