H3 Further Reaction Mechanisms (9813)
Learn Hammond reasoning and compare SN1, SN2, E1 and E2 pathways through 14 ordered H3 lessons with worked mechanisms and practice.
Use energy profiles, rate data, curved arrows and stereochemical evidence together to explain why substitution or elimination follows a particular pathway. The topic starts with the energy profile, applies it to the substitution pathway, and then to elimination and its competition with substitution.
Prerequisites
- H2 nucleophiles, electrophiles, leaving groups and curly-arrow conventions.
- Rate equations, reaction order, rate constants and activation-energy profiles.
- Carbocation stability, alkene substitution and stereochemical projections.
Lessons (Recommended Order)
Follow the lessons in order on a first pass: the Hammond postulate underpins both mechanism families, and the elimination lessons compare their pathways with substitution. For revision, go straight to the lesson for the statement you need.
3.1 Basic Physical Organic Chemistry
3.2 Nucleophilic Substitution
- Explain nucleophile effects on substitution rate
- Explain leaving-group effects on substitution rate
- Explain substituent effects on substitution rate
- Compare SN1 and SN2 profiles, rate laws and steady-state reasoning
- Compare SN1 and SN2 stereochemistry and ion-pair effects
- Apply substituent effects to SN1 and SN2
- Evaluate competition between SN1 and SN2
3.3 Elimination
Common Exam Traps
- Choosing a mechanism from the substrate label alone instead of using all supplied evidence.
- Drawing a curved arrow from an atom rather than from the electron pair or bond that moves.
- Treating Zaitsev, Hofmann, syn or anti as unconditional rules without checking structure and geometry.
Learning goals
- Apply the Hammond postulate to reaction mechanisms
- Explain nucleophile effects on substitution rate
- Explain leaving-group effects on substitution rate
- Explain substituent effects on substitution rate
- Compare SN1 and SN2 profiles, rate laws and steady-state reasoning
- Compare SN1 and SN2 stereochemistry and ion-pair effects
- Apply substituent effects to SN1 and SN2
- Evaluate competition between SN1 and SN2
- Apply syn/anti elimination and stereoselectivity
- Apply Zaitsev/Hofmann regioselectivity
- Compare E1 and E2 profiles and rate laws
- Compare E1 and E2 regioselectivity
- Apply substrate effects in E2/SN2 competition
- Apply base effects in E2/SN2 competition
Syllabus statements covered
- understand and apply the Hammond postulate to the study of reaction mechanisms
- explain how the relative rate of nucleophilic substitution is affected by the nature of the: — nucleophile
- explain how the relative rate of nucleophilic substitution is affected by the nature of the: — leaving group
- explain how the relative rate of nucleophilic substitution is affected by the nature of the: — substituents
- describe and compare the mechanisms and kinetics of SN1 and SN2 reactions, in terms of: — the energy profile and rate law, including steady state approximation in SN1 [mathematical treatment of steady state is not required]
- describe and compare the mechanisms and kinetics of SN1 and SN2 reactions, in terms of: — stereochemistry, including ion pair interactions in SN1
- describe and compare the mechanisms and kinetics of SN1 and SN2 reactions, in terms of: — substituent effects
- explain the factors affecting competition between SN1 and SN2 mechanisms [solvent effects are not required]
- understand and apply the following concepts to the study of elimination reactions: — syn- and anti-elimination; and its effect on stereoselectivity
- understand and apply the following concepts to the study of elimination reactions: — regioselectivity: Zaitsev (thermodynamic) and Hofmann (kinetic) product(s)
- describe and compare the mechanisms and kinetics of E1 and E2 reactions, in terms of: — the energy profile and rate law
- describe and compare the mechanisms and kinetics of E1 and E2 reactions, in terms of: — regioselectivity
- explain the E2/SN2 competition, in terms of: — substrate effects
- explain the E2/SN2 competition, in terms of: — base effects
Check what I know
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Recent attempts
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Beyond the syllabus: optional enrichment that does not count towards your progress.
Check what I know
Start here to see which parts you already know.
About 8 minutes
Time is up, but your answers have not been submitted yet. Check your connection and try again.
Answer 5 short questions. It shows what to work on next and doesn't count towards mastery.
Recent attempts
History is stored only in this browser.
No completed attempts are saved yet.
Beyond the syllabus: optional enrichment that does not count towards your progress.
Practise
Work through questions with marking and feedback as you learn.
About 10 minutes
Time is up, but your answers have not been submitted yet. Check your connection and try again.
Questions are picked at random each time you start. You'll see the answer after each question. It's for practice only and doesn't count towards mastery.
Recent attempts
History is stored only in this browser.
No completed attempts are saved yet.
Beyond the syllabus: optional enrichment that does not count towards your progress.
Practise after feedback
After a check, practise the skills it showed you need to work on.
About 10 minutes
Time is up, but your answers have not been submitted yet. Check your connection and try again.
Questions are picked at random each time you start. You'll see the answer after each question. It's for practice only and doesn't count towards mastery.
Recent attempts
History is stored only in this browser.
No completed attempts are saved yet.
Beyond the syllabus: optional enrichment that does not count towards your progress.
Check what I know
Start here to see which parts you already know.
About 8 minutes
Time is up, but your answers have not been submitted yet. Check your connection and try again.
Answer 5 short questions. It shows what to work on next and doesn't count towards mastery.
Recent attempts
History is stored only in this browser.
No completed attempts are saved yet.
Beyond the syllabus: optional enrichment that does not count towards your progress.
Practise
Work through questions with marking and feedback as you learn.
About 10 minutes
Time is up, but your answers have not been submitted yet. Check your connection and try again.
Questions are picked at random each time you start. You'll see the answer after each question. It's for practice only and doesn't count towards mastery.
Recent attempts
History is stored only in this browser.
No completed attempts are saved yet.
Beyond the syllabus: optional enrichment that does not count towards your progress.
Practise after feedback
After a check, practise the skills it showed you need to work on.
About 10 minutes
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Questions are picked at random each time you start. You'll see the answer after each question. It's for practice only and doesn't count towards mastery.
Recent attempts
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No completed attempts are saved yet.
Beyond the syllabus: optional enrichment that does not count towards your progress.
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Answer 4 questions. You'll see your score, the answers and explanations at the end. Your result can count towards your course progress.
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Beyond the syllabus: optional enrichment that does not count towards your progress.
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Check my progress
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Answer 5 questions. You'll see your score, the answers and explanations at the end. Your result can count towards your course progress.
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Beyond the syllabus: optional enrichment that does not count towards your progress.
Review
Come back later to see whether your learning has lasted.
About 10 minutes
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Answer 5 questions. You'll see your score, the answers and explanations at the end. A scheduled review counts towards your course progress only when it is due.
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Check my progress
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Answer 5 questions. You'll see your score, the answers and explanations at the end. Your result can count towards your course progress.
Recent attempts
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Beyond the syllabus: optional enrichment that does not count towards your progress.
Review
Come back later to see whether your learning has lasted.
About 10 minutes
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Answer 5 questions. You'll see your score, the answers and explanations at the end. A scheduled review counts towards your course progress only when it is due.
Recent attempts
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No completed attempts are saved yet.
Beyond the syllabus: optional enrichment that does not count towards your progress.