Historical Context & Motivation
For centuries, chemists knew that substances reacted with one another, but they had no way to explain why certain atoms bonded together while others did not. The breakthrough came when scientists realized that the electrons surrounding an atom's nucleus hold the key to chemical reactivity. In particular, the concept of electron-pair sharing — the idea that atoms can form bonds by sharing pairs of electrons — transformed our understanding of how molecules form and how reactions proceed. This section traces the milestones that led to our modern picture of electron-pair sharing reactions.
The central question that this topic addresses is: How can we use the movement of electron pairs to predict products, explain mechanisms, and solve IB Chemistry problems? By mastering curly arrow notation and the nucleophile–electrophile framework, you gain a powerful toolkit for understanding nearly every organic and many inorganic reactions.
Core Principles of Electron-Pair Sharing Reactions
Electron-pair sharing reactions revolve around a simple idea: a species with an available electron pair (a nucleophile) donates that pair to a species that is electron-deficient (an electrophile). This electron-pair donation creates a new covalent bond. The reverse process — bond breaking where one species keeps the electron pair — is called heterolytic fission. Together, these ideas allow us to trace the flow of electrons through any reaction mechanism.
Nucleophile
Electrophile
Curly Arrow Notation
Heterolytic Bond Fission
Lewis Acid–Base Framework
Visualizing Electron-Pair Movement
The diagram below illustrates the fundamental mechanism of an electron-pair sharing reaction. A hydroxide ion (OH⁻) acts as a nucleophile, donating a lone pair to a carbon atom that bears a partial positive charge (the electrophile). The curly arrow shows the direction of electron flow — from the nucleophile to the electrophilic carbon. This is the core idea behind nucleophilic substitution, one of the most important reaction types you will encounter in IB Chemistry.
Notice two critical features in this diagram. First, every curly arrow begins at a site of electron density — either a lone pair or a bond — and ends at an atom that can accept those electrons. Second, the number of arrows tells you how many bond-making and bond-breaking events occur in a single mechanistic step. In IB Chemistry, you must be able to draw these arrows accurately to earn full marks on mechanism questions.
Mechanistic Framework: How Electron Pairs Drive Reactions
While electron-pair sharing reactions are not typically described by mathematical equations in the way kinetics or thermodynamics problems are, there is a logical framework you must follow. Every mechanism question in IB Chemistry requires you to (1) identify the nucleophile, (2) identify the electrophile, (3) draw curly arrows showing electron-pair movement, and (4) show the products with correct charges and bonds.
Formal Charge Conservation
Bond Order Change During Electron-Pair Sharing
Key Mechanistic Steps in IB Chemistry
- Nucleophilic substitution (SN2): The nucleophile attacks the electrophilic carbon in a single concerted step while the leaving group departs. Two curly arrows are drawn.
- Nucleophilic addition: A nucleophile attacks an electron-poor carbon in a C=O group, breaking the π bond. Commonly seen in reactions of aldehydes and ketones.
- Electrophilic addition: An electrophile attacks the electron-rich C=C double bond of an alkene. The π electrons act as the nucleophilic source.
- Lewis acid–base coordination: A lone pair on a Lewis base is donated to an empty or partially empty orbital on a Lewis acid, forming a dative (coordinate) bond. Only one curly arrow is needed.
Classifying Electron-Pair Sharing Reactions
The IB syllabus groups electron-pair sharing reactions into several categories. The diagram below maps these reaction types visually, showing how the nucleophile and electrophile interact in each case. Understanding these categories will help you quickly identify which mechanism applies in a given problem.
| Reaction Type | Nucleophile | Electrophile | # Curly Arrows |
|---|---|---|---|
| SN2 substitution | OH⁻, CN⁻, NH₃ | C bonded to leaving group (e.g. C–Br) | 2 |
| Nucleophilic addition | HCN, NaBH₄ | Carbonyl carbon (C=O) | 2 |
| Electrophilic addition | C=C π electrons | H⁺ (from HBr, HCl) | 2+ |
| Lewis acid–base coordination | NH₃, H₂O, Cl⁻ | BF₃, AlCl₃, metal ions | 1 |
Worked Example: Drawing a Mechanism
Let's walk through a typical IB exam-style problem: drawing the mechanism for the reaction of bromoethane (CH₃CH₂Br) with aqueous sodium hydroxide (NaOH) to produce ethanol (CH₃CH₂OH) and sodium bromide (NaBr). This is an SN2 nucleophilic substitution reaction.
Comparing Electron-Pair Sharing Reaction Types
One of the most important skills in IB Chemistry is distinguishing between different types of electron-pair sharing reactions. The table below compares three major categories, highlighting what makes each one unique. Understanding these differences helps you select the correct mechanism when you encounter an unfamiliar reaction in an exam.
| Feature | Nucleophilic Substitution | Electrophilic Addition | Lewis Acid–Base |
|---|---|---|---|
| Substrate | Haloalkane (C–X) | Alkene (C=C) | Any Lewis acid |
| Who attacks? | Nucleophile (e⁻ pair donor) | Electrophile (e⁻ pair acceptor) | Lewis base (e⁻ pair donor) |
| Bond changes | One bond forms, one bond breaks | π bond breaks, two σ bonds form | One dative bond forms |
| Products | Substituted product + leaving group ion | Saturated addition product | Adduct (coordinate compound) |
| IB Example | OH⁻ + CH₃Br → CH₃OH + Br⁻ | HBr + C₂H₄ → C₂H₅Br | NH₃ + BF₃ → H₃N–BF₃ |
Connection to Advanced Theory
The electron-pair sharing framework you learn in IB Chemistry is the foundation for much more complex ideas in university-level organic chemistry and biochemistry. At higher levels, mechanisms involve multiple steps, intermediate species (like carbocations or carbanions), and competing pathways. However, the fundamental principle remains the same: electrons flow from regions of high density to regions of low density, and curly arrows track that flow.
| IB Level (Reactivity 3.4) | University Level (Organic Chemistry) |
|---|---|
| SN2: single concerted step | SN1 vs SN2 competition, stereochemistry (inversion vs retention) |
| Electrophilic addition to alkenes (two-step) | Markovnikov vs anti-Markovnikov selectivity, rearrangements |
| Lewis acid–base with BF₃ or metal ions | Transition metal catalysis, organometallic mechanisms |
| Nucleophilic addition to C=O (one step) | Nucleophilic acyl substitution, condensation reactions, enolate chemistry |
If you pursue chemistry at university, you will also study how orbital interactions (HOMO–LUMO theory) provide a quantum mechanical explanation for why nucleophiles donate and electrophiles accept electron pairs. For now, focus on confidently identifying nucleophiles, electrophiles, and drawing accurate curly arrows — these skills will serve you well for years to come.
Practice Problems
Summary: Electron-Pair Sharing Reactions
Electron-pair sharing reactions are driven by the interaction between nucleophiles (electron-pair donors / Lewis bases) and electrophiles (electron-pair acceptors / Lewis acids). Curly arrows trace the movement of electron pairs from source to destination: the tail begins at the lone pair or bond providing the electrons, and the head points to the atom or bond accepting them. This notation allows you to draw mechanisms for nucleophilic substitution (SN2), nucleophilic addition, electrophilic addition, and Lewis acid–base coordination reactions.
When solving IB mechanism problems, follow these steps: (1) identify the nucleophile and electrophile, (2) draw curly arrows from the electron-pair source to the acceptor, (3) show any heterolytic bond fission with an arrow from the bond to the departing atom, and (4) verify that charge is conserved across reactants and products. Mastering this framework gives you a universal language for explaining how and why chemical reactions happen.