R3.4 Electron-pair sharing reactions
Practice exam-style IB Chemistry questions for Electron-pair sharing reactions, aligned with the syllabus and grouped by topic.
Question 1
What is the nucleophile in the reaction below?
A.
CH₃CH₂NH₃⁺
B.
NH₃
C.
Br⁻
D.
CH₃CH₂Br
Question 2
What is the leaving group in the reaction below?
A.
OH⁻
B.
C₂H₅I
C.
I⁻
D.
C₂H₅OH
Question 3
Which equation represents heterolytic fission of a carbon–chlorine bond?
A.
CH₃Cl → C + H₃Cl
B.
CH₃Cl → CH₃• + Cl•
C.
CH₃Cl → CH₃⁻ + Cl⁺
D.
CH₃Cl → CH₃⁺ + Cl⁻
Question 4
What is the electrophile in the reaction below?
A.
The C=C bond in ethene
B.
Hδ⁺ in HBr
C.
Brδ⁻ in HBr
D.
CH₃CH₂Br
Question 5
What is the organic product when ethene reacts with bromine in the dark?
A.
1,1-Dibromoethane
B.
Ethane
C.
Bromoethane
D.
1,2-Dibromoethane
Question 6
In the reaction BF₃ + NH₃ → F₃B←NH₃, what are the Lewis acid and Lewis base?
A.
BF₃ and NH₃ are both Lewis bases.
B.
BF₃ and NH₃ are both Lewis acids.
C.
BF₃ is the Lewis acid and NH₃ is the Lewis base.
D.
NH₃ is the Lewis acid and BF₃ is the Lewis base.
Question 7
What is the charge on the complex ion [Fe(CN)₆]³⁻ assigned to the iron ion if each cyanide ligand has charge 1−?
A.
+4
B.
+2
C.
+3
D.
+6
Question 8
Define the term nucleophile.
[1]
State one neutral nucleophile.
[1]
Question 9
Deduce the organic product for each reaction.
CH₂=CH₂ + HBr →
[1]
CH₃CH=CHCH₃ + Br₂ →
[1]
CH₂=CH₂ + H₂O →, H⁺ catalyst
[1]
Question 10
What is the product of the hydration of hex-3-ene under acidic conditions?
A.
Hexan-2-ol
B.
Hexan-1-ol
C.
Hexan-3-ol
D.
3-Bromohexane
Question 11
Bromine water is decolourized by propene in the dark but not by propane in the dark. What best explains this observation?
A.
Propane has only single bonds and is thermodynamically stable.
B.
Propene has a C=C bond with high electron density.
C.
Propene reacts with bromine by nucleophilic substitution.
D.
Propane forms carbocations more readily than propene.
Question 12
Which statement distinguishes an S_N2 reaction from an S_N1 reaction?
A.
S_N2 rate depends only on the halogenoalkane concentration.
B.
S_N2 has a carbocation intermediate; S_N1 has none.
C.
S_N1 requires backside attack in a single transition state.
D.
S_N2 is concerted; S_N1 proceeds through a carbocation intermediate.
Question 13
For comparable primary halogenoalkanes reacting with hydroxide ions, what is the expected order of substitution rate?
A.
RCl > RI > RBr
B.
RI > RBr > RCl
C.
RBr > RCl > RI
D.
RCl > RBr > RI
Question 14
What is the major product when propene reacts with HBr under normal electrophilic addition conditions?
A.
1-Bromopropane
B.
1,2-Dibromopropane
C.
2-Bromopropane
D.
Propan-1-ol
Question 15
What electrophile reacts with benzene in nitration?
A.
NO₃⁻
B.
SO₄²⁻
C.
HNO₃
D.
NO₂⁺
Question 16
Bromoethane reacts with aqueous sodium hydroxide.
Write an equation for the organic reaction.
[1]
Identify the nucleophile and leaving group.
[1]
State the direction of electron-pair movement from the C–Br bond during substitution.
[1]
Question 17
Distinguish between homolytic and heterolytic fission in terms of electrons and products. [2]
Question 18
Consider the reaction NH₃ + BF₃ → H₃N→BF₃.
Identify the electron-pair donor.
[1]
Identify the electron-pair acceptor.
[1]
State whether the donor is acting as a nucleophile or an electrophile.
[1]
Question 19
Chloromethane can undergo heterolytic fission of the C–Cl bond.
Write an equation for this heterolytic fission.
[1]
State where a curly arrow should start and end for this bond breaking.
[1]
Explain why the chlorine-containing product is negative.
[1]
Question 20
For each reaction, identify the Lewis acid and Lewis base.
H⁺ + H₂O → H₃O⁺
[1]
AlCl₃ + Cl⁻ → AlCl₄⁻
[1]
State the relationship between nucleophiles and Lewis bases.
[1]
Question 21
Deduce the charge or metal oxidation state for each complex.
Charge on [Cu(NH₃)₄] if Cu is +2.
[1]
Charge on [NiCl₄] if Ni is +2.
[1]
Oxidation state of Fe in [Fe(OH)(H₂O)₅]²⁺.
[1]
Question 22
A student tests four hydrocarbons by shaking each with bromine water in the dark. The table shows the observations.
| Hydrocarbon | Molecular formula | Bromine water after shaking in dark |
|---|---|---|
| A | C5H12 | orange-brown |
| B | C4H8 | colourless |
| C | C6H14 | orange-brown |
| D | C7H16 | orange-brown |
Identify the compound most likely to be an alkene.
[1]
State the type of reaction causing the observation in (a).
[1]
Explain why an alkane does not give the same observation in the dark.
[1]
Question 23
The displayed structures of four species are shown.
Identify one negatively charged nucleophile.
[1]
Identify one neutral nucleophile.
[1]
Identify the species that cannot donate a lone pair from nitrogen.
[1]
State the feature that allows a species to act as a nucleophile.
[1]
Question 24
Which species is least likely to act as a nucleophile at nitrogen?
A.
CH₃NH₂
B.
CN⁻
C.
(CH₃)₄N⁺
D.
NH₃
Question 25
A tertiary halogenoalkane reacts with water. Which rate equation is most consistent with an S_N1 mechanism?
A.
rate = k[RX][H₂O]
B.
rate = k[RX]²
C.
rate = k[H₂O]
D.
rate = k[RX]
Question 26
In the first step of electrophilic addition of Br₂ to ethene, where should the first curly arrow start?
A.
At the C=C bond
B.
At a C–H bond
C.
At the Br–Br bond
D.
At a bromide ion
Question 27
Explain why alkenes undergo electrophilic addition with bromine water in the dark, while alkanes do not. [3]
Question 28
Pent-2-ene reacts separately with bromine, hydrogen bromide and steam in acidic conditions.
State the type of reaction with bromine.
[1]
Write the product formed with bromine.
[1]
State the product expected from addition of hydrogen bromide at SL without considering major/minor products.
[1]
State the functional group formed on hydration.
[1]
Question 29
Describe the S_N2 reaction of 1-bromobutane with OH⁻.
State the rate equation.
[1]
Describe the bond-making and bond-breaking in the single step.
[1]
State the stereochemical consequence at a chiral reacting carbon.
[1]
Explain why a primary halogenoalkane favours this mechanism.
[1]
Question 30
Describe the S_N1 reaction of 2-chloro-2-methylpropane with water.
State the rate equation.
[1]
Write the species formed in the slow first step.
[1]
State the role of water in the second step.
[1]
Explain why a tertiary halogenoalkane favours S_N1.
[1]
Question 31
Explain the relative rates of substitution for CH₃CH₂Cl, CH₃CH₂Br and CH₃CH₂I with the same nucleophile. [3]
Question 32
2-Methylpropene reacts with HBr.
Deduce the major organic product.
[1]
Identify the more stable carbocation formed in the mechanism.
[1]
Explain why this carbocation is more stable.
[1]
Question 33
A reaction scheme shows chloroethane reacting with three possible reagents: OH⁻, NH₃ and Br₂.
Identify which reagent can act as a nucleophile in nucleophilic substitution of chloroethane.
[1]
State the leaving group in the substitution reaction.
[1]
Deduce the organic product when OH⁻ is used.
[1]
Explain, using electron-pair movement, how the C–Cl bond breaks.
[1]
Question 34
A diagram shows two possible ways for the C–Br bond in bromomethane to break.
Identify the pathway showing heterolytic fission.
[1]
State the products of heterolytic fission of CH₃Br.
[1]
Explain why heterolytic fission is represented with a double-barbed curly arrow rather than fish-hook arrows.
[1]
Question 35
A flow chart shows conversions of ethene to three organic products using HBr, Br₂ and H₂O/H⁺.
Match HBr to the correct product.
[1]
Match Br₂ to the correct product.
[1]
Match H₂O/H⁺ to the correct product.
[1]
State the common reaction type for all three conversions.
[1]
Explain why ethene is susceptible to this reaction type.
[1]
Question 36
A table gives formulas of complex ions and ligand charges.
| Complex ion formula | Ligands present | Charge of each ligand |
|---|---|---|
| [Co(H₂O)₆] | H₂O | 0 |
| [CoCl₄] | Cl⁻ | −1 |
| [Cr(NH₃)₅Cl]²⁺ | NH₃, Cl⁻ | 0, −1 |
Deduce the charge on [Co(H₂O)₆] if cobalt is +2.
[1]
Deduce the charge on [CoCl₄] if cobalt is +2.
[1]
Deduce the oxidation state of chromium in [Cr(NH₃)₅Cl]²⁺.
[1]
State the role of the ligands in these complex ions according to Lewis theory.
[1]
Question 37
Describe the electrophilic addition mechanism for the reaction of ethene with HBr.
State the electron-pair donor and electrophile in the first step.
[1]
Describe the bond breaking in HBr.
[1]
Identify the intermediate formed after protonation.
[1]
State the role of Br⁻ in the final step.
[1]
Question 38
Benzene reacts with a charged electrophile, E⁺, by electrophilic substitution.
Explain why benzene tends to substitute rather than add.
[1]
Describe the first step of the mechanism.
[1]
Describe how aromaticity is restored.
[1]
State why the first step is often rate-determining.
[1]
Question 39
The graph shows the relative initial rates for reactions of three primary halogenoalkanes with aqueous hydroxide ions under identical conditions.
Identify the halogenoalkane with the greatest rate.
[1]
State the order of leaving group ability shown.
[1]
Explain the trend using C–X bond strength.
[1]
Suggest why iodide is a more stable leaving group than chloride.
[1]
Question 40
Energy profiles for two nucleophilic substitution reactions are shown.
Identify which profile represents an S_N1 mechanism.
[1]
State what the valley between the two maxima represents.
[1]
State the rate equation expected for the S_N2 profile.
[1]
Explain why the S_N2 profile has only one maximum.
[1]
Question 41
A mechanism diagram for electrophilic substitution of benzene by E⁺ is shown with three arrows labelled P, Q and R.
Identify the arrow showing attack of benzene on E⁺.
[1]
Identify the intermediate shown after the first step.
[1]
State what is removed in the second step.
[1]
Explain why substitution rather than addition restores stability.
[1]
Question 42
Chloroethane reacts with hydroxide ions in aqueous solution.
Identify the nucleophile and leaving group, and write the organic equation.
[1]
Explain the electron-pair movement in this nucleophilic substitution reaction, including the role of the polar C–Cl bond.
[1]
Question 43
Ethene reacts with bromine, hydrogen bromide and water under suitable conditions.
Deduce the organic product for each reaction.
[1]
Discuss why these reactions are classified as electrophilic addition reactions.
[1]
Question 44
Compare heterolytic fission with homolytic fission, using bromomethane and bromine as examples.
Define heterolytic fission and write an equation for heterolytic fission of CH₃Br.
[1]
Compare the arrows and products used for heterolytic and homolytic fission.
[1]
Question 45
Lewis acid-base theory can describe reactions in both organic and inorganic chemistry.
Define Lewis acid and Lewis base, and identify each in BF₃ + NH₃ → F₃B←NH₃.
[1]
Discuss how coordination bond formation links Lewis acid-base reactions, nucleophiles/electrophiles and complex ions.
[1]
Question 46
A product analysis for addition of HBr to an unsymmetrical alkene is shown.
| Product | Relative abundance / % |
|---|---|
| 2-bromopropane | 92 |
| 1-bromopropane | 8 |
Identify the major product from the data.
[1]
Deduce which carbocation intermediate leads to the major product.
[1]
Explain why this carbocation is favoured.
[1]
State the prediction rule illustrated by the data.
[1]
Question 47
Four species are proposed as nucleophiles: OH⁻, NH₃, H₂O and (CH₃)₄N⁺.
State the essential feature of a nucleophile and identify two species from the list that have this feature.
[1]
Evaluate the statement: "All nucleophiles must be negatively charged." Use the species in the list to support your answer.
[1]
Question 48
Primary and tertiary halogenoalkanes undergo nucleophilic substitution by different mechanisms.
State the rate equations for an S_N2 reaction and an S_N1 reaction.
[1]
Compare and contrast the mechanisms of substitution in a primary halogenoalkane and a tertiary halogenoalkane, including intermediates, transition states and stereochemistry.
[1]
Question 49
An unsymmetrical alkene, 2-methylbut-2-ene, reacts with HBr.
Deduce the major product and name the rule used to predict it.
[1]
Explain the mechanism leading to the major product in terms of electrophilic attack, carbocation stability and nucleophilic attack.
[1]
Question 50
Benzene reacts with NO₂⁺ to form nitrobenzene by electrophilic substitution.
Outline the two main steps in the electrophilic substitution mechanism of benzene with a charged electrophile, E⁺.
[1]
Evaluate why benzene undergoes substitution rather than addition, and why the first step is usually rate-determining.
[1]
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