Topic R3.3 · SL and HL

Electron sharing reactions: notes and practice questions

5 min read Jump to 20 practice questions

Summary

This topic explains electron sharing reactions involving highly reactive radicals.
A radical is a molecular entity with an unpaired electron.
Radicals are produced by homolytic fission, where a covalent bond breaks, with each atom retaining one electron.
Single-barbed arrows (fish hooks) are used to show the movement of a single electron.
Radical substitution reactions with alkanes and halogens involve initiation, propagation, and termination steps.
Alkanes are kinetically stable due to the strength and non-polar nature of C-C and C-H bonds.

How it is examined

Two hours of teaching, so a small share of marks, but it is examined every session because it is easy to write clean marking points for. The standard shape is "State the equation for the initiation step" [1], "State two propagation steps" [2], "State an equation for a termination step" [1]. Radical dots are part of the mark: an equation written without them can be refused. The "why a mixture of products" answer wants further substitution, not just "many reactions happen".

Given in the booklet

Average bond enthalpies, which support the "why does UV break Cl-Cl and not C-H" reasoning. The radical dot notation and the three stage names are recall.

Key ideas

3.3.1 A radical is a molecular entity that has an unpaired electron. Radicals are highly reactive. Students identify and represent radicals, for example `·CH₃` and `Cl·`.
3.3.2 Radicals are produced by homolytic fission, for example of halogens, in the presence of ultraviolet (UV) light or heat. Students explain, including with equations, the homolytic fission of halogens, known as the initiation step in a chain reaction.
3.3.3 Radicals take part in substitution reactions with alkanes, producing a mixture of products. Students explain, using equations, the propagation and termination steps in the reactions between alkanes and halogens.

At HL

None for Reactivity 3.3.

Guiding questions

What happens when a species possesses an unpaired electron?

Linking questions

Structure 2.1 How is it possible for a radical to be an atom, a molecule, a cation or an anion? Consider examples of each type.
Structure 2.2 What is the reverse process of homolytic fission? Chlorine radicals released from CFCs are able to break down ozone, O₃, but not oxygen, O₂, in the stratosphere. What does this suggest about the relative strengths of bonds in the two allotropes?
Reactivity 1.2 Why do chlorofluorocarbons (CFCs) in the atmosphere break down to release chlorine radicals but typically not fluorine radicals?
Reactivity 2.2 Why are alkanes described as kinetically stable but thermodynamically unstable?

Practice questions

24 questions · 9 easy · 15 medium

Showing 20 of 20

Question 1

EasyPaper 2 · calculator1 mark

Free radicals are highly reactive species characterized by the presence of an unpaired electron. They are involved in many atmospheric processes. Which of the following species is not a free radical?

A. $Cl$

B. $O_3$

C. $NO$

D. $CH_3$

Question 2

MediumPaper 1A · calculator1 mark

Which alcohol and conditions would produce the highest yield of butanal?

Alcohol

Conditions

A. Butan-1-ol

Reflux

B. Butan-2-ol

Reflux

C. Butan-1-ol

Distillation

D. 2-methylpropan-2-ol

Distillation

Question 3

EasyPaper 2 · calculator1 mark

Ethane, $C_2H_6$ , is exposed to chlorine gas, $Cl_2$ , in the presence of ultraviolet (UV) light. Which equation represents the initial reaction that occurs via a free radical mechanism?

A. $C_2H_6 + Cl_2 \rightarrow C_2H_5Cl + HCl$

B. $C_2H_4 + Cl_2 \rightarrow C_2H_4Cl_2$

C. $C_2H_5Cl + NaOH \rightarrow C_2H_5OH + NaCl$

D. $2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O$

Question 4

MediumPaper 2 · calculator1 mark

Which structural isomer of hexane, $C_6H_{14}$ , will form exactly two different structural isomers of monochlorohexane, $C_6H_{13}Cl$ , upon free-radical substitution?

A. Hexane
B. 2-Methylpentane
C. 2,2-Dimethylbutane
D. 2,3-Dimethylbutane

Question 5

EasyPaper 2 · calculator1 mark

Which equation correctly represents the homolytic fission of a chlorine molecule, $Cl_2$ , during the initiation step of a free-radical substitution reaction?

A. $Cl_2 \rightarrow 2Cl•$

B. $Cl_2 \rightarrow Cl^+ + Cl^-$

C. $Cl_2 + 2e^- \rightarrow 2Cl^-$

D. $Cl_2 \rightarrow 2Cl^+ + 2e^-$

Question 6

MediumPaper 2 · calculator1 mark

What is the major organic product formed when but-1-ene reacts with hydrogen bromide?

A. 1-bromobutane

B. 2-bromobutane

C. 1,2-dibromobutane

D. 2,2-dibromobutane

Question 7

EasyPaper 1A · calculator1 mark

Which equation represents a termination step in the free-radical substitution of ethane with bromine?

A. $Br_2 \rightarrow 2Br\bullet$

B. $C_2H_6 + Br\bullet \rightarrow C_2H_5\bullet + HBr$

C. $C_2H_5\bullet + Br\bullet \rightarrow C_2H_5Br$

D. $C_2H_5\bullet + Br_2 \rightarrow C_2H_5Br + Br\bullet$

Question 8

MediumPaper 2 · calculator5 marks

(a)

Ethane, $C_2H_6$ , reacts with bromine, $Br_2$ , in the presence of ultraviolet (UV) light to form bromoethane, $C_2H_5Br$ , via a free-radical substitution mechanism. The first step of the mechanism is initiation.

(i) Define the term homolytic fission.

(ii) State the number of electrons and the overall charge of a bromine radical.

(iii) Write an equation for the initiation step of this reaction.

[3]

(b)

(b) The mechanism continues with propagation and termination steps.

(i) Write one equation for a propagation step.

(ii) Write one equation for a termination step that results in the formation of butane, $C_4H_{10}$ .

[2]

Question 9

EasyPaper 1A · calculator1 mark

Which equation represents a termination step in the free-radical substitution of ethane with bromine?

A. $Br_2 \rightarrow 2Br\bullet$

B. $C_2H_6 + Br\bullet \rightarrow \bullet C_2H_5 + HBr$

C. $\bullet C_2H_5 + Br\bullet \rightarrow C_2H_5Br$

D. $\bullet C_2H_5 + Br_2 \rightarrow C_2H_5Br + Br\bullet$

Question 10

MediumPaper 1A · calculator1 mark

Which compound(s) will decolorize warm, acidified potassium manganate(VII) solution?

I. $CH_3CH_2CH(CH_3)CH_2OH$

II. $CH_3C(OH)(CH_3)CH_2CH_3$

III. $CH_3CH_2CH(OH)CH_2CH_3$

A. I only

B. I and III only

C. II and III only

D. I, II and III

Question 11

EasyPaper 1A · calculator1 mark

Which compound is produced by the oxidation of propan-2-ol with acidified potassium dichromate(VI)?

A. Propanal

B. Propanoic acid

C. Propanone

D. Propene

Question 12

MediumPaper 1A · calculator1 mark

Which radical is most likely to be formed by the homolytic fission of one covalent bond in a molecule of 1-bromo-1-chloro-2,2,2-trifluoroethane, $CF_3CHBrCl$ , when exposed to ultraviolet radiation?

A. $\cdot CF_3CHCl$

B. $\cdot CF_3CHBr$

C. $\cdot CF_2CHBrCl$

D. $\cdot CF_3CBrCl$

Question 13

EasyPaper 1A · calculator1 mark

The reaction between methane and chlorine in the presence of UV light proceeds via a free-radical mechanism. Which species is formed in the initiation step?

A. $Cl^-$

B. $CH_3\cdot$

C. $HCl$

D. $Cl\cdot$

Question 14

MediumPaper 1A · calculator1 mark

Propene, $C_3H_6$ , is an important feedstock in the chemical industry.

Which row correctly identifies the major organic product formed when propene reacts with the given reagent?

A. Reagent: $H_2O$ (acid catalyst), Product: Propan-1-ol

B. Reagent: $HBr$ , Product: 2-bromopropane

C. Reagent: $Br_2$ , Product: 1,1-dibromopropane

D. Reagent: $H_2$ (Ni catalyst), Product: Prop-1-ene

Question 15

EasyPaper 1A · calculator1 mark

Which compound is formed from the complete oxidation of $CH_3CH_2CH_2CH_2OH$ ?

A. $CH_3CH_2CH_2CHO$

B. $CH_3CH_2COCH_3$

C. $CH_3CH_2CH_2COOH$

D. $CH_3COOCH_2CH_3$

Question 16

MediumPaper 1A · calculator1 mark

Which product is formed when butan-1-ol is heated under reflux with an excess of acidified potassium dichromate(VI) solution?

A. $CH_3CH_2CH_2CHO$

B. $CH_3CH_2CH_2COOH$

C. $CH_3COCH_2CH_3$

D. $CH_3CH_2COOCH_3$

Question 17

EasyPaper 1A · calculator1 mark

Which species acts as the nucleophile in the reaction below?

$CH_3CH_2Cl + NH_3 \rightarrow CH_3CH_2NH_3^+ + Cl^-$

A. $CH_3CH_2Cl$

B. $NH_3$

C. $CH_3CH_2NH_3^+$

D. $Cl^-$

Question 18

MediumPaper 1A · calculator1 mark

Which reaction represents the reduction of a functional group?

A. $CH_3CH_2CHO \rightarrow CH_3CH_2COOH$

B. $CH_3COCH_3 \rightarrow CH_3CH(OH)CH_3$

C. $CH_3CH_2OH \rightarrow CH_3CHO$

D. $CH_2=CH_2 \rightarrow HOCH_2CH_2OH$

Question 19

MediumPaper 1A · calculator1 mark

Which alcohol(s) will decolorize acidified potassium permanganate(VII) solution upon warming?

I. $CH_3CH_2CH_2CH_2CH_2OH$

II. $CH_3C(OH)(CH_3)CH_2CH_3$

III. $CH_3CH_2CH(OH)CH_2CH_3$

A. I only

B. III only

C. I and III only

D. I, II and III

Question 20

MediumPaper 1A · calculator1 mark

Bromochlorofluoromethane, $CHBrClF$ , is a compound that was once used in fire extinguishers. Its release into the atmosphere contributes to ozone depletion. Which organic radical is most likely to form from the homolytic fission of one covalent bond in a molecule of $CHBrClF$ when exposed to ultraviolet radiation?

A. $\cdot CHClF$

B. $\cdot CHBrF$

C. $\cdot CHBrCl$

D. $\cdot CBrClF$

4 more Electron sharing reactions questions in the app

Every answer is marked mark by mark, IB-style, and the AI tutor helps when you are stuck.

Where marks are lost

Reaching for "human error" or "only one trial." A source of error has to be a specific step in the method, not a general apology for the result.
Joining the dots instead of drawing a curve.
Naming a chemical instead of the property that distinguishes it, or vice versa. Answering with the nearest fact that comes to mind rather than the fact the command term and stem jointly ask for is a recurring way to answer a question that was not, quite, the one asked.

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What does Electron sharing reactions cover in IB Chemistry?

This topic explains electron sharing reactions involving highly reactive radicals. A radical is a molecular entity with an unpaired electron. Radicals are produced by homolytic fission, where a covalent bond breaks, with each atom retaining one electron.

Is Electron sharing reactions SL or HL?

Both. SL and HL students study Electron sharing reactions, and HL goes further: None for Reactivity 3.3.

How do I revise Electron sharing reactions for IB Chemistry?

Start from the core idea: this topic explains electron sharing reactions involving highly reactive radicals. In the exam: two hours of teaching, so a small share of marks, but it is examined every session because it is easy to write clean marking points for. The standard shape is "State the equation for the initiation step" [1], "State two propagation steps" [2], "State an equation for a termination step" [1]. Then practise exam-style questions, easiest first, writing out every step of your working before you check it.

How does FourtyFive help me practise Electron sharing reactions?

FourtyFive has 24 Electron sharing reactions questions. Every answer you write is marked mark by mark, IB-style, and you see where each mark was won or lost. Every part has a hint, the AI tutor helps you through the step you are stuck on, and your Study Profile picks what to practise next.

Is FourtyFive free for Electron sharing reactions practice?

Yes. A free account gives you 50 marked answers a month, and you do not need a card to sign up.

Can I handwrite Electron sharing reactions answers on an iPad?

Yes. In the FourtyFive iPad app you write your working by hand with Apple Pencil, the way you would on paper, and it is marked the same way.

Electron sharing reactions: notes and practice questions

How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

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Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

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Where marks are lost

Practise this topic as a session

Questions,
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How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

4 more Electron sharing reactions questions in the app

Where marks are lost

Questions,answered.

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bank built for you.