The main purpose of this experiment was to determine the equilibrium constants of deprotonation reactions of 2-naphthol using UV-Vis and fluorescence spectroscopy.

by kanaanghadygmailcom (student)

Deprotonation of 2-Naphthol in Ground and Excited States.

Ghady Kanaan
CHE 441- 001
Experiment 5
Lab Partners: Wyatt Ross Ringo, Abby Darpel

Experiment Date: April 6th , 2021

Submission Data: April 13th , 2021

Introduction

The main purpose of this experiment is to determine the equilibrium constants and enthalpies of the deprotonation reaction of 2-Naphthol. This experiment dives deep into proving that thermodynamical quantities of a specific compound are correlated to their energy states.

This experiment relies on spectroscopy to determine unknown quantities. An equation that comes to mind that relates energy and light is Einstein’s equation (1):

This equation uses a Plank’s constant, the speed of light, and which is the wavelength in meters. The value obtained for the equation above is the energy of a photon, in order to obtain the energy per mole of reaction, simply use Avogadro’s number.

Transitions between electronic states rely heavily on the movement of electrons. The energy difference for these valance-shell electrons happens in the ultraviolet and visible light spectrums, hence UV-Vis’s spectroscopy. Organic molecules are generally in the group electronic state, S0, (1). With the help of quantum mechanical rules, the electronic excitation from ground state to excited state would be stronger since the states have the same electron spin multiplicity (1). If the energy transition involves photons, S0 → S1 upward transition, as seen in Diagram 1, show the absorption of light leading the molecule to be in a higher state. On the other hand, if the molecule shows emission of light, S1 → S0, this can be measured with fluorescence. Comparing the different phenomenon, absorption is a one step process while fluorescence is a two-step process because light must first be absorbed to excite the molecule before it can fluoresce (1),(4).

Absorption bands at higher energy correlate to transitions located mostly in the vibrational levels of the ground state at , all the way to vibrational levels at the excited state. As anything with high amounts energy goes, excited states are unstable when compared to ground electronic states. Thus, allowing molecules at excited states to undergo reactions that will allow them to get rid of the excess energy bottled up.

This experiment requires the use of 2-Naphthol, when 2-Naphthol is placed in an aqueous solution, it acts as a weak acid due to the hydroxy functional group present in the compound(3). Recall the weak acid general equation:

When 2-Naphthol is placed in an aqueous solution, it exists in equilibrium with and its conjugated base (2):

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When 2-Naphthol is placed in an aqueous solution, it exists in equilibrium with and its conjugated base (2):

Diving deeper into 2-Naphthol quantum properties, in its ground electronic state, 2-Naphthol is a singlet under S0. This means that all of the molecule’s electrons are spin-spin paired. In this lab, a deep analyzation of the electronic state transition of 2-Naphtol. Mainly, this lab focuses on the transition from its group state S0 to the first excited state S1 (1). Two different types of spectroscopy will be put to use in this lab. UV-vis absorption spectroscopy will focus on measuring the group state of the molecule being tested, on the other hand, fluorescence spectroscopy is used to measure the properties of the excited state due to emission. In order to achieve the purpose of this experiment, the absorption spectrum of 2-Naphthol require the use of five solution with varying pH. With 2-Naphthol’s pKa ranging to be around 9.51 (3), three of the five solution will have a pH similar to 2-Naphtol, while the other two solution will be on the opposite ends of the pH spectrum. The most acidic solution, Hydrochloric acid, will have a pH of lower than 1, while the solution with the highest pH, Sodium Hydroxide, will have a pH of higher than 10. These two extreme solutions are necessary, since they allow 2-Naphthol to exist in its protonated for and deprotonated form, respectively.

Experimental

Materials:

Procedure:

Begin the experiment by calibrating the pH probe. To start, open the Logger Pro software and make sure the pH probe is connected to the Vernier tool. Obtain the acidic buffer, and then place the pH probe in the buffer. Wait for the pH reading to stabilize, then enter 4 for the value and save. Repeat these steps with the basic buffer solution of pH 10.

Next, prepare the buffer solutions. To prepare the neutral solution, pipette 5 mL of NH4Cl with 5 mL of NH4OH. For the slightly acidic solution, pipette 7 mL of NH4Cl with 3 mL of NH4OH, and for the slightly basic solution, pipette 7 mL of NH4OH and 3 mL of NH4Cl. To make the acidic solution, it is solely comprised of HCl, while the basic solution is made with NaOH. Proceed with measuring the pH of each buffer solution. Place the probe in the prepared solution, record the readings in the lab notebook, make sure to rinse the probe before and after each dip in the prepared solutions.

Next, the UV-Vis spectroscopy must be conducted. Prior to running the spectra, the spectrometers parameters must be set. Locate the VisionPro software and open it. The proceed to set the following parameters under “Scan Method”:

Set the Data Mode to “Absorbance”
Set the Start Wavelength to 300 nm and the End Wavelength to 400 nm
Set the Scan Speed to “Intelliscan”
The Scan Interval was set to “Normal”
Finally, set the Graph Low to 0.000 Å and the Graph High to 1.000 Å.

Now in the cuvette, place 2 mL of the prepared solution along with 20 µL of 2-naphthol. Place the cuvette in the spectrometer and select “RUN”. Repeat thus step with the rest of the prepared solution and make sure the data was saved.

Finally, Fluorescence spectroscopy must be conducted for HCl and NaOH. Before running the spectrometers, the following parameters must be set:

Set the Repeat Number to 1
Set the Repeat Interval Time to 1 min
Set the PMT Voltage to 500
Set the Scan Speed to “User Defined”
The Integration Time was set to 0.5 nm.
The Ex lit and Em slit were set to 5 nm
Type EM Interval was set to 0.5 nm.

Now in a cuvette, pipette 2 mL of HCl along with 20 µL of 2-naphthol. Place the cuvette in the slot in the spectrometer and wait till a spectrum is obtained. Repeat this step with NaOH and make sure all the data is saved.

Results

Figure 1 UV-Vis Spectras of the Experimental Solutions

Figure 2 Fluorescence Spectrum of the Most Acidic and Basic Solutions

Table 1 UV-Vis and pH Result Summary

Table 2 Final Results with Corresponding Literature Values

Calculations and Error

To calculate the pKa of the solution, a variation of the Henderson-Hasselbach Equation must be used:

Deconstructing the equation above, [A-] is the concentration of the conjugate base, while [HA] is concentration of the acid. However, for the purpose of this experiment, a variation of the equation above must be used to solve for the pKa:

Looking at the equation above, Aj refers to the maximum absorbance of the solution for j=2,3,4. For the solution 2, the pKa is calculated as follows:

The same steps were used to find the pKa for solution 3 and 4

To find the error of the pKa use the propagation of error formula:

The process above was repeated for the pKa value of solution 3 and 4

Using the pKa values obtained from the results above can be used to find the equilibrium constant of the excited state, pKa*, which can be calculated using the equation below (1):

Deconstructing the equation above, the NA represents Avogadro’s number which is: , while h is Planck’s constant: , c is the speed of light which is values at: , and R is the ideal gas constant at , and T is the room temperature in Kelvin at 296.6 K. and are the average wavenumbers from the absorption and emission spectrum which can calculated the following way:

The average must be calculated for HCl and NaOH:

Given all these values above, pKa* for solution 2,3, and 4 can be calculated:

Following the same steps as above the values of pKa* for solution 3 and 4 were found:

Using the values obtained and the given error, the propagation of error for pKa* can be calculated:

The process above was repeated for the pKa* value of solution 3 and 4

Calculate the average pKa and pKa* as well its uncertainty:

Follow the same steps above for pKa*:

To find the standard deviation of the averages, the following equation must be used:

The steps above were repeated to find the standard deviation in pKa*:

Discussion

The main purpose of this experiment was to determine the equilibrium constants of deprotonation reactions of 2-naphthol using UV-Vis and fluorescence spectroscopy. This experiment ran well and our average pKa value agrees with the literature value. The experimental value was found to be at 9.712 ± 0.21, while the literature value was 9.50 ± 0.2 (4). While the calculated pKa* had a significant deviation from the literature value. The experimental pKa* was 9.6475 and the literature value was 2.97 ± 0.03. Our values experience a slight deviation in pKa and a very noticeable difference in pKa* due to some unexpected errors that were encountered in the lab. Keeping in mind that the values for fluorescence spectroscopy were obtained from the TA due to the Fluorescence Spectrometer not functioning properly during our experimental trial. A possible source of error could’ve begun with the pH probe. After calibration, the pH reading was unstable, and the recorded value was the average readout. To eliminate this error, obtain a better and more stable pH probe and calibrate properly. In addition, proper calibration for each instrument is required and must be precise. A slight deviation in one value, may grow exponentially throughout the experiment.

Isosbestic points occur when wavelengths of different pH have the same absorbance on the UV-Vis spectrum. Isosbestic points occur when the absorbance of AH and B are the same at a given wavelength. Analyzing the data graph above, the graph experiences 2 isosbestic points at 307 nm and 315 nm.

All in all, this experiment ran well, and the results above can be replicated in a better environment to obtain more accurate results.

References
(1) Department of Chemistry. Chemistry 441G Physical Chemistry Laboratory Manual. January

2021. (accessed February 9, 2021).

(2) Y.-S. Lee, H. Yu, O.-H. Kwon, D.-J. Jang Excited-state deprotonation dynamics of

2-naphthol in NaX nanoreactors,

(3) National Center for Biotechnology Information (2021). PubChem Compound Summary for

CID 8663, 2-Naphthol. (Accessed April 6, 2021)

(4) Rosebrook, Donald D, and Brandt, Warren W. "Determination of Excited State PKa Values

Using Photopotentiometry." Journal of Physical Chemistry (1952) 70, no. 12 (1966): 3857-862.

The main purpose of this experiment was to determine the equilibrium constants of deprotonation reactions of 2-naphthol using UV-Vis and fluorescence spectroscopy.

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