Electron-pair sharing reactions: notes and practice questions
- This topic covers advanced electron-pair sharing reactions, including Lewis acid-base theory, coordination compounds, and detailed organic reaction mechanisms.
- Lewis acids are electron-pair acceptors; Lewis bases are electron-pair donors.
- Nucleophiles are Lewis bases, and electrophiles are Lewis acids, forming coordination bonds.
- Ligands donate electron pairs to transition element cations to form complex ions; deduce their charge.
- Nucleophilic substitution reactions (S1 and S2) for halogenoalkanes, including S2 stereospecificity, are covered.
- Electrophilic addition to alkenes involves mechanisms and carbocation stability for unsymmetrical alkenes.
- Benzene undergoes electrophilic substitution with charged electrophiles.
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⁺.
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 · 3 easy · 4 mediumQuestion 1
EasyPaper 2 · calculator1 markWhat is the oxidation state of cobalt in the coordination compound ?
A. +1
B. +2
C. +3
D. +4
First, identify the overall charge of the compound. Then, determine the charge of the counter-ion outside the square brackets. This will allow you to find the charge of the complex ion. Remember that ethylenediamine (en) is a neutral ligand.
Question 2
MediumPaper 2 · calculator1 markThe reaction mechanism involves the formation of a carbocation intermediate. Which of the following haloalkanes would react fastest with water via an mechanism?
A. 1-chloropropane
B. 2-chloropropane
C. 2-chloro-2-methylpropane
D. Chloroethane
The rate of an reaction depends on the stability of the carbocation intermediate formed in the rate-determining step. Consider the type of carbocation (primary, secondary, or tertiary) that would be formed from each haloalkane.
Question 3
EasyPaper 1A · calculator1 markWhat is the role of ammonia in the reaction of 2-bromobutane with excess ethanolic ammonia?
A. Electrophile and Lewis acid
B. Nucleophile and Lewis acid
C. Electrophile and Lewis base
D. Nucleophile and Lewis base
Consider the definitions of a nucleophile and a Lewis base. Does ammonia possess a lone pair of electrons that it can donate? Is it attracted to a region of positive charge (a nucleus)?
Question 4
MediumPaper 2 · calculator1 markPentene, , has two straight-chain isomers. How many of these isomers produce an optically active alcohol upon hydration with steam and an acid catalyst?
A. 0
B. 1
C. 2
D. It depends on the E/Z configuration of the starting isomer.
First, draw the structures of the two straight-chain isomers of pentene. Then, for each isomer, predict the product(s) of hydration (addition of ). Remember Markovnikov's rule. Finally, examine the structure of each alcohol product to see if it contains a chiral centre.
Question 5
EasyPaper 1A · calculator1 markWhat is the oxidation state of in ?
A.
B.
C.
D.
First, identify the overall charge of the complex ion by considering the counter-ion. Then, sum the known oxidation states of the ligands and set the total equal to the complex ion's charge to solve for the oxidation state of the central metal.
Question 6
MediumPaper 1A · calculator1 markWhich statements are correct for the complex ion ?
I. Cyanide ions are behaving as Lewis bases.
II. The oxidation state of iron is .
III. The coordination number of the iron ion is 6.
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
Consider the definitions of a Lewis base, how to calculate the oxidation state of the central metal ion in a complex, and the definition of coordination number. The charge on a cyanide ion () is .
Question 7
MediumPaper 1A · calculator1 markWhich term cannot be used to describe the chemical behaviour of water, ?
A. Lewis acid
B. Ligand
C. Brønsted-Lowry base
D. Nucleophile
Consider the definitions of each term. A Lewis acid is an electron pair acceptor. A ligand donates an electron pair to a metal ion. A Brønsted-Lowry base is a proton acceptor. A nucleophile is an electron-rich species that attacks an electrophile. Does water have lone pairs of electrons to donate, or does it have empty orbitals to accept electron pairs?
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