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Organic Chemistry II | Lecture | Laboratory

Organic Chemistry Laboratory II
Determination of pKa Values of Substituted Benzoic Acids
Experiment Description

Introduction

Students will work in pairs to determine the pKa (acidity constant) of benzoic acid or a substituted benzoic acid by titration with NaOH. The following experiment is modified from a procedure described in “Organic Laboratory: Microscale and Standard Scale Experiments”, 4^th edition by John A. Landgrebe (1993). The pKas will be determined for the twelve benzoic acids given in the Table 1. Students will measure the equivalence points of benzoic acids using a computer and the interface probes for measuring pH. The pKas that are measured will be related to the specific substituent of the benzene ring and compared to the pKa of benzoic acid to determine if the substituent is electron-donating or electron-withdrawing. Using the experimental data and the Hammett equation, students will also determine the sigma value for each substituent . All groups will work together to generate a plot of sigma values for all substituents. The following substituted benzoic acids will be used. The pKa values for these compounds were found in the “CRC Handbook of Tables for Organic Compound Identification”, 3rd edition, pp. 428-433.

Acid	Literature pKa	Acid	Literature pKa
Benzoic Acid	4.20	2-Nitrobenzoic Acid	2.22
Salicylic Acid	2.75	3-Nitrobenzoic Acid	3.44
3-Hydroxybenzoic Acid	3.90	2-Toluic Acid	3.91
4-Hydroxybenzoic Acid	4.61	3-Toluic Acid	4.27
2-Chlorobenzoic Acid	2.92	4-Toluic Acid	4.37
3-Chlorobenzoic Acid	3.82	4-Chlorobenzoic Acid	3.98

Table 1: Acidity constants of Benzoic Acid and Substituted Benzoic Acids

Procedure
Calibrate the pH Probe

Plug the pH probe into the GO-LINK, then plug the GO-LINK into a USB port of your computer. Start loggerpro (on PC– Start->search-> type logger then select logger pro). The program should be displaying an uncalibrated pH value in the lower left. Unscrew the vial from the bottom of the pH probe and slide the cap up the probe. Rinse the electrode thoroughly with de-ionized (DI) water into a large beaker. NEVER LET THE GLASS BULB ON THE BOTTOM OF THE PROBE DRY OUT. NEVER TOUCH THE GLASS BULB TO ANY SURFACE. In loggerpro go to Experiment → calibrate → Golink: 1 pH. A screen will pop up. Click “calibrate now”. Place the probe in the vial containing the pH 10 buffer and swirl gently. Wait till the voltage in the center right stabilizes, then in the “Reading 1” box type “10.0” (without the quotes) and click keep. Rinse the probe THOUROUGHLY with DI water. Place the probe in the pH 4 buffer and again wait for the voltage to stabilize. Put “4.0” (without the quotes) in “Reading 2” and click keep. Then click the “done” button. At this point the pH display should be reading 3.97 – 4.03. Rinse the probe thoroughly and place it in the pH 7 buffer for a few seconds. If the pH doesn't read 6.95 – 7.05, contact your instructor.

Dispense the Acid
Solutions of the benzoic acids have been prepared in 50% ethanol. Dispense exactly 50 ml of the acid solution using volumetric pipets to measure the 50 ml into a 150ml beaker containing a magnetic stirring bar. Two acids will be set out on each bench. Each acid will be titrated twice. Two students at the bench will titrate one acid and two students at the bench will titrate the other acid. Record the name of the acid you use in your notebook.

Recording the Data

Download and open the Excel file here to record your data. Save the file as an Excel Workbook using your last name and followed by titrationdata. (eg. hass_titrationdata.xls). Enter your name in the “name” cell on the spreadsheet. Enter the name of your compound in the “Compound” cell. Put the theoretical pKa value for your compound from the table above into the “pKa-Theo” cell. A graph on the right in the Excel file appears. The blue line on the graph is the theoretical curve for your compound if the titration was in pure water, not 50% ethanol. The red line is the predicted titration curve in 50% ethanol. You must enter data into the two columns, volume and pH. Enter the volume reading on the buret, (not the quantity you added). As you enter your data, a yellow line will appear that represents your experimental tiration curve. Do not change other cells, as that might skew your results.

Titrating the Acid
Place the beaker on a stirplate underneath the burette containing 50 mM NaOH. Clamp the pH probe on a slight angle into the solution. (Figure 1). The “crown” on the bottom of the electrode must be fully immersed in the solution, BUT high enough to not get hit by the stirbar. If the probe is getting hit by the moving stirbar, it will cause erroneous results, damage to the probe, and a frowny-face to appear on your instructor. Begin stirring the solution.

Once the pH stabilizes, record the initial pH reading of the benzoic acid solution and the volume reading on the burette into your spreadsheet. Note the numbers get bigger as you add more solution. NaOH will be added to the benzoic acid solution in volumetric increments (starting with 0.3mL) that produce a change in pH of less than 0.2 pH units (~0.5-1.5 ml). The solution will be allowed to equilibrate until stable then a pH reading will be taken.

Initially add ~0.3 mL volumes of NaOH. Note volume and pH each time. When the pH change is > 0.10 pH unit increase volume added to 0.5mL each time. If the pH change becomes close to or greater than 0.2, decrease volumes added to ~0.3 mL. When the change in pH becomes < 0.05 for three consecutive readings (~pH 12.1), stop the titration.

On the right, in yellow, your experimental titration curve will appear in the Excel file. Save the file.

Figure 1: Titration Set-Up

Determine the Endpoint and the Initial Acid Concentration from the Titration Curve

The endpoint and the concentration of the acid must be determined. The endpoint is defined as the quantity of base (typically in ml) needed to neutralize (or completely ionize) the acid. Since we are using monoacid compounds the stoichiometry of base to acid is 1:1. For each mole of NaOH used, one mole of acid is ionized.

The endpoint and the concentration of the benzoic acid solution can determined from the titration curve in the following way. Using the graph (not the table), identify the volume of base added at the lower inflection point of the curve (i.e., where it just starts to go straight up). This value can be found by dragging the cursor over that point and the volume value will be represented by the first number. Record this volume. Using the graph again, identify the volume of base added at the higher inflection point of the curve (i.e., where it just starts to curve and level out). Record this volume. Average the two values. This will be your endpoint NaOH volume. The concentration of acid can be determined from the endpoint and the concentration of the NaOH solution (0.05M). Calculate the number of moles of NaOH that was used to completely ionize the monoacidic benzoic acid.

Determine the midpoint and the pKa of the acid
The pKa of the acid must be determined. The titration curve is also used to determine the pKa. First, the midpoint, the volume of base needed to ionize half the acid present, is calculated from the endpoint. The midpoint can be determined by dividing the endpoint by 2. For example, if the volume of base is 4 ml, the midpoint is 2 ml.

The pKa of the acid is then determined from the pH of the solution at the midpoint, and the Hendersohn-Hasselbach equation. Determine the pH at the midpoint using the titration curve. At this pH, 50% of the acid is ionized (conjugate base) and 50% is unionized (acid). At this pH, [CB]/[A] =1.

The pKa of the acid is also used to determine the pKa. First, the midpoint, the volume of base needed to ionize half the acid present, is calculated from the endpoint. The midpoint can be determined by dividing the endpoint by 2. For example, if the volume of base is 4 ml, the midpoint is 2 ml. The pKa of the acid is then determined from the pH of the solution at the midpoint, and the Hendersohn-Hasselbach equation. Determine the pH at the midpoint using the titration curve. At this pH, 50% of the acid is ionized (conjugate base) and 50% is unionized (acid). At this pH, Ka = [CB]/[A] =1. Enter the pKa value that you calculated into the “pKa-pred” cell in the spreadsheet. The red line now indicates the the predicted titration curve for your pKa value you entered. Save the file.

Mean and Standard Deviation of pKa Values Determined by the Class
THIS MUST BE DONE BEFORE LEAVING THE LAB!

Each lab section will determine the pKa values for all 12 of the benzoic acids listed in Table 1. This data will be shared by the entire class and will be needed for you to complete the post-lab assignment. In oder to do this, all students need to upload their data to a website that can be accessed at: http://www.trentnet.net/acp-pka. Upon entering this site, you will be required to enter your ACPHS email account email (full name ie. Matt.groening@acphs.edu) twice. Enter your lab section by letter (A, B, C, D etc... ). (Click here if you do not know/remember the letter designation of your lab day). Enter your bench by letter, and postion on the bench (A-west-1, etc). You will then need to enter the name of the acid that you titrated, the experimental endpoint that you measured, the experimental midpoint volume, and the pKa value that you calculated. After all students from all lab sections have entered their data, a class Excel file will e compiled and will be available for students to download on or after March 20, 2013. You will need to access this file to complete the remaining part of the experiment, i.e., calculating the Ka and sigma values.

Download the Class Data File and Calculating Ka and Sigma (s) Values

Downloading the Class File
Download the compiled data file from: http://www.trentnet.net/acp-pKa-data.cvs and save the file as an Excel Workbook to your computer as class_pKadata.xls. This file will contain all of the class pKa data. Open the this file (class_pKadata.xls) and enter control-a (hold ctrl and a at the same time), then control-c (control and c at the same time). Open the spreadsheet you used to collect your titration data (lastname_titrationdata.xls) Go to the “sigmas” tab on this spreadsheet (see Figure at right) and put the cursor into cell A1 on sigma page. Enter control-v (control and v at the same time). Most of the data analysis will be done automatically on the sigma page of this spreadsheet including the average pKa value determined by all students in all sections in the class and from this data, the sigma values for each substitutent studied in this experiment. A description of how these values are determined is shown below. Save the file as an Excel Workbook (lastname_titrationdata.xls.).

The Excel file should automatically calculate the average pKa values for each acid that was determined by the class. The average Ka values are then determined from the mean pKa values (pKa = -log Ka) . Sigma (s) values for substituents of substituted benzoic acids are then determined from the Ka values and the Hammett equation. Rho is a constant equal to 1.46 and is used for benzoic acid and all substituted benzoic acids.

Sample Sigma Value Calculation
A example calculation of the sigma value for a meta- chloro- substitutent (m-chlorobenzoic acid) is given on the right. The literature pKa value of benzoic acid is 4.2 and for m-chlorobenzoic acid, the pKa value is 3.77 (determined in water).

When all of the sigma values are calculated, they are plotted relative to each other. The electron-dontaing substituenmts have negative sigma values (to the left on the graph) and the electron-withdrawing groups have positive sigma values and are plotted to the right on the graph.

Complete the datasheet as the post-lab assignment for this experiment.

Waste Disposal
The pH of aqueous solutions should be checked prior to disposal. Acidic solutions should be disposed of in "acidic aqueous waste" and basic solutions in "basic aqueous waste".