R3.4
Electron-pair sharing reactions
4 SL · +7 HL (SL 3.4.1-3.4.5) (HL 3.4.6-3.4.13)
Unverified
← View on the mapParent topic: What are the mechanisms of chemical change?
Guiding question
Guiding questionWhat happens when reactants share their electron pairs with others?
The one big idea
Nucleophiles donate electron pairs, electrophiles accept them. Heterolytic fission underlies nucleophilic substitution and electrophilic addition. At HL, Lewis acid–base theory, SN1 vs SN2 mechanisms, Markovnikov addition and electrophilic substitution of benzene deepen the model.
What this rests on
Our editorial judgement.The dependencies in this section are this site’s own assessment of what a sub-topic rests on. They have notbeen verified by a chemistry teacher and are not part of the official IB guide. By contrast, section 8 (Linking questions) reproduces the IB’s own wording verbatim.
- EditorialS2.2 The covalent modelHL onlyLewis acid-base reactions form coordination bonds (S2.2.3); benzene's resonance stabilisation drives electrophilic substitution (HL S2.2.11-12, R3.4.13). Reciprocal linking questions.
- EditorialS3.1 The periodic table: Classification of elementsHL onlyTransition-element properties (HL S3.1.8-10) are needed for complex-ion/ligand chemistry (HL R3.4.8); group 17 trends explain leaving-group ability (HL R3.4.10).
- EditorialS3.2 Functional groups: Classification of organic compoundsHalogenoalkane substitution cannot be discussed without the functional-group classification.
- EditorialR1.2 Energy cycles in reactionsHL onlyBond enthalpy (R1.2.1) determines C-X leaving-group ability (HL R3.4.10; explicit linking question).
- EditorialR2.2 How fast? The rate of chemical changeHL onlyRate equations, order and intermediates vs transition states (HL R2.2) distinguish SN1 from SN2 (HL R3.4.9); one of the most important HL edges in the course.
- EditorialR3.3 Electron sharing reactionsThe guide contrasts homolytic (radical) with heterolytic fission explicitly.
What rests on this
No dependents recorded.
Core concepts that must be mastered
- Nucleophiles and electrophiles(3.4.1)A nucleophile donates both bonding electrons to its reaction partner — the electrophile. You must recognise nucleophiles (include neutral species like H₂O, NH₃ and negatively charged species like OH⁻, CN⁻, Cl⁻) and electrophiles (include neutral species like BF₃ and positively charged species like NO₂⁺, H⁺). A nucleophile donates an electron pair; an electrophile accepts one. (The formal Lewis acid–base framing of these roles is developed at HL below, in 3.4.6.) A nucleophile attacks a δ+ carbon or a carbocation — it does not attack the nucleus.
- Nucleophilic substitution and heterolytic fission(3.4.2, 3.4.3)In nucleophilic substitution, a nucleophile donates an electron pair to form a new bond as another bond breaks, producing a leaving group. Heterolytic fission — both bonding electrons remain with one fragment — is the bond-breaking mode here, contrasting with the homolytic fission of radical reactions (R3.3). You must use curly arrows for electron-pair movement: the tail sits on a lone pair or a bond, the head points to where the electron pair moves. The curly arrow shows the movement of electron pairs, not atoms. Further mechanistic detail is not required at SL.
- Electrophilic addition(3.4.4, 3.4.5)An electrophile accepts both bonding electrons from its partner. Alkenes are susceptible to electrophilic attack because of the high electron density of the C=C bond — the π bond is above and below the plane and is a good electron source. This leads to electrophilic addition: you must deduce equations for reactions of alkenes with water, halogens and hydrogen halides. Examples: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br; CH₂=CH₂ + H₂O → CH₃CH₂OH. Mechanisms are not assessed at SL. Bromine water is decolourised by alkenes but not by alkanes — this is the standard test for unsaturation.
- HL: Lewis acid–base theory and complex ions(3.4.6–3.4.8)A Lewis acid is an electron-pair acceptor; a Lewis base is an electron-pair donor. This generalises the Brønsted–Lowry definition (R3.1): every Brønsted base is a Lewis base, but a Lewis base need not accept a proton. Nucleophiles are Lewis bases and electrophiles are Lewis acids. A Lewis base + Lewis acid → a coordination bond — a covalent bond where both shared electrons come from one species (defined in S2.2.3). Coordination bonds form when ligands donate an electron pair to transition element cations, forming complex ions: [Cu(H₂O)₆]²⁺, [Fe(CN)₆]³⁻. You must deduce the charge on a complex ion given the formula and the ligands present.
- HL: SN1 and SN2 mechanisms(3.4.9, 3.4.10)Nucleophilic substitution includes reactions of halogenoalkanes with nucleophiles. You must describe and explain the mechanisms for primary and tertiary halogenoalkanes. SN2 (primary): a concerted one-step mechanism. The nucleophile attacks the δ+ carbon from the side opposite the leaving group, the C–X bond breaks as the C–Nu bond forms, and the leaving group departs — all in one step. The SN2 mechanism is stereospecific: the configuration at the carbon inverts (Walden inversion) because the nucleophile must attack from the back. SN1 (tertiary): a two-step mechanism. The leaving group departs first, forming a carbocation intermediate; the nucleophile then attacks the planar carbocation from either side. The intermediate carbocation is flat, so the product is a racemic mixture if the carbon was chiral. The tertiary carbocation is stabilised by the three alkyl groups (inductive effect), which is why SN1 is favoured for tertiary halogenoalkanes. Secondary halogenoalkanes can undergo both SN1 and SN2. The rate equation for SN1 is rate = k[halogenoalkane] (first order); for SN2 it is rate = k[halogenoalkane][nucleophile] (second order) — this connects to the kinetics in R2.2. Substitution rate is also influenced by the identity of the leaving group: a better leaving group is a weaker base (I⁻ is better than Cl⁻ because the C–I bond is weaker and I⁻ is more stable). Roles of solvent and mechanism on rate are not assessed.
- HL: Electrophilic addition and substitution mechanisms(3.4.11–3.4.13)Alkenes readily undergo electrophilic addition. You must describe and explain the mechanisms of reactions between symmetrical alkenes and halogens, water and hydrogen halides. For bromine: the π electrons attack Br₂, forming a cyclic bromonium ion intermediate, then Br⁻ attacks from the opposite side (anti-addition). For unsymmetrical alkenes, relative carbocation stability determines the major product — Markovnikov behaviour. The electrophile (H⁺) adds to give the more stable carbocation: tertiary > secondary > primary. H adds to the carbon with more hydrogens. The explanation is carbocation stability, not a rule to memorise. Electrophilic substitution includes reactions of benzene with electrophiles. Benzene does not undergo addition like an alkene — addition would destroy the delocalised π system and its resonance stabilisation. Instead it undergoes substitution, preserving the aromatic ring. You must describe and explain the mechanism of the reaction between benzene and a charged electrophile E⁺: the π electrons attack E⁺, forming a carbocation intermediate (the ring temporarily loses aromaticity), then a proton is lost to restore the delocalised system. Formation of the electrophile will not be assessed — for nitration, NO₂⁺ is generated from concentrated HNO₃ and H₂SO₄, but you will not be asked how. The features of benzene (six identical C–C bonds, delocalised electrons, resonance stabilisation from S2.2.12) explain why it resists addition despite being highly unsaturated.
- How this sub-topic connectsElectron-pair sharing is the third and most mechanistically rich of the three mechanism sub-topics. Lewis acid–base theory generalises Brønsted–Lowry (R3.1) — every Brønsted base is a Lewis base. The SN1 and SN2 rate equations connect to kinetics (R2.2: first vs second order, rate-determining step, intermediates vs transition states). Benzene's resistance to addition comes from the resonance model (S2.2: delocalisation, bond length evidence). Markovnikov addition and carbocation stability link to the covalent model (S2.2: hyperconjugation and inductive effects). Coordination bonds and complex ions connect to transition element chemistry (S3.1 HL: ligands, d-orbital splitting, colour). The contrast between homolytic fission (R3.3: radicals) and heterolytic fission (here: ions) is the fundamental fork in reaction mechanisms.
Quantitative non-negotiables
- Recognise nucleophiles and electrophiles, including neutral and charged species.
- Deduce equations for nucleophilic substitution with electron-pair movement described.
- Deduce equations for electrophilic addition of alkenes with water, halogens and hydrogen halides.
- Explain heterolytic fission with curly arrows for electron-pair movement.
- At HL: apply Lewis acid–base theory to identify the role of each reacting species.HL
- At HL: deduce the charge on a complex ion given the formula and ligands present.HL
- At HL: describe and explain SN1 (tertiary) and SN2 (primary) mechanisms, including the stereospecificity of SN2.HL
- At HL: predict the major product of unsymmetrical alkene addition using carbocation stability (Markovnikov).HL
- At HL: describe and explain the mechanism of electrophilic substitution of benzene.HL
Common failure modes
M-69 — Curly arrows show where atoms move.
Confidence: verified
Why it’s wrong: Curly arrows show the movement of electron pairs (double-headed) or single electrons (fishhook, single-headed).
Correction: The tail must sit on a lone pair or a bond, never on an atom or a positive charge.
M-70 — A nucleophile is attracted to the positive charge, so it attacks the nucleus.
Confidence: verified
Why it’s wrong: A nucleophile donates an electron pair to an electron-poor centre.
Correction: It attacks a δ+ carbon or a carbocation. It is a Lewis base.
M-73 — Benzene reacts by addition like an alkene.
Confidence: verified
Why it’s wrong: Benzene undergoes electrophilic substitution, preserving the delocalised π system.
Correction: Addition would destroy the resonance stabilisation.
M-74 — In HBr addition to propene, the H goes on whichever carbon.
Confidence: verified
Why it’s wrong: Markovnikov — the electrophile adds to give the more stable carbocation (tertiary > secondary > primary).
Correction: H adds to the carbon with more hydrogens. The explanation is carbocation stability, not a rule to memorise.
What "HL standard" actually looks like
HLHL adds Lewis acid–base theory and coordination bonds, complex ion charge, the concerted SN2 vs two-step SN1 mechanisms (with stereospecificity), leaving-group effects on rate, full electrophilic addition mechanisms (including Markovnikov via carbocation stability), and electrophilic substitution of benzene. +7 HL hours — the heaviest HL block in Reactivity alongside R3.1.
Linking questions
Official IB.The questions in this section are the IB’s own linking questions, reproduced verbatim from the guide. They are not this site’s editorial judgement — see section 3 for that distinction.
- Official IB
How can isotope tracers provide evidence for a reaction mechanism?
- Official IB
Why do Lewis acid–base reactions lead to the formation of coordination bonds?
- Official IB
What are the structural features of benzene that favour it undergoing electrophilic substitution reactions?
- Official IB
What is the nature of the reaction between transition element ions and ligands in forming complex ions?
- Official IB
How does the strength of a carbon–halogen bond affect the rate of a nucleophilic substitution reaction?
- Official IB
Which mechanism in the hydrolysis of halogenoalkanes involves an intermediate?
- Official IB
What are the rate equations and units of k for the reactions of primary and tertiary halogenoalkanes with aqueous alkali?
- Official IB
Why has the definition of acid evolved over time?
- Official IB
Why are some reactions of alkenes classified as reduction reactions while others are classified as electrophilic addition reactions?
- Official IB
What is the difference between the bond-breaking that forms a radical and the bond-breaking that occurs in nucleophilic substitution reactions?
- Official IB
Why is bromine water decolourized in the dark by alkenes but not by alkanes?
- Official IB
Why are alkenes sometimes known as "starting molecules" in industry?
- Official IB
What is the relationship between Brønsted–Lowry acids and bases and Lewis acids and bases?
- Official IB
Do coordination bonds have any different properties from other covalent bonds?
- Official IB
What differences would be expected between the energy profiles for SN1 and SN2 reactions?
- Official IB
What are the rate equations for these SN1 and SN2 reactions?
- Official IB
How useful are mechanistic models such as SN1 and SN2?
- Official IB
Why is the iodide ion a better leaving group than the chloride ion?
- Official IB
What are the features of benzene, C₆H₆, that make it not prone to undergo addition reactions, despite being highly unsaturated?
- Official IB
Nitration of benzene uses a mixture of concentrated nitric and sulfuric acids to generate a strong electrophile, NO₂⁺. How can the acid/base behaviour of HNO₃ in this mixture be described?
Bridge: GCSE → IB HL
Derived. These are the bridge items tagged to this sub-topic— places where the GCSE model gets redefined, genuinely new territory, or carry-over strengths. The tagging is this site’s editorial judgement.
- K-11 — Basic organic families and the idea of a homologous seriesCarry-over
- N-03 — Reaction mechanisms and curly arrows — a causal, temporal story about invisible entitiesNew
- R-07 — Acids are redefined three times overRedefined
- R-16 — Organic chemistry stops being descriptive and becomes mechanisticRedefined