Electron-pair sharing reactions: notes and practice questions
- This topic covers nucleophilic substitution and electrophilic addition reactions, focusing on electron-pair sharing.
- A nucleophile is a reactant that forms a bond by donating both bonding electrons.
- An electrophile is a reactant that forms a bond by accepting both bonding electrons.
- In nucleophilic substitution, a nucleophile donates an electron pair to form a new bond, and another bond breaks, producing a leaving group.
- Heterolytic fission is the breakage of a covalent bond where both bonding electrons remain with one fragment, forming ions.
- Alkenes undergo electrophilic addition due to the high electron density of their carbon-carbon double bond.
- Deduce equations and represent electron movement using curly arrows for these reactions.
How it is examined
At HL this is the highest-value drawing subtopic in Paper 2. A full mechanism is typically [3] and every arrow is a marking point: correct start (from a lone pair or a bond, not from an atom), correct finish (to an atom or between atoms), partial charges where the question needs them, and the intermediate or transition state drawn. May 2025 HL Paper 2 TZ1 asked candidates to predict the product of ethene with bromine [1] then draw the mechanism showing the movement of electron pairs [3], and separately to name the mechanism of a halogenoalkane reaction [1], draw the transition state [1] and deduce the rate equation [1]. At SL the same chemistry appears only as products and equations. Generating an SL question that asks for a mechanism or a curly arrow diagram is out of syllabus.
Average bond enthalpies for the carbon-halogen comparison. Nothing else. Mechanism drawing, curly arrow conventions, carbocation stability order and Markovnikov reasoning are all recall.
- 3.4.1 A nucleophile is a reactant that forms a bond to its reaction partner (the electrophile) by donating both bonding electrons. Students recognize nucleophiles in chemical reactions.
- 3.4.2 In a nucleophilic substitution reaction, a nucleophile donates an electron pair to form a new bond, as another bond breaks producing a leaving group. Students deduce equations with descriptions and explanations of the movement of electron pairs in nucleophilic substitution reactions. The guide illustrates this with a general scheme: a nucleophile `Nu:⁻` attacks a substrate `R-X:` where a curly arrow runs from the nucleophile's lone pair to the carbon and a second curly arrow runs from the C-X bond to X, giving `R-Nu` and `:X:⁻`. Nu is the nucleophile, R is the electrophile, X is the leaving group.
- 3.4.3 Heterolytic fission is the breakage of a covalent bond when both bonding electrons remain with one of the two fragments formed. Students explain, with equations, the formation of ions by heterolytic fission.
- 3.4.4 An electrophile is a reactant that forms a bond to its reaction partner (the nucleophile) by accepting both bonding electrons from that reaction partner. Students recognize electrophiles in chemical reactions.
- Further details of the mechanisms are not required at SL (3.4.2). SL students meet the general nucleophilic substitution scheme and write equations; they are not asked to produce the mechanism.
- The mechanisms of these reactions will not be assessed at SL (3.4.5, electrophilic addition). Same rule.
- HL: different halogenoalkanes should include RCl, RBr, RI. The roles of the solvent and the reaction mechanism on the rate will not be assessed (3.4.10).
- HL: the formation of the electrophile will not be assessed (3.4.13). The benzene mechanism starts from a given E⁺.
- 3.4.6 A Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair donor. Students apply Lewis acid-base theory to inorganic and organic chemistry to identify the role of the reacting species.
- 3.4.7 When a Lewis base reacts with a Lewis acid, a coordination bond is formed. Nucleophiles are Lewis bases and electrophiles are Lewis acids. Students draw and interpret Lewis formulas of reactants and products to show coordination bond formation.
- 3.4.8 Coordination bonds are formed when ligands donate an electron pair to transition element cations, forming complex ions. Students deduce the charge on a complex ion, given the formula of the ion and ligands present.
- 3.4.9 Nucleophilic substitution reactions include the reactions between halogenoalkanes and nucleophiles. Students describe and explain the mechanisms of the reactions of primary and tertiary halogenoalkanes with nucleophiles.
Guiding questions
- What happens when reactants share their electron pairs with others?
Linking questions
- Reactivity 3.3 What is the difference between the bond-breaking that forms a radical and the bond-breaking that occurs in nucleophilic substitution reactions? Why is bromine water decolourized in the dark by alkenes but not by alkanes?
- Structure 2.4 Why are alkenes sometimes known as "starting molecules" in industry?
- Reactivity 3.1 (HL) What is the relationship between Brønsted-Lowry acids and bases and Lewis acids and bases? 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?
- Structure 2.2 (HL) Do coordination bonds have any different properties from other covalent bonds? What are the features of benzene, C₆H₆, that make it not prone to undergo addition reactions, despite being highly unsaturated?
- Reactivity 2.2 (HL) What differences would be expected between the energy profiles for SN1 and SN2 reactions? What are the rate equations for these SN1 and SN2 reactions?
- Structure 3.1 (HL) Why is the iodide ion a better leaving group than the chloride ion?
- Nature of science, Reactivity 2.2 (HL) How useful are mechanistic models such as SN1 and SN2?
Practice questions
7 questions · 4 easy · 3 mediumQuestion 1
EasyPaper 2 · calculator1 markWhat is the role of boron trifluoride, , in the following reaction?
A. It is a Lewis acid.
B. It is a Lewis base.
C. It is a Brønsted–Lowry acid.
D. It is a Brønsted–Lowry base.
Consider the definitions of Lewis acids and bases in terms of electron pair donation and acceptance. Which species in the reaction is accepting an electron pair?
Question 2
MediumPaper 2 · calculator7 marksA student investigates the rate of hydrolysis of two structural isomers of a chloroalkane with the formula : 1-chlorobutane and 2-chloro-2-methylpropane. The reaction is carried out by warming each chloroalkane with aqueous potassium hydroxide.
(a) For each isomer, state whether it is a primary, secondary or tertiary haloalkane.
(b) Predict, with a reason, which isomer will react faster with aqueous potassium hydroxide.
(c) Explain your prediction in part (b), referring to the mechanisms of nucleophilic substitution.
(d) State the IUPAC name of the organic product formed from the reaction of 2-chloro-2-methylpropane with aqueous potassium hydroxide.
Recall the definitions of primary, secondary, and tertiary haloalkanes. This is based on the number of carbon atoms directly bonded to the carbon atom that is bonded to the halogen.
Consider the stability of the carbocation intermediate that can be formed from each isomer. How does this relate to the reaction mechanism and rate?
Compare the mechanism, which is favoured by tertiary haloalkanes, with the mechanism. Key factors to discuss are the stability of intermediates and steric hindrance.
The reaction is a nucleophilic substitution where the chlorine atom is replaced by a hydroxyl group (). Name the resulting alcohol.
Question 3
EasyPaper 2 · calculator1 markWhat type of reaction occurs when iodoethane reacts with aqueous potassium cyanide?
A. Electrophilic addition
B. Nucleophilic substitution
C. Free radical substitution
D. Reduction
Consider the functional groups of the reactants. Iodoethane is a haloalkane and potassium cyanide provides a species with a lone pair of electrons and a negative charge. What role does this species play in the reaction?
Question 4
MediumPaper 2 · calculator1 markWhat is the role of iron(III) bromide, , in the bromination of benzene?
A. It is consumed in the reaction.
B. It acts as a Brønsted-Lowry acid.
C. It polarizes the bromine molecule.
D. It stabilizes the benzene ring.
Benzene requires a strong electrophile to react. How does help to turn the non-polar molecule into a suitable electrophile?
Question 5
EasyPaper 2 · calculator1 markEthanol can be synthesized by warming chloroethane with aqueous sodium hydroxide. Which term best describes this type of reaction?
A. Free-radical substitution
B. Electrophilic addition
C. Nucleophilic substitution
D. Reduction
Consider the nature of the attacking species (from sodium hydroxide) and what happens to the chloroethane molecule (is something added across a double bond, or is one group replaced by another?).
Question 6
MediumPaper 2 · calculator5 marksBoron trifluoride, , is a versatile industrial catalyst. It reacts with ammonia, , to form a stable adduct as shown in the equation below.
(a) Define the term Lewis acid.
(b) Explain, with reference to the electronic structures of the reactants, why this reaction is classified as a Lewis acid-base reaction. Identify the Lewis acid and the Lewis base.
(c) Explain why this reaction is not considered a Brønsted-Lowry acid-base reaction.
A Lewis acid is defined by its role in electron pair exchange during a chemical reaction.
Consider the valence electron configuration of the central atoms in both reactant molecules. Which one has a lone pair to donate, and which one has an incomplete octet?
Recall the definition of a Brønsted-Lowry acid and base. What particle must be transferred for a reaction to be classified this way?
Question 7
EasyPaper 1A · calculator1 markWhich species can act as a nucleophile?
A.
B.
C.
D.
A nucleophile is a species that donates a pair of electrons to form a new covalent bond. Look for a species that is electron-rich, for example, one with a lone pair of electrons or a negative charge.
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