The role of electronic structure in chemical reactions
The essence of any chemical transformations lies in changes to the state of electrons. It is the unique electronic configuration of crystal structures and molecules that determines parameters such as atom valency, electrical charges, as well as the strength and spatial orientation of the resulting bonds.
Based on the nature of the ongoing electronic processes, all chemical reactions are generally divided into two major classes:
- Oxidation-reduction (redox) reactions. These are based on the transfer of a varying number of electrons from a reducing agent to an oxidizing agent. The main result of such a transfer is a change in the formal integer charges of atoms (oxidation states).
- Acid-base reactions. Their mechanism is fundamentally different: whole electron pairs are transferred between interacting atoms, while the oxidation states of the elements remain unchanged. Understanding this mechanism is critically important because acid-base characteristics are typical of a vast range of compounds.
Four key theories
Historically, four approaches to defining acids and bases have emerged. They are classified depending on the reaction conditions (e.g., the presence of a solvent).
- Arrhenius theory (1887). Works under any conditions.
- Acid: a molecular substance containing hydrogen atoms capable of being replaced by metals or entering the structure of complex cations (e.g., ammonium).
- Base: a hydroxide-containing substance that reacts with an acid in an aqueous environment.
- Essence of interaction: the formation of a salt and water (classic equation: $H^+ + OH^- = H_2O$).
- Cady and Elsey solvent system theory (1928). Applicable to protic solvents.
- Based on changes in the acidity index of the medium.
- Acid: a particle that lowers the pH of a solution (creates an acidic medium).
- Base: a particle that raises the pH of a solution (creates an alkaline medium).
- Brønsted–Lowry protolytic theory (1923). Also used for protic solvents.
- Describes reactions as a proton transfer process.
- Acid: acts as a proton donor.
- Base: acts as a proton acceptor.
- The interaction leads to the formation of conjugate acid-base pairs.
- Lewis electronic theory (1923). Applicable under any conditions.
- Acid: a particle that accepts an electron pair (acceptor).
- Base: a particle that donates an electron pair (donor).
- As a result, a coordinate covalent (dative) bond is formed.
Universality of Lewis theory and complexation
Among all concepts, the Lewis theory is the most comprehensive. It describes the standard mechanism of elementary chemical transformations not only in liquids, but also in gases and solids. If we analyze reactions through the lens of electron pair donors and acceptors, it becomes clear that all other theories are completely non-contradictory and easily reduce to the Lewis concept.
A striking example is the reaction of sulfur trioxide ($SO_3$) with water. In this process, the oxide acts as a typical Lewis acid, and water as a base, resulting in the synthesis of sulfuric acid.
Complexation also fits perfectly into the framework of acid-base interactions. Consider the reaction of copper ions ($Cu^{2+}$) with ammonia ($NH_3$). In simplified terms, this is a combination reaction where the copper ion is the acid and ammonia is the base: $Cu^{2+} + 4NH_3 = [Cu(NH_3)_4]^{2+}$
However, in reality (in aqueous solution), copper ions are initially hydrated — bound to water molecules. Therefore, the process is a more complex substitution reaction replacing a weaker base (water) with a stronger one (ammonia): $[Cu(H_2O)_4]^{2+} + 4NH_3 = [Cu(NH_3)_4]^{2+} + 4H_2O$
Briefly: answers to topical questions
What factors affect the strength of acids and bases in aqueous and non-aqueous solutions?
The strength of acids and bases is influenced by molecular structural features and electrostatic interactions.
- Influence of the aromatic ring — reduces basic properties (e.g., pyridine is weaker than aliphatic amines).
- Conjugation effect — if the nitrogen electron pair is engaged in conjugation (as in pyrrole), basic properties are not exhibited.
- Electrostatic attraction — makes it difficult for a proton to dissociate from a negatively charged anion compared to a neutral molecule during stepwise dissociation.
- Strength of the conjugate pair — the stronger the acid, the weaker its conjugate base, and vice versa.
What are examples of amphoteric compounds from the standpoint of protolytic theory?
From the perspective of protolytic theory, amphoteric compounds (ampholytes) are particles capable of both donating and accepting a proton. Such substances include:
- Water (H2O) — a bifunctional compound exhibiting the properties of both an acid and a base.
- Ammonia (NH3) — capable of acting as an ampholyte in proton transfer reactions.
- Hydroanions — negatively charged hydrogen-containing ions.
Amphotericity is also characteristic of amino acids, proteins, oxides, hydroxides, and salts (e.g., ammonium acetate).
How to calculate the pH and pOH of buffer solutions based on acid-base equilibria?
The calculation of hydrogen and hydroxyl indices for buffer systems is performed using the Henderson–Hasselbalch equations.
- Acidic buffers — calculated using the formula: pH = pKₐ + lg([salt]/[acid]). The use of amounts of substance or products of volumes and concentrations is permitted.
- Basic buffers — first, the hydroxyl index is calculated: pOH = pKᵦ + lg([salt]/[base]). Then, the transition to the hydrogen index is made using the formula: pH = 14 − pOH.
When calculating systems with polyprotic acids, only the first dissociation step is taken into account, as subsequent ones are significantly weaker.
Lewis works with ELECTRONS: Acid = Acceptor (takes a pair), Base = Donor (gives a pair). Brønsted works with PROTONS using the same donor/acceptor principle, but in reverse.
Class and exam questions
Do different acid-base theories contradict each other?
No, they do not contradict each other at all. Different theories simply describe the behavior of acids and bases (antagonist particles) under various conditions and from different perspectives.
What is the main difference between acid-base reactions and redox reactions?
In acid-base reactions, electron pairs are transferred while the formal charges (oxidation states) of atoms remain unchanged. In redox reactions, a different number of electrons is transferred, which always leads to a change in oxidation states.
How does Lewis theory explain complex formation?
According to Lewis, complexation is a reaction where the central metal ion (e.g., copper) is an acceptor acid, and the ligands (ammonia, water) act as electron pair donor bases.
What else to study on this topic
- Mechanism of coordinate covalent bond formation according to Lewis theory
- Conjugate pairs in the Brønsted–Lowry protolytic theory
- Influence of electronic structure on the spatial orientation of bonds
- Ion hydration processes in aqueous solutions
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