How Molecule's Structure affect the pKa - Lab Report Example

How Molecules Structure affect the pKa of Its Ground and Excited s: The relationship between the Structure of a Molecule and the pKa value of its ground and Excited states [Name] [Institution] Abstract The objective of this laboratory experiment was to determine how a molecules structure can affect the pKa of Its ground and excited States. The experiment was based on an analysis of the photoacidity of 2-naphthol. The molecule 2-naphthol in its protonated form was particularly used with a pH value range of between 8.6 and 10.2 as was determined by a spectrophotometer. The results indicated a pKa value of 7.12. How Molecules Structure affect the pKa of Its Ground and Excited States: The relationship between the Structure of a Molecule and the pKa value of its ground and Excited states Introduction The structure of a molecule is widely believed to have a considerable influence on the pKa of Its Ground and Excited States. This is particularly attributed to the energy relationships between the ground and excited states of various organic acids in both their deprotonated and protonated forms. According to many experts, the electronic structure of a molecule can determine both its physical and chemical properties as well as the potential charge distribution, ionization potential, geometry, electronic affinity and ultimately, chemical reactivity. In this regard, any change in the electronic structure of a molecule may be expected to alter the chemical or physical properties. Generally, this is what usually occurs when molecules are raised to an electronically excited state through the absorption of quantum light such as photons with an energy that matches the energy gap between the exited and ground states (Chang, 2000). 2-naphthol, also known as β-naphthol or ArOH is a fluorescent, colorless and water soluble solid that can effectively be used to help determine the potential energy relationships between the protonated and deprotonated forms of organic acids and their ground and excited states. 2- napthol Photoacidity is one of the best ways of determining how a molecule’s structure can potentially impact on the pKa of its ground and excited States. For example, in many of the organic molecules containing an even number of electrons, an arrangement characterized by the ground state having its entire electron spins paired the net spin angular momentum turning to zero usually occurs. This is arrangement is commonly referred to as a singlet state (Perrin and Dempsey, 2001). The absorption of light by a given molecules like the 2-naphthol can significantly affect its chemistry by altering its geometry as well as electron density. At a particular pH, the value of pKa of ArOH is closely related to its absorbance A at a specified wavelength λsp as given by Where: AArOH and 1 ArOA −= the absorbance of the acid and its conjugate base. Experiment The reagents that were used in the experiment included 1.0 M ammonium hydroxide buffer solution (NH4OH), 1.0 M ammonium chloride (NH4Cl), 0.10 M hydrochloric acid (HCl), 0.10 M NaOH, and 1x10-3 M 2-naphthol (ArOH in ethanol). Buffer 50 ml NH4OH , 50 ml NH4Cl PH=9.52. To prepare the solution, the stock 2-naphthol was diluted with hydrochloric acid and sodium hydroxide buffer whereby the HCl provided resulted protonated molecule (ArOH) while the sedum hydroxide buffer resulted in the deprotonated molecule (ArO-). On the other hand, the pH of the solutions was checked and the UV-visible spectra were then generated using pectrophotometer. The resultant data was then recorded and analyzed. Results/Discussion Fig 1: In dilute hcl In the dilute acid, the HCl provided resulted protonated molecule (ArOH) thereby impacting on the molecule ArOH)structure of Fig 2: In Sodium Hydroxide (NaOH) Fig 3: In buffer The results of the first part of the experiment showed a significant shift from the protonated form of 2-naphthol towards the deprotonated form of 2-naphthol. As can be seen in the above results, electronic structure of a molecule can determine both its physical and chemical properties as well as the potential charge distribution, ionization potential, geometry, electronic affinity and ultimately, chemical reactivity (Atkins and Paula, 2012). This may be attributed to the fact that many of the organic molecules containing an even number of electrons, an arrangement characterized by the ground state having its entire electron spins paired the net spin angular momentum turning to zero usually occurs (Laws and Brand, 2009). Fig 4: spectrum of ArOH and ArO On the other hand, the second part of the experiment involved the calculation and determination of how the pKa differed with the pKa*. The equation used was as follows: Where: A= absorbance at 343 nm AArOH= absorbence of the protonated form AArO -=Absorbance of the deprotonated form As shown in the graph in figure 4 below, the value of pKa was found to be 7.13. Fig 4: Alpha values vs. pH In order to obtain pKa*, the values obtained from the graph above can be calculated using the following equation: Where: NA = Avogadro’s number h= Planck’s constant R = 8.314 J/mol T=298.15 K Although this value is not consistent with the results of other previous experiments as reported in the literature, this may be attributed to some errors that may have occurred during the experiment. One potential source of error in the experiment is the fact that the 2-naphthol is only somewhat soluble in water (Halpern and McBane, 2006). Generally, the best method for calculating the pK * of ArOH is known as Foster cycle based on the equation below: Using the absorbance and the fluorescence spectra of ArOH, the value of pKa* was found to be 4.2. Conclusion Based on the results of the experiment, it can be concluded that that the chemical and physical properties of 2-naphthol were altered whenever the electronic structure of a molecule was changed through the absorption of quantum light such as photons with an energy that matches the energy gap between the exited and ground states. References Atkins, P. W., Paula, J. (2012). Physical Chemistry. 11th Edition. Oxford, UK: Oxford University Press. Chang, R. (2000). Physical chemistry for the chemical and biological sciences. Sausalito, California: University Science Books. Laws, W. R., Brand, L. (2009). Analysis of two-state excited-state reactions. The fluorescence decay of 2-naphthol. J. Phys. Chem. 83(20), 795-802. Halpern, A. M., McBane, G. (2006). Experimental physical chemistry. New York: W. H. Freedman and Company. Perrin, D. & Dempsey, B. (2001). pKa Prediction for Organic Acids and Bases. New York: Chapman & Hall. Read More