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Polarimetry and stereochemistry: the optical rotation of Vitamin C as a function of pH. Johan A. E-mail: J. Linthorst uu. A novel experiment was prepared to help chemistry educators to improve the stereochemistry curriculum of secondary school students and undergraduate students.
Based on polarimetry and Vitamin C, chemistry educators are now provided with an experiment that takes into account the delocalization of electrons, and the interactions with plane-polarized light in atomic bonds. Keywords: Polarimetry, stereochemistry, organic chemistry, image, Vitamin C, curriculum, secondary school, undergraduate, enantiomers, optical activity.
Bensaude-Vincent and Simon proposed a new philosophical perspective on the negative image of chemistry with their Chemistry: The Impure Science Linthorst, a. For centuries chemists have been studying objects which are not visible. According to Bensaude-Vincent and Simon, this contributed to the negative image of chemistry Linthorst b; Linthorst, Tai and Sadler statistically investigated the learning process of students who were subjected to context-rich chemistry curricula Linthorst, In fact, they found no convincing evidence for a better understanding of chemistry and its concepts by students in comparison with traditional curricula.
So following Bensaude-Vincent, Simon, Tai and Sadler, we search for instructional demonstrations that might contribute to a better conceptualization of chemical theories and models by students without solely focusing on contexts.
This article is an example of such an approach. Stereochemical concepts are important in the chemical arena because they are employed extensively, e. Just like in some other European countries, at Dutch secondary schools, students following pre-university education, are subjected to different aspects of stereochemistry, e. Consequently, chemistry educators are concerned with the teaching of stereochemistry.
For example, Lipkowitz et al. In their synthesis they determined enantiomeric excess with gas chromatography through the use of a chiral stationary phase. In comparison with Lipkowitz, et al. They argued that the difficulty of learning stereochemistry is partly due to 'the limited ability of some students to visualize molecular structures in three dimensions'.
Following Bensaude-Vincent and Simon , this should ultimately be explained by the fact that atoms and molecules are not visible. Nevertheless, Cody, et al. In one of the most cited stereochemical articles aimed at chemistry educators, Barta and Stille introduced their "hands on" approach in which students literally have to use their hands to visualize chiral molecules.
Lewis developed an abstract method with his geometrical 'proto-center concept'. Just like the methods of Barta, Stille and Cody, et al. Traditionally, chirality is taught on the basis of asymmetric carbons in a molecule.
These carbons are configurationally stable and have four different groups. Considering chemistry didactics, an asymmetric carbon might then be visualized with four different colored balls "covalently" attached to the carbon. Clearly, amongst others Cody, et al. From secondary school through undergraduate organic chemistry curricula, molecules with one or more asymmetric carbons, dominate in the teaching of chirality. Take for example 2-bromobutane that has one asymmetric carbon Figure 1.
The mirror images of both 2-bromobutane molecules are not superimposable, and so these molecules are enantiomeric. Both these enantiomers rotate plane-polarized light, but one rotates it to the left and the other one rotates it to the right, which forms the basics of polarimetry Volhardt and Schore, ; Hesse, et al.
In comparison with the left-handed 2-bromobutane C 4 H 9 Br molecule, but under the same conditions e. Analogous effects occur in the case of the right-handed ones. In comparison with 2-bromopentane, the asymmetric carbon C 2 of 2-bromobutane has an ethyl substituent instead of a n -propyl substituent, the other substituents are the same.
With respect to the difference in magnitude of the specific rotations: why should a secondary school or undergraduate student accept this difference?
One possibility is: by convention, or say a normative idea presented by the teacher. So, considering the colored balls, n -propyl has another color than ethyl. Unfortunately, such an explanation ignores the physical phenomena in which the electric component of plane-polarized light interact with the electrons in molecules and ions.
Indeed, drawing from our own experiences, some students might focus on their conceptualization of the atoms that are directly covalently bonded to the asymmetric C 2 instead of atoms and bonds along a substituent chain. For both 2-bromobutane and 2-bromopentane, the atoms bonded directly to the asymmetric center are the same. From this point of view, the colored balls,based on a convention, are not satisfactory for all students.
Therefore, we present a demonstration experiment that has the potential to be used by teachers as an adjunct to the other described methods, e. This approach is in line with findings of educational psychologists, who advocated the use of experiments and related classroom discussions in science education instead of presenting normative ideas to students Linn and Eylon, For this experiment we used Vitamin C, a compound that appears in several fruits, and which is used in the regeneration of Vitamin E in human bodies Volhardt and Schore, Moreover, Vitamin C appears to be suitable for our purpose to focus on the delocalization of electrons in chiral compounds with regards to learning stereochemistry and polarimetry.
In several beakers, 20,00 mL of this solution is added with a pipette; a volume of 5,00 mL of HCl 4 M or NaOH 4 M or 6 M is added to each beaker, with distilled water in varying ratios. For this end, a Novex Disc Polarimeter The pH is determined with a Vernier Labquest2 interface. Concentrated sodium hydroxide and hydrochloric acid might cause severe skin burns and damage the eyes. Therefore, protecting gloves and goggles should be worn.
Vitamin C, also known as ascorbic acid H 2 Z , is electrically neutral and there is no net charge to be stabilized. Consequently, in H 2 Z dominates one resonance form in which the oxygens have no formal positive charge Figure 2. The negative charge of ascorbate is stabilized by resonance, which explains the acidic properties of Vitamin C Figure 2. Of course, Z 2- is also stabilized by resonance. Vitamin C contains two asymmetric centers on C 4 and C 5 and is therefore optically active. As shown in Figure 2 , after deprotonation the covalent bonds of C 4 and C 5 remain intact.
Nevertheless, these bonds are affected after deprotonation as shown in Figure 3. The observed optical rotation is plotted against the pH, which results in a sigmoid shape of the curve in the zone near the mentioned pK a values.
This shows that there is no straightforward relation between the absolute configuration and the sign of rotation Volhardt and Schore, Contrary to the colored balls, this observation is a springboard for discussion in the classroom that provides a closer look on the matter Linn and Eylon, By now, the teacher has a didactic tool to explain that the mutual distance of electrons in Vitamin C, and its corresponding repulsive forces through the atomic bonds, changes with deprotonation and delocalization.
Consequently, the distribution of electrons around the asymmetric centers alters and thereby the interaction of plane-polarized light with these electrons also changes. In sum, with respect to chirality the groups of C 4 and C 5 in molecular Vitamin C are not the same for ionized Vitamin C.
This observation might induce that students are not solely focusing on the atoms directly attached to C 4 and C 5 , but also on atoms and bonds along a substituent chain. For the students it makes sense now to accept the conventional difference in colored balls of ethyl and n -propyl, as explained in the case of 2-bromobutane and 2-bromopentane.
Without the introduction of complex molecular orbital discussions, this experiment relates macroscopic phenomena and non-visible particles in a way that is disregarded in stereochemistry curricula. This might be changed now with this demonstration, and could eventually be extended with classroom discussions that are concerned with other topics, e. Linthorst the New Scholars Award Engberts University of Groningen, The Netherlands is kindly acknowledged for his comments on an earlier version of this article.
Barta, N. Bekkers, R. Groningen, The Netherlands: Noordhoff Publishers, Bensaude-Vincent, B. Cody, J. Franken, P. Chemie Overal Sk Vwo Deel 2. Handbook of Chemistry and Physics, 93 rd Edition, The online-edition, retrieved June 15th, , from URL www.
Lewis, D. Linn, M. In: Alexander, P. Linthorst, J. Lipkowitz, K. Tai, R. Volhardt, K. Freeman and Company, Wamser, C.
Servicios Personalizados Revista. Similares en SciELO. Abstract A novel experiment was prepared to help chemistry educators to improve the stereochemistry curriculum of secondary school students and undergraduate students. Introduction Bensaude-Vincent and Simon proposed a new philosophical perspective on the negative image of chemistry with their Chemistry: The Impure Science Linthorst, a. Hazards Concentrated sodium hydroxide and hydrochloric acid might cause severe skin burns and damage the eyes.
Results and discussion Vitamin C, also known as ascorbic acid H 2 Z , is electrically neutral and there is no net charge to be stabilized. Author information J. References Barta, N.