Determination of the Resonance Stabilization Energy

of Benzene by Bomb Calorimetry

Author: J. M. McCormick

Last Update: October 15, 2008

Introduction

The structure and chemical reactivity of benzene was very puzzling to chemists for a number of years.  They knew that it contained six carbon atoms and six hydrogen atoms, but not how the atoms were connected to give such a high degree of unsaturation.  Kekulé famously solved the problem by postulating that benzene has a cyclic structure and three alternating double bonds.  While this did explain some aspects of benzene's properties and reactivity, it raised more even more questions.  These questions were eventually solved and lead to our modern picture of benzene.  The simplest model of benzene's structure has six π electrons delocalized over the six carbon atoms giving six identical C-C bonds.  For more details on the development of the resonance theory of benzene, the reader is directed to any introductory organic chemistry text, for example reference 1.¹  For a brief introduction to Kekulé's life and work please see reference 2.²

One particularly important piece of evidence that lead to modification of Kekulé's original theory, and eventually to our modern concept of resonance, was benzene's anomalous enthalpy of hydrogenation.  Based on the enthalpy of hydrogenation (i. e., the heat released when the compound reacts with H₂) of cyclohexene and 1,3-cyclohexadiene, the enthalpy of hydrogenation for 1,3,5-cyclohexatriene should be about -85.8 kcal/mole.¹  However, benzene's enthalpy of hydrogenation is significantly less exothermic than this, indicating that benzene's delocalized structure is significantly more stable than the classic Kekulé structure.  In this exercise you will determine the resonance stabilization energy of benzene by bomb calorimetry following the procedure of Halpern and McBane^3,4 with minor modifications, which are outlined below.  The theoretical background for bomb calorimetry and the methods used are given in the literature^3-6 and online by clicking here.

Procedure

The general procedure of Halpern and McBane for the combustion of benzoic acid³ will be used for both standardization of the calorimeter and the combustion of 1, 5, 9-trans, trans, cis-1, 5, 9-cyclododecatriene (CDDT).² For additional safety precautions, see references 7 and 8.^7,8

You will be using a Parr 1341 plain jacket calorimeter and Parr model 1108 oxygen combustion bomb similar to those shown in Halpern and McBane.⁴  A side and a top view of the bomb itself are shown in Fig. 1.  The oxygen regulator that is used to fill the bomb, which is not described in Halpern and McBane, is shown in Fig. 2.

Figure 1.  Side view (left)⁷ and top view schematic (right) of the Parr model 1108 oxygen combustion bomb.

Figure 2. Oxygen regulator used to fill the Parr 1108 combustion bomb.⁷

Modifications for calibration of the calorimeter using the combustion of benzoic acid (by step number in Halpern and McBane's experiment 5) are:

1. You will make the pellet yourself using the press in the laboratory.  It is important that you do not overcompress the sample (pellets that are too solid will not burn completely).  You only need to apply enough pressure to give a pellet that will hold together during handling.

 

3. Omit this step.

 

4. Insertion of the bomb head is easier if the pressure release valve is open.

 

5. Connect the oxygen inlet line to the bomb's fill port.  With the pressure release valve open, start a gentle flow of oxygen through the bomb by opening the regulator valve slightly.  Let the oxygen flow for a few seconds and then close the bomb's pressure release valve. Slowly fill the bomb to no more than 30 atm of pressure.  At 30 atm, close the regulator valve and trip the regulator's pressure release valve to vent oxygen from the line between the regulator and the bomb.  If you exceed 30 atm the regulator's pressure release valve should open to vent oxygen.  When this happens, shut off the oxygen supply valve, fully open the regulator's vent valve, then vent the bomb and fill again.  Note that if you exceed 30 atm the regulator's pressure release valve will probably need to be reset which requires disassembly of the regulator and will cost you valuable time.

 

Once you have filled the bomb the first time, vent it and refill to 30 atm.  You are then ready to proceed to the combustion step.  Helpful hint: it is wise to check the condition of the ignition wire using a digital voltmeter (DVM) to measure the resistance across the ignition leads often times during the filling procedure and when it is placed in the calorimeter bucket.

 

6-9.  Depending on which experiment you have been assigned, you will be using either the Parr 1661 calorimetry thermometer (Bomb Calorimeter 1) or the Parr 6772 calorimetry thermometer (Bomb Calorimeter 2).  Click on the appropriate link to view operating instructions for the assigned thermometer.  Note that each handles data output in a slightly different way, but whichever thermometer you use remember to save the data either to your Y: or removable data storage device.  LoggerPro has a number of useful features for manipulating and analyzing data, but all final figures should come from Excel.

 

Once you have made an initial run with benzoic acid and one with CDDT, you should examine your data and re-evaluate the time intervals between each temperature measurement.  You may be able to take less data, and therefore take less time to complete a measurement, but still maintain high precision in the results.

 

Note that it is easier to put the bucket in the calorimeter first and then place the bomb in the bucket (there is a handy tool to help you lower the bomb into place).  Check the resistance between the ignition leads one last time.  Next attach the ignition leads to the bomb (be sure that the ignition unit is unplugged to prevent any unexpected ignitions) and finally add the 2 L of water.  Close the lid, attach the belt to the stirrer and the motor, and place the temperature probe in position.

 

11. The time interval is already set, do not change it mid-run.

 

12. Be careful where you set the calorimeter lid if the temperature probe is still attached.  We have a metal ring that may be attached to a ring stand that can be used to hold the calorimeter's lid while it is not in use.

 

14. It is more precise, and easier, to weigh the remaining pieces of the fuse wire.

 

15.  At least two (preferably three) calibration runs must be done per day, and these should alternate with sample runs.  It is advised that you start with a benzoic acid calibration run, then proceed to a CDDT sample run, and then another calibration run, etc.

 

You are to omit the sections on the combustion of naphthalene, sucrose and breakfast cereal in experiment 5, as well as the supplemental exercises in both experiment 5 and 6 on computational chemistry.  However, you should read the Data Analysis section in experiment 5 as it contains useful information that you will need.

The procedure for the combustion of CDDT is given in Halpern and McBane as experiment 6.⁴ CAUTION! Use no more than the recommended amount of CDDT, and it is advised that you use no more than 80% of the amount given in reference 1 for your first run.

Results and Analysis

Perform the calculations (including the corrections) suggested in the procedure,^3,4 and report all values at 95% confidence. Compare your measured resonance stabilization energy with the generally accepted value(s).  Propagate the uncertainties in the measured quantities through to the resonance stabilization energy of benzene.  Identify what measured quantities contributed the most to the uncertainty in your final result.  You do not need to answer any of the questions at the end of either exercise.  However, you should look at them as they may give you topics to consider as you write your laboratory report.

References

1. Solomons, T. W. G. Organic Chemistry, 4^th Ed.; Wiley: New York, 1988, p. 487-504.

 

2. Lipeles, E. S. J. Chem. Educ. 1981, 58, 624-625. Click here to view as a PDF file (Truman addresses and J. Chem. Educ. subscribers only).

 

3. Halpern, A. M. and McBane, G. C. Experimental Physical Chemistry, 3^rd Ed.; W. H. Freeman: New York, 2006, p. 6.1-6.5.

 

4. Halpern, A. M. and McBane, G. C. Experimental Physical Chemistry, 3^rd Ed.; W. H. Freeman: New York, 2006, p. 5.1-5.15.

 

5. Garland, C. W.; Nibler, J. W. and Shoemaker, D. P. Experiments in Physical Chemistry, 7^th Ed.; McGraw-Hill: New York, 2003, p. 145-151.

 

6. Atkins, P. and de Paula, J. Physical Chemistry, 8^th Ed.; W. H. Freeman: New York, 2006, p. 37-56.

 

7. Parr Bulletin 1341, Plain Jacket Calorimeter; Parr Instrument Co.: Moline, IL, 1977.

 

8. Wilson, L. Y. and Tatum, R. J. Chem. Educ. 1985, 62, 902.  Click here to view as a PDF file (Truman addresses and J. Chem. Educ. subscribers only).