The Heat of Protonation of Pyridine and Chloro Substituted Pyridines
Robert L. srnlth and H. R. Pinnick, Jr. Southeast Missouri State University Cape Girardeau. MO 63701
I
A physical chemistry laboratory experiment
Students in the t w ..i c a l undereraduate organic chemistrv course are always well instructed in the concepts of inductive and resonanre effects. These effects are used in explaining aspects of the chemical reactivity 01 \.ivious organic molecules. However, it is seldom that a student in a n undergraduate program isconfnmted u,ith an exerrise that directly and uneuuivucallv demonstrates these effects. It occurred co the aithors t h a t a n experiment suitable for t h e undergraduate physical chemistry laboratory t o illustrate the above-mentioned nrincioles would be the calorimetric determination of the heats of protonation of pyridine, 2-chloropyridine, and 3-chloro~vridine. ., A proc1:durc has been developed utilizing the continuous t i t m t ~ o nmethod ( I ) . This technique involves the determination of the initial energy equivalent of the Dewar vessel containina a n aqueous solution of t h e organic base followed t,y ir slow fitration of pcrchloric acid from a motor driven hurrr, with the bridgeoutput displayed a s a function of time on a recorder chart. The risultini thermoeram is analvzed bv evaluating the lead and trail slopes and reading a series of t e m ~ e r a t u r evalues (in chart unit divisions) a t eaual time intervals, usually 0.5 or 1.0 min, along the buret addition part of the trace. By evaluating the heat capacity a t each point, a n array of heat terms result which must be corrected for the heat of stirring-heat leak effect, the heat of dilution of the titrant, the heat effect due t o any difference in temperature between the titrant and Dewar solution a t the beginning of the titration, and the heat of water formation, if applicable. Using equilibrium constant data from the literature the enthalpy change is calculated from
-.
where 0; is the corrected heat a t each data point and ni is the numbe;of moles of protonated base forked. A computer Droaram written in FORTRAN (2) is available for processing i h e h a t a . T h e printout contains the enthalpy and entropy values, the species distribution, the heat correction terms, the corrected heats, and the error-square sum U = E(Qnp - Qcstc)2
(2)
Experimental The pyridine was obtained from Fisher (ACS certified grade) and the 2- and 3-chloropyridines were obtained from Aldrieh (99%purity). Each base was distilled one time under nitrogen at atmospheric pressure. Perchloric acid was standardized with soda-ash. The calorimeter was a Tronac Model 450 isaperibol titration calorimeter equipped with a 10 ml buret. Auxiliary equipment were a HewlettPackard 3490A multimeter, a Rolla Engineering and Development model 400 digital clock, and a 1 mV Bristol model 560 recorder.
320 1 Journal of Chemical Education
Convenient concentrations were 0.05 M perchloric acid with about 0.1 M pyridine and 3-chloropyridine. The 2-chloropyridine titration required 0.5 Mperchlorieacid or higher and 0.22M base, which is near the solubility limit at 25% One minute time intervalsfar data points typically yielded 13 for pyridine, 16 far 3-chloropyridine,and 20 for 2-chloropyridine. Volumes of titrant added were 3.2 to 3.8 ml. Results The table contains the values as the mean of five experimentsalong with the standard deviations. The equilibrium constants, as log K, were taken as 5.17 for pyridine (3),2.84 for 3-ehlaropyridine ( 4 ) ,and 0.49 for 2-chloropyridine (4). The error-square sum reported is the normalized value, U', i.e., eqn. (2) divided by the number of data points. Discussion The standard deviation for the chloropyridines is not an accurate measure of the uncertainty in the enthalpy value. That is, if a heat value can be resolved to about 0.01 calorie, the error for the 3-chloropyridine case is about 9%and around 15%for the 2-chloropyridine, based on the magnitudes of the measured heats. Also, it is probably necessary to use additional purification procedures on the chloropyridines if highly accurate enthalpy values are to be obtained. Nevertheless, the experiment demonstrates dramatically the positional effect of the cblorosubstituent on the basicity of the pyridine nitrogen atom. Some of thequestions that the student can beasked aboutthe experiment are (1) What effect does the error in the measurement of the heat value have on the error limits of the results?, (2) What are the implications of the trend in the entropy values?, and (3) Is there a possible sterie effect in the protonation of 2-chloropyridine? Acknowledgment The authors would like to express their appreciation to the students in the current year's physical chemistry laboratory course and to Mr. Randy Weiss who did the distillation work. ~~~
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Literature Cited (1) Chnnte"%e".J. J.. Ruckman. J.,Eatough,D. &and 1zatf.R. M., Th~rmochrmicoAlto,
1212 (19691. (4) Christensen.J. J.. Smith, D. E.. Slade,D.,and Izaff R. M., Thermochimico Acts. 6.35 (19721.
The Heats-of Protonation of Pyridines -
-
8858
.- .- . - -
pyridine 3-chlorapyridine 2-chloropytidine -
-AHO (kcallmole)
4.97 + 0.13 2.98 f 0.05 0.37f 0.01
A 9 (eu.)
+
7.0 0.2 2.99 f 0.05 1.0f 0.1
U'X 10'
--
0.0781 0.580
4.37
