Reactivity 3.3

Electron sharing reactions

Guiding question:

What happens when a species possesses an unpaired electron?

Time:

SL and HL - 2 h

AHL - none

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Prior knowledge (from HL): Why do some of the fragments seen on a mass spectrum have a dot next to them (•)?

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Prior knowledge: Species with unpaired electrons have very high potential energy. How might this affect their reactivity?

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SL content

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Part A

Radicals

R3.3.1 and R3.3.2

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CH₃Cl → •CH₃ + Cl•

How might we produce species with unpaired electrons?

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How do we identify radicals?

A radical is…

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Identify the radical in each pair of species:

1. CH₄ CH₃
2. Cl Cl₂
3. NO N₂O
4. C₃H₈ C₃H₈⁺
5. NO₃ NO₃^-

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How are radicals produced?

Cl一Cl →

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CH₃一Cl →

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Quick check

Which of the following is true for the homolytic fission of Br₂?

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Quick check

Which bond in bromochlorofluoromethane would be broken by UV radiation with the longest wavelength?

1. C一H
2. C一F
3. C一Cl
4. C一Br

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Practice questions

1. Chlorofluorocarbons (CFCs) have been used as refrigerants and aerosol propellants. Dichlorodifluoromethane, known as Freon, is one example of a CFC.

1. Deduce the equation that shows the production of a chlorine radical when Freon is exposed to UV light. [1]
2. State the name for the bond breaking processing occurring in part a. [1]
3. Chlorine radicals in the atmosphere can catalyze the decomposition of ozone, O₃ , as shown below.

Step 1: Cl•(g) + O₃(g) → O₂(g) + ClO•(g)

Step 2: ClO•(g) + O•(g) → O₂(g) + Cl•(g)

State the overall reaction taking place and outline why Cl• is considered a catalyst.[2]

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Practice questions

2.

1. Draw a Lewis formula for ozone, O₃ , and molecular oxygen, O₂. [1]

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• (HL) Draw a resonance hybrid for ozone. [1]

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• (HL) Suggest why UV light is able to breakdown ozone, O₃, in the atmosphere but not O₂. [2]

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Practice questions

3. This question is about the ozone depletion potential (ODP) of refrigerants (R) in the atmosphere.

1. Identify the class of these compounds. [1]
2. Suggest why ODP values do not have units. [1]
3. Suggest 1 other consideration when selecting a refrigerant. [1]

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NOS: International collaboration

198 countries signed the Montreal Protocol to minimise the use of ozone depleting substances.

Evaluate the impact of international agreements like this one.

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Extension: Radical mechanisms during combustion

Combustion reactions have a variety of radical involved in their reaction mechanism.

Suggest some radicals that might be formed during the combustion of a hydrocarbon.

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Part B

Radical substitution reactions

R3.3.3

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Retrieval practice: Draw and annotate an energy profile for a combustion reaction.

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(Extension: Which molecule might be an exception?)

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Why might alkanes be chemically unreactive?

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Radical substitution reactions between alkanes and halogens

Example: CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

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Quick check

Suggest reactions in each stage for: C₂H₆(g) + Br₂(g) → C₂H₅Br(g) + HBr(g)

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Practice questions

1. Trichlorofluoromethane, known as CFC-11, is used as a precursor for refrigerants.
1. Use curly arrows to show what happens when CFC-11 is exposed to UV light in the atmosphere. [2]

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2. State the name of the process outlined above. [1]

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• Suggest one reason that this process is problematic. [1]

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Practice questions

2. Propane can undergo a radical substitution to form chloropropane.

1. Suggest the initiation step for this reaction. [1]
2. State 2 possible propagation step involved in this reaction. [2]
3. State 2 possible termination steps. [2]
4. Over time, products such as trichloropropane can be formed. Use IUPAC nomenclature to name 3 possible structural isomers of C₃H₅Cl₃. [1]

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Practice questions

3. In addition to catalyzing the breakdown of the ozone layer, CFCs are also potent greenhouse gases.

1. State the type of electromagnetic radiation absorbed by greenhouse gases. [1]
2. Suggest why a nonpolar molecule like tetrachloromethane is able to absorb the radiation identified in part a. [1]

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NOS: Representing chemical processes

Suggest a limitation to the representation of the radical substitution of methane below:

CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

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Conceptual review

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What happens when a species possesses an unpaired electron?

Suggest why the concentration of organic chlorine containing compounds in the atmosphere is monitored.

What does this graph suggest about the role of the Montreal Protocol (which committed to reducing the use of ozone-depleting substances)?

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Venn diagram (SL): Initiation, propagation and termination

1. Always produces a radical
2. Always involves a molecule as a reactant
3. Combination of two radicals
4. Involves UV radiation
5. Demonstrates catalytic role of a radical

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Initiation

Propagation

Termination

Venn diagram (SL): Ozone depletion vs global warming

1. Caused by an increase in greenhouse gases
2. Atmospheric issues
3. Accelerated by CFCs
4. Involve radicals
5. Impact atmospheric absorption of UV
6. Caused by increasing absorption of IR

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Ozone depletion

Global warming

Key terminology

• Radical
• Homolytic fission
• Single-barbed arrows (fish hook)
• UV radiation
• Initiation
• Propagation
• Termination
• Substitution

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Quizlets

• R3.3 SL - Electron sharing reactions

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