H3 further reaction mechanisms
Use Hammond reasoning and compare SN1, SN2, E1 and E2 pathways through worked mechanisms and practice.
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Before you begin
This topic explains why a haloalkane reacts by one mechanism rather than another. It starts with energy profiles and the Hammond postulate, uses them to explain the rates of SN1 and SN2 reactions, compares the two mechanisms by their kinetics and stereochemistry, then does the same for E1 and E2 elimination and its competition with substitution.
Be comfortable with: curly arrows, electrophiles and nucleophiles, halogenoalkanes, SN1, SN2 and elimination, rate equations and orders, mechanisms and the rate-determining step, and Newman projections and conformations.
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
Lessons
Work through them in order.
3.1 Basic Physical Organic Chemistry
3.2 Nucleophilic Substitution
- Explain nucleophile effects on substitution rateExplain how the nucleophile's identity and concentration change the rate of substitution.
- Explain leaving-group effects on substitution rateExplain how the leaving group changes substitution rate, comparing C–Cl, C–Br and C–I.
- Explain substituent effects on substitution rateRank the substitution rates of methyl to tertiary substrates once the SN1 or SN2 pathway is known.
- Compare SN1 and SN2 profiles, rate laws and steady-state reasoningMatch SN1 and SN2 to their energy profiles and rate equations, including steady-state reasoning.
- Compare SN1 and SN2 stereochemistry and ion-pair effectsPredict inversion in SN2 and racemisation in SN1, including the effect of ion pairs.
- Apply substituent effects to SN1 and SN2Explain why alkyl groups slow SN2 but speed up SN1, from methyl to tertiary substrates.
- Evaluate competition between SN1 and SN2Decide whether SN1 or SN2 dominates by weighing substrate, nucleophile and leaving group together.
3.3 Elimination
- Apply syn/anti elimination and stereoselectivityPredict which alkene stereoisomer an E2 elimination gives from the anti-periplanar geometry.
- Apply Zaitsev/Hofmann regioselectivityPredict whether the more or less substituted alkene forms from the β-hydrogens and the base.
- Compare E1 and E2 profiles and rate lawsMatch E1 and E2 to their rate equations, energy profiles and curly-arrow mechanisms.
- Compare E1 and E2 regioselectivityCompare how E1 and E2 decide which alkene forms from the same haloalkane.
- Apply substrate effects in E2/SN2 competitionPredict whether E2 or SN2 wins as the substrate changes from methyl to tertiary.
- Apply base effects in E2/SN2 competitionPredict whether E2 or SN2 wins as the base changes, from its strength and bulk.
Practise and check
Or choose
Topic reference
Quick reference
| Question | First move | What to show |
|---|---|---|
| Describe a transition state | compare its energy with the minima on either side | endothermic step: late, product-like; exothermic step: early, reactant-like |
| SN1 or SN2 from rate data | change one concentration at a time | rate = k[RX][Nu⁻]: SN2; rate = k[RX]: SN1 |
| Substituent effect on rate | say which mechanism first | SN2: steric hindrance at the back of the carbon; SN1: carbocation stability |
| Stereochemistry of substitution | single enantiomer in; what comes out? | SN2: inversion (reassign R/S); SN1: both, often slight excess inversion |
| E2 alkene stereochemistry | rotate until C–H and C–X are anti | carry the other groups into the alkene; then assign E/Z |
| Which alkene is major? | list every β-carbon carrying an H | small base: Zaitsev; bulky base: Hofmann; in a ring, trans-diaxial only |
| E2 or SN2? | can the reagent reach Cα? Is there a β-H? | tertiary or bulky base: E2; primary with a good nucleophile: SN2 |
Core knowledge to remember
- One-step mechanisms (SN2, E2) have one transition state and no intermediate. Two-step mechanisms (SN1, E1) have a carbocation intermediate in a minimum between two maxima.
- SN2: rate = k[RX][Nu⁻]. SN1: rate = k[RX]. E2: rate = k[RX][base]. E1: rate = k[RX].
- The carbocation in SN1 and E1 stays at a very low, nearly constant concentration: the steady-state approximation.
- Alkyl groups at the reacting carbon slow SN2 (steric hindrance) but speed up SN1 (carbocation stability).
- SN2 inverts the configuration; SN1 gives both configurations, often with a slight excess of inversion because of ion pairing.
- E2 is normally anti: the C–H and C–X bonds that break are at 180°. In a cyclohexane ring, they must be trans-diaxial.
- The Zaitsev product is the more substituted, thermodynamic alkene; the Hofmann product is the less substituted, kinetic alkene, favoured by bulky bases.
- A bulky base favours E2 over SN2; a tertiary haloalkane with a strong base gives E2.
Common mistakes
- Choosing a mechanism from the class of haloalkane alone, instead of using all the evidence given.
- Drawing a curly arrow from an atom instead of from a lone pair or a bond.
- Giving one order of reactivity for haloalkanes without saying which mechanism.
- Saying that SN2 always changes R to S.
- Counting the three curly arrows of E2 as three steps.
- Treating Zaitsev, Hofmann, syn or anti as fixed rules without checking the structure and geometry.