THE ELECTROSTATIC FORCE APPLIED TO TEACHING ORGANIC CHEMISTRY IN UNDERGRADUATE CLASSES

Firme, Caio L.

doi:10.21577/0100-4042.20240014

Caio L. Firme ^**e-mail: caio.firme@ufrn.brfirme.caio@gmail.com

Instituto de Química, Universidade Federal do Rio Grande do Norte (UFRN), 59078-970 Natal - RN, Brasil

https://orcid.org/0000-0002-5203-6262

*e-mail: caio.firme@ufrn.br firme.caio@gmail.com

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Figure 1
Results of students’ answers to the first two binary questions of the questionnaire to undergraduate students of the Federal University of Rio Grande do Norte, Brazil about (A) Coulomb’s law (question 1) and (B) its application to organic chemistry (question 2)

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Figure 2
(A) Schematic representation of the hydrogen bond in octanoic acid and sodium octanoate interacting with water along with their solubilities in water and Equations 8 and 9. (B) Equations 10-12 and schematic representation of the hydrogen bond between a pair of ethanol and a pair of ethanethiol molecules of the question 4 from the questionnaire. (C) Equations 13 and 14 associated with the π bond of C=C and C=O groups, respectively. (D) Resonance structures of butadiyne (2), hexatriyne (3) and octatetrayne (4) and the structure of ethyne (1) for comparison reason along with their corresponding Equations (15-18) of the electrostatic force of π bonds. The numbering of the equations was adapted to this work

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Figure 3
(A) Multiple-choice alternatives to question 3 of the questionnaire where the numbering of the equations was adapted to this work; (B) Multiple-choice alternatives from question 4 of the questionnaire in . (C) Multiple-choice alternatives from question 5 of the questionnaire. (D) Multiple-choice alternatives from question 6 of the questionnaire in where the numbering of the equations was adapted to this work

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Figure 4
Points scored versus number of participants in the questionnaire

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Table 1
First two questions (binary questions) of the Google Forms questionnaire applied to undergraduate students of chemistry course in Brazil

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Table 2
Second part of Google Forms questionnaire applied to undergraduate students of chemistry course in Brazil. The numbering of the equations and figures is according to the sequence of the questions of this work

(1)

(2)

(3)

(4)

(5)

(6)

(7)

Question number

Question

(1)

Coulomb’s law of electrostatic force (F = KQq/r²2 Hernani; Ulum, L. L.; Mudzakir, A.; J. Phys.: Conf. Ser. 2020, 1521, 042087. [Crossref]
Crossref... ), where Q and q are both positive or negative charges or one positive and one negative, can be applied in chemistry to show the strength of an attractive interaction between a negative charge and positive charge from a chemical bond (covalent or ionic) or from an intermolecular interaction.
The negative charge can be a sigma or pi bond electron, the partial negative atomic charge (δ^-) of an electronegative atom bonded to other more electropositive atoms, or the negative atomic charge of an anion (represented by the formal charge q-). The positive charge can be the positive atomic partial charge (δ⁺) of an electropositive atom joined to other more electronegative atoms, the positive atomic charge of a cation (represented by the positive formal charge q+), or effective atomic number, Z_eff, of an atomic nucleus. The r is the distance separating the charges, and K is a constant. This equation can be applied to only two charges. The atomic nucleus can be thought of as only a single charge of Z_eff value because the protons in the nucleus are clustered in a very small volume relative to the electrosphere.

Did you find it easy to explain how the electrostatic force is adapted to chemistry?

(2)

Read the following explanation: “Ammonia, NH₃, has a lower boiling point than water, H₂O, because the hydrogen bond between ammonia molecules (electrostatic interaction between H and N) is weaker than the hydrogen bond between water molecules (electrostatic interaction between H and O). This occurs because oxygen is more electronegative than nitrogen, causing the partial negative atomic charge, in modulus, on O (oxygen atom) to be greater than on N (nitrogen atom), leading to greater electrostatic strength of hydrogen bonding between water molecules than between ammonia molecules”.
Understand “in module” as a mathematical operation to remove the negative sign from a variable or a number.
Understand “partial atomic charge” as the charge of an atom, less than 1 and greater than 0, in modulus, due to its heterogeneous covalent bond with other atoms more or less electropositive or electronegative than it.

Did you find this explanation easy to understand?

Question number	Question
(3)	Solubility according to electrostatic force Read the following text: Solubility measures how much a solute dissolves in a given amount of a solvent, for example, water, at a given temperature. It mainly depends on the intermolecular interactions between the solute and the solvent. Figure 2A^a a The reference of the figures is in accordance with the sequence of the numbering of this work. shows the hydrogen bond between water and two different solutes: carboxylic acid (octanoic acid), and its corresponding salt (sodium octanoate). Each molecule of a carboxylic acid and its salt can have multiple hydrogen bonding interactions with water molecules, but for the sake of simplicity, only one hydrogen bonding interaction has been chosen. The solubility of the carboxylic acid (octanoic acid) in water and its salt (sodium octanoate) are shown in Figure 2A^a a The reference of the figures is in accordance with the sequence of the numbering of this work. . We observed that octanoic acid is almost insoluble in water (0.68 g L^-1 of water), and the water solubility of sodium octanoate is 70 times greater. Figure 2A^a a The reference of the figures is in accordance with the sequence of the numbering of this work. shows the electrostatic force equations related to the two hydrogen bonds indicated in the text above (one between octanoic acid and water, in Equation 8^b b The numbering of the equations in each question is in accordance with the sequence of the numbering of this work. , and the other between sodium octanoate and water, in Equation 9^b b The numbering of the equations in each question is in accordance with the sequence of the numbering of this work. ) in a system of two charges: one positive (the partial positive atomic charge of the hydrogen atom in water, δ⁺) and the other negative (the partial negative atomic charge δ^-, of the neutral oxygen atom in the acid, and the formal charge, q^-, of the negatively charged oxygen atom of the salt) separated by a distance r. In these electrostatic force equations, K is a constant. Assume r of equal value for both systems. We are making the following association: the greater the electrostatic strength of the hydrogen bond, the greater the solubility. It is important to note that: the formal negative charge (q-) of oxygen is greater, in modulus, than the partial atomic charge (δ) of the neutral oxygen atom. Remember that when we say “in module”, we remove the negative sign from a variable or number. Mark the correct alternative to explain the difference in solubility based on the equations of the electrostatic force of hydrogen bonding between octanoic acid (octanoic acid) and water (water) (Equation 8) and between sodium octanoate (sodium octanoate) and water (Equation 9).
(4)	Boiling point according to electrostatic force Read the following text: The boiling point depends on the intermolecular interactions of the pure substance. Here, we make the following association: the greater the electrostatic strength of the hydrogen bond, the greater the boiling point. Figure 2B^a a The reference of the figures is in accordance with the sequence of the numbering of this work. shows the electrostatic force equation applied to a system of two atomic charges, one positive and the other negative, q⁺ and q^-, separated by a distance r (Equation 10^b b The numbering of the equations in each question is in accordance with the sequence of the numbering of this work. ). In this electrostatic force equation, K is a constant. This equation can be rewritten for the hydrogen bonding interaction between two ethanol molecules (Equation 11^b b The numbering of the equations in each question is in accordance with the sequence of the numbering of this work. ) and between two ethanethiol molecules (Equation 12^b b The numbering of the equations in each question is in accordance with the sequence of the numbering of this work. ), where δ⁺ and δ^- are the positive and negative partial atomic charges, respectively, with r being the distance between the H and O (or S) atom. Below the equations, it is represented (by dashed lines) the hydrogen bond between two molecules of ethanol and two molecules of ethanethiol. Note that the electronegativity of oxygen, χ_(o), is greater than that of sulfur, χ_(s), and that the boiling point, Bp, of ethanol is greater than that of ethanethiol. For each pair of statements below, one is right, and one is wrong. Indicate which is right and which is wrong.
(5)	Double bond strength Read the following text: It is known that each term of the electrostatic force equation is applied to only two charged particles. For example, in a single C-C bond, composed of a sigma (σ) bond, there are 4 terms of the electrostatic force equation associated with that bond. Each electron in the σ bond interacts with each carbon nucleus in isolation. Being 2 nuclei and 2 electrons, it leads to 4 terms of the equation. When analyzing a double bond, either C=C or C=O, 8 terms of the electrostatic force equation appear for each of these bonds: 4 terms for the sigma bond (σ) and 4 terms for the pi bond (π). In Equations 13^b b The numbering of the equations in each question is in accordance with the sequence of the numbering of this work. and 14^b b The numbering of the equations in each question is in accordance with the sequence of the numbering of this work. , in each of its terms, there are two charges in the numerator represented by e (the electron charge) and Z_eff (effective atomic number of an atom). In the denominator of each term, there is the squared distance between the average position of the electron and the atomic nucleus, r²2 Hernani; Ulum, L. L.; Mudzakir, A.; J. Phys.: Conf. Ser. 2020, 1521, 042087. [Crossref] Crossref... (see Figure 2C^a a The reference of the figures is in accordance with the sequence of the numbering of this work. ). To simplify the reasoning, we consider that the r²2 Hernani; Ulum, L. L.; Mudzakir, A.; J. Phys.: Conf. Ser. 2020, 1521, 042087. [Crossref] Crossref... remains constant for the two bonds C=O and C=C. Here is what is asked: Establish the electrostatic force factor explaining why the carbonyl (C=O) bond is stronger than in the vinyl group (C=C).
(6)	Resonance theory and stability Read the following text: The resonance theory makes it possible to compare the stability of conjugated systems. The amount of resonance structures indicates the most delocalized conjugate system and, consequently, the most stable system. The best-known method for obtaining the stability of a molecular system is the heat of combustion reaction. The parameterized heat of combustion, ∆Hp_(comb), indicates the following order of stability: octatetrayne > hexatriyne > butadiyne > ethyne, which is directly related to the number of their resonance structures: 4 (octatetrayne), 3 (hexatriyne), 2 (butadiyne) and 1 from ethyne, representing a localized system (see Figure 2D^a a The reference of the figures is in accordance with the sequence of the numbering of this work. ). The resonance hybrid and the resonance structure are considered equivalent when there is only one structure. This occurs in localized systems like ethyne. In Figure 2D^a a The reference of the figures is in accordance with the sequence of the numbering of this work. , the ethyne molecule (1) and the resonance structures, ψ, of butadiyne (2), hexatriyne (3), and octatetrayne (4) are shown. The stability trend octatetrayne > hexatriyne > butadiyne > ethyne is mainly associated with the π bond system and can be rationalized by the π bond strength in the conjugated oligoynes and the inverse relationship between strength and energy. As all carbon atoms have the same geometry (digonal or linear), the distance between the carbon nucleus and the average position of the π electron, r, is almost the same for all the analyzed oligoynes. The same reasoning is used to claim that all Z_eff (C), the effective atomic number of the carbon atom, in all oligoynes are the same. For ethyne, butadiyne, hexatriyne, and octatetrayne, there is 2c-2e bonding (two centers-two π electrons), 4c-4e bonding (four centers-four π electrons), 6c-6e bonding, and 8c-8e bonding system, respectively, where “c” is center and “e” is π electron. Understand “center” as a carbon atom. Since conjugated systems are fully delocalized, each resonating π electron interacts with each digonal (or linear) carbon atom. So, for ethyne, there is an electrostatic interaction between π electron and a digonal carbon multiplied by 4 (2 × 2), i.e. 2 π electrons multiplied by 2 carbons (Equation 15^b b The numbering of the equations in each question is in accordance with the sequence of the numbering of this work. ). Note that in the last two paragraphs above, we considered only the π electrons that move along the resonance structures. For example, in butadiyne, there are two π bonds for each triple bond, but we are only considering one of the π bonds of each triple bond to refer to the 4c-4e bond since only one π bond of each triple bond moves in the resonance structures. Give the values that multiply the electrostatic force equations of the π system for butadiyne, hexatriyne and octatetrayne, Equations 16-18^b b The numbering of the equations in each question is in accordance with the sequence of the numbering of this work. , respectively, based on what was shown for ethyne in the paragraph above.

\begin{aligned} F_{α (A)} = F_{α (A) e} + F_{α (A) n} = \\ F_{α (A)} = - Z_{eff(A)} \int \frac{ρ (r) (R_{A} - r)}{{| R_{A} - r |}^{3}} d r + Z_{eff (A)} \sum_{B \neq A} \frac{Z_{eff(B)} (R_{A} - R_{B})}{{| R_{A} - R_{B} |}^{3}} \end{aligned}

[1] *e-mail: caio.firme@ufrn.br firme.caio@gmail.com