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IB Chemistry • Reactivity: What are the mechanisms of chemical change?

Apply Electron-Pair Sharing Reactions — Apply Reactivity 3.4—Electron-pair sharing reactions in problem-solving and explanations

Master how nucleophiles and electrophiles drive organic and inorganic reactions through electron-pair sharing mechanisms.

Explore the lesson 15 min read
IB Chemistry emblem with test tubes, a laboratory flask, and international globe
Inside this lesson 10 sections 5 questions 7 figures

SECTION 1

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.

1916 • Lewis Electron-Pair Theory

Gilbert N. Lewis proposed that atoms achieve stable configurations by sharing electron pairs, laying the groundwork for understanding covalent bonding and Lewis dot structures.

1923 • Lewis Acid–Base Theory

Lewis extended his model to define acids as electron-pair acceptors and bases as electron-pair donors, unifying many reaction types under a single framework.

1933 • Ingold's Nucleophile–Electrophile Concept

Christopher Ingold classified reagents as nucleophiles (electron-pair donors that seek positive centers) and electrophiles (electron-pair acceptors that seek negative centers), providing the language we still use today.

1950s • Curly Arrow Mechanisms

Organic chemists popularized the use of curly arrows (curved arrows) to show the movement of electron pairs during bond-making and bond-breaking steps, making reaction mechanisms visually intuitive.

2023 • IB Chemistry Curriculum Refresh

The IB Diploma Programme reorganized its chemistry syllabus around the Reactivity theme, placing electron-pair sharing reactions at the heart of Reactivity 3.4 to emphasize mechanistic thinking in problem-solving.

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.

SECTION 2

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.

1

Nucleophile

An electron-rich species that donates a lone pair or bonding pair to form a new bond. Examples include OH⁻, NH₃, and H₂O. A nucleophile is always a Lewis base.
2

Electrophile

An electron-poor species that accepts an electron pair to form a new bond. Examples include H⁺, carbocations (R⁺), and BF₃. An electrophile is always a Lewis acid.
3

Curly Arrow Notation

A double-headed curved arrow represents the movement of an electron pair. The arrow tail starts at the electron-pair source and the head points to where the pair moves. Single-headed (fishhook) arrows represent single electrons in radical mechanisms.
4

Heterolytic Bond Fission

When a covalent bond breaks and both electrons go to one atom, forming ions. The curly arrow points away from the bond to the atom that keeps the pair. This is the reverse of bond formation by electron-pair sharing.
5

Lewis Acid–Base Framework

Every electron-pair sharing reaction can be described as a Lewis acid–base interaction. The Lewis base donates the pair (nucleophile), and the Lewis acid accepts it (electrophile). This unifies coordination chemistry, organic reactions, and more.
✦

KEY TAKEAWAY

Think of a nucleophile as someone holding out a gift (an electron pair) and the electrophile as the person reaching out to receive it. The curly arrow is like an arrow on a delivery map — it shows exactly where the electron pair travels from donor to acceptor. If you can follow the arrows, you can predict new bonds, broken bonds, and the products of a reaction.

SECTION 3

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.

Nucleophilic Substitution: Curly Arrow MechanismOH⁻ attacks the electrophilic carbon, displacing Br⁻REACTANTSOH⁻Nucleophile(Lewis base)HHCδ+BrLeaving groupδ−electron pairdonationbond breaksPRODUCTSHHCHONew bond formedCH₃OHBr⁻Bromide ion(departed with e⁻ pair)+RULE:Curly arrows always start at the electron-pair source (nucleophile / lone pair / bond)and point toward the electron-pair acceptor (electrophile / atom gaining the pair).
The left side shows the reactants: OH⁻ (nucleophile, cyan) donates its lone pair via a curly arrow to the electrophilic carbon (pink, δ+). A second curly arrow shows the C–Br bond breaking heterolytically. The right side shows the products: methanol (CH₃OH) and bromide ion (Br⁻).

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.

SECTION 4

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

FORMAL CHARGE

FC=V−N−B/2FC = V − N − B/2

FC = formal charge, V = number of valence electrons in the free atom, N = number of non-bonding (lone pair) electrons, B = number of bonding electrons. The total formal charge of reactants must equal the total formal charge of products.

Bond Order Change During Electron-Pair Sharing

BOND ORDER CHANGE

ΔBonds=(bondsformed)−(bondsbroken)ΔBonds = (bonds formed) − (bonds broken)

Each curly arrow either forms one new bond (arrow points to an atom) or breaks one existing bond (arrow points away from a bond). In a balanced mechanism, the total change in formal charge is zero across all species.

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.
⚠

IB Exam Tip

When drawing mechanisms, always show lone pairs on the nucleophile. IB examiners look for the curly arrow originating from the lone pair, not from the atom symbol itself. Arrows that start at the wrong place will lose marks even if the products are correct.

SECTION 5

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.

Classification of Electron-Pair Sharing ReactionsReactivity 3.4 — IB ChemistryElectron-Pair SharingNucleophilic AttackElectrophilic AttackLewis Acid–BaseSN2 SubstitutionNu⁻ attacks C bearinga leaving group2 curly arrowsNucleophilic AdditionNu⁻ attacks C=Oπ bond breaks2 curly arrowsElectrophilic AdditionE⁺ attacks C=Cπ electrons donate2+ curly arrowsCoordinationLone pair → emptyorbital (dative bond)1 curly arrowExamples from IB ChemistryOH⁻ + CH₃Br → CH₃OH + Br⁻HBr + CH₂=CH₂ → CH₃CH₂BrNH₃ + BF₃ → H₃N–BF₃NaCN + CH₃CH₂Br → CH₃CH₂CN + NaBr
This classification tree shows three main branches of electron-pair sharing reactions. Nucleophilic attacks (cyan) include SN2 substitution and nucleophilic addition. Electrophilic attacks (pink) involve an electrophile targeting a π bond. Lewis acid–base coordination (amber) forms dative bonds with a single curly arrow.
Reaction TypeNucleophileElectrophile# Curly Arrows
SN2 substitutionOH⁻, CN⁻, NH₃C bonded to leaving group (e.g. C–Br)2
Nucleophilic additionHCN, NaBH₄Carbonyl carbon (C=O)2
Electrophilic additionC=C π electronsH⁺ (from HBr, HCl)2+
Lewis acid–base coordinationNH₃, H₂O, Cl⁻BF₃, AlCl₃, metal ions1

Summary of electron-pair sharing reaction types relevant to IB Chemistry

SECTION 6

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.

SN2 Mechanism: OH⁻ + CH₃CH₂Br → CH₃CH₂OH + Br⁻

  1. 1

    Step 1 — Identify the Nucleophile

    The hydroxide ion (OH⁻) is the nucleophile because it has a lone pair of electrons available to donate. It is electron-rich due to its negative charge. In your mechanism, draw the OH⁻ ion with its lone pairs clearly shown.
    Nucleophile: OH⁻ (lone pair donor)
  2. 2

    Step 2 — Identify the Electrophile

    The electrophilic centre is the carbon atom bonded to bromine in CH₃CH₂Br. Because bromine is more electronegative than carbon, the C–Br bond is polar, and the carbon carries a partial positive charge (δ+). This makes it attractive to the nucleophile.
    Electrophile: C(δ+) in C–Br
  3. 3

    Step 3 — Draw Curly Arrow 1 (Bond Formation)

    Draw a curly arrow starting from a lone pair on the oxygen of OH⁻ and pointing to the carbon atom bearing the bromine. This arrow represents the donation of the electron pair and the formation of the new C–O bond. Make sure the arrow tail starts at the lone pair, not at the O symbol.
    Arrow 1: lone pair on O → electrophilic C (new C–O bond)
  4. 4

    Step 4 — Draw Curly Arrow 2 (Bond Breaking)

    Draw a second curly arrow starting from the C–Br bond and pointing to the bromine atom. This represents heterolytic fission of the C–Br bond, where both bonding electrons move to bromine. Bromine departs as Br⁻. This arrow shows why this is a substitution — the OH replaces the Br.
    Arrow 2: C–Br bond → Br (Br departs as Br⁻)
  5. 5

    Step 5 — Show Products and Verify Charges

    The products are ethanol (CH₃CH₂OH) and bromide ion (Br⁻). Check that charge is conserved: you started with OH⁻ (charge −1) and CH₃CH₂Br (charge 0), giving a total charge of −1. The products are CH₃CH₂OH (charge 0) and Br⁻ (charge −1), totaling −1. Charges balance.
    Products: CH₃CH₂OH + Br⁻ (total charge = −1 ✓)
⚠

Common Mistake

Students often forget to show the second curly arrow (the leaving group departing). In an SN2 mechanism, both arrows occur simultaneously in a single step. If you only show one arrow, IB examiners will deduct marks because you haven't fully described the electron-pair redistribution.

SECTION 7

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.

FeatureNucleophilic SubstitutionElectrophilic AdditionLewis Acid–Base
SubstrateHaloalkane (C–X)Alkene (C=C)Any Lewis acid
Who attacks?Nucleophile (e⁻ pair donor)Electrophile (e⁻ pair acceptor)Lewis base (e⁻ pair donor)
Bond changesOne bond forms, one bond breaksπ bond breaks, two σ bonds formOne dative bond forms
ProductsSubstituted product + leaving group ionSaturated addition productAdduct (coordinate compound)
IB ExampleOH⁻ + CH₃Br → CH₃OH + Br⁻HBr + C₂H₄ → C₂H₅BrNH₃ + BF₃ → H₃N–BF₃

Comparison of three electron-pair sharing reaction categories tested in IB Chemistry

✦

KEY TAKEAWAY

Think of reaction classification like sorting mail. The "address" on each reaction (haloalkane? alkene? Lewis acid?) tells you which "mailbox" (mechanism) it belongs to. Once you correctly identify the substrate, the curly arrows practically draw themselves. Always start by identifying the functional group of the substrate — this determines the mechanism.

SECTION 8

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 stepSN1 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 ionsTransition metal catalysis, organometallic mechanisms
Nucleophilic addition to C=O (one step)Nucleophilic acyl substitution, condensation reactions, enolate chemistry

How IB-level electron-pair sharing concepts expand at the university level

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.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

In the reaction between ammonia (NH₃) and boron trifluoride (BF₃), identify which species acts as the nucleophile and which acts as the electrophile. Explain your reasoning using the terms 'electron-pair donor' and 'electron-pair acceptor'.
Show answer
Ammonia (NH₃) is the nucleophile because it has a lone pair on the nitrogen atom that it can donate — it is the electron-pair donor (Lewis base). Boron trifluoride (BF₃) is the electrophile because boron has an incomplete octet with only six electrons in its valence shell, making it an electron-pair acceptor (Lewis acid). The lone pair on nitrogen is donated into the empty p orbital on boron, forming a dative (coordinate) bond. This produces the adduct H₃N–BF₃.

Question 2: BASIC CALCULATION

The reaction OH⁻ + CH₃Cl → CH₃OH + Cl⁻ is an SN2 reaction. Calculate the formal charge on the oxygen atom in OH⁻ and in the product CH₃OH. Confirm that total charge is conserved.
Show answer
In OH⁻: Oxygen has 6 valence electrons. It has 3 lone pairs (6 non-bonding electrons) and is involved in 1 bond (2 bonding electrons). FC = 6 − 6 − 2/2 = 6 − 6 − 1 = −1. So the formal charge on O in OH⁻ is −1. In CH₃OH: Oxygen has 2 lone pairs (4 non-bonding electrons) and is involved in 2 bonds (4 bonding electrons). FC = 6 − 4 − 4/2 = 6 − 4 − 2 = 0. So the formal charge on O in CH₃OH is 0. Charge conservation: reactant side has −1 (from OH⁻) + 0 (from CH₃Cl) = −1. Product side has 0 (from CH₃OH) + −1 (from Cl⁻) = −1. Total charge is conserved.

Question 3: INTERMEDIATE

Draw the curly arrow mechanism for the nucleophilic substitution reaction between cyanide ion (CN⁻) and 1-bromopropane (CH₃CH₂CH₂Br). Describe each arrow you would draw, and state the products.
Show answer
Arrow 1: A curly arrow starts from the lone pair on the carbon atom of CN⁻ (the nucleophilic end) and points to the carbon bonded to bromine in CH₃CH₂CH₂Br. This represents the formation of a new C–C bond. Arrow 2: A second curly arrow starts from the C–Br bond and points to the bromine atom, representing the heterolytic breaking of the C–Br bond as both electrons go to Br. Products: CH₃CH₂CH₂CN (butanenitrile) and Br⁻. Note that CN⁻ attacks through carbon (it is an ambident nucleophile), but carbon is the preferred nucleophilic atom for SN2 reactions. Charge is conserved: −1 on the left, −1 on the right.

Question 4: APPLIED

Ethene (CH₂=CH₂) reacts with hydrogen bromide (HBr) in an electrophilic addition reaction. The first step involves the H⁺ end of the polar H–Br bond acting as the electrophile. Explain why the π electrons of the C=C double bond act as the nucleophile, and describe what happens in the first mechanistic step including the curly arrow(s) you would draw.
Show answer
The C=C double bond contains a π bond, which is a region of high electron density located above and below the plane of the two carbon atoms. This electron-rich region acts as the nucleophile — the π electrons are the electron-pair source. In the first step, a curly arrow is drawn from the C=C π bond to the H atom of H–Br, representing the π electrons forming a new C–H bond. Simultaneously, a second curly arrow is drawn from the H–Br bond to Br, showing the heterolytic fission of H–Br as both bonding electrons move to bromine, forming Br⁻. After this step, one carbon gains a hydrogen (now bonded to three groups) and the other carbon becomes a carbocation (C⁺) because it lost its share of the π electrons. This carbocation is the intermediate, and Br⁻ subsequently attacks it in the second step to form the final product, bromoethane (CH₃CH₂Br).

Question 5: CRITICAL THINKING

A student claims that water (H₂O) can act as both a nucleophile and an electrophile in different reactions. Evaluate this claim by providing one example where water acts as a nucleophile and one where it acts as an electrophile. For each example, explain which electron pair is involved and draw a description of the curly arrow(s).
Show answer
The student's claim is correct. Water as a nucleophile: In the hydrolysis of a haloalkane such as CH₃Br, H₂O attacks the electrophilic carbon by donating a lone pair from oxygen to the carbon. A curly arrow goes from the lone pair on O in H₂O to the C in CH₃Br, and a second arrow goes from the C–Br bond to Br. Products: CH₃OH₂⁺ + Br⁻ (the protonated alcohol later loses H⁺ to form CH₃OH). Water as an electrophile: In the reaction between a very strong base like the amide ion (NH₂⁻) and water, the NH₂⁻ donates its lone pair to one of the hydrogen atoms on water. A curly arrow goes from the lone pair on N in NH₂⁻ to the H in H₂O, and a second arrow goes from the O–H bond to O. Products: NH₃ + OH⁻. Here water is the electrophile because it accepts the electron pair at its hydrogen atom. This dual behavior illustrates the amphoteric nature of water and shows that nucleophile/electrophile roles depend on the reaction partner, not an inherent fixed property.

SUMMARY

Summary: Electron-Pair Sharing Reactions

Summary

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.

Varsity Tutors • IB Chemistry • Apply Electron-Pair Sharing Reactions