New organic mechanisms
Introduced| English | Meaning |
|---|---|
| electrophilic substitution/ɪˌlektrəʊˈfɪlɪk ˌsʌbstɪˈtjuːʃn/ | A reaction in which an electrophile replaces an atom or group in a molecule. |
| electrophile/ɪˌlektrəʊˈfaɪl/ | A species that accepts an electron pair to form a covalent bond. |
| delocalised/dɪˈlɒkəlaɪzd/ | Describing electrons that are spread over several atoms rather than confined to one bond or atom. |
| nitration/naɪˈtreɪʃn/ | A reaction that introduces a nitro group into a molecule. |
| nitrobenzene/ˌnaɪtrəʊˈbenziːn/ | nitrobenzene |
Two new mechanisms
- A Level adds two reaction mechanisms to the ones you already know.
- Electrophilic substitution 亲电取代 is how benzene reacts.
- Addition–elimination appears with carbonyls and acyl chlorides.
Electrophilic substitution is when:
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Benzene reacts by electrophilic substitution, keeping its stable ring while swapping an H for the electrophile.
Benzene reacts mainly by:
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Substitution keeps the stable delocalised ring intact, unlike addition which would destroy it.
Benzene undergoes electrophilic ______ rather than addition.
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This preserves the stable ring.
Reduction adds hydrogen (or removes oxygen) to a molecule.
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The opposite of oxidation.
Electrophilic substitution
- An electrophile 亲电试剂 replaces a hydrogen atom on a benzene ring.
- This is the characteristic way benzene reacts — the stable delocalised 离域 ring is kept.
- Benzene won't do addition (that would destroy the ring), so it substitutes instead.
Benzene: sigma framework plus a delocalised pi system
Electrophilic substitution on benzene
Step through how benzene reacts. An electrophile swaps for a hydrogen — keeping the stable ring — instead of adding across it.
In an addition–elimination reaction:
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Addition–elimination (e.g. with 2,4-DNPH or acyl chlorides) adds then eliminates a small molecule.
Match each term to its meaning.
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Each item links the term to its correct meaning.
Addition–elimination
- A molecule first adds on, then a small molecule is removed.
- Seen with 2,4-DNPH (with carbonyls) and with acyl chlorides (which lose HCl).
Two aromatic mechanisms: electrophilic substitution and addition-elimination
Worked example: nitration 硝化
Nitrating benzene (concentrated HNO₃ + H₂SO₄, 50 °C) — an electrophilic substitution:
- The acids react to make the electrophile, NO₂⁺.
- NO₂⁺ takes two electrons from the ring → an unstable intermediate.
- The intermediate loses H⁺, restoring the stable ring.
- Product: nitrobenzene 硝基苯 — one H has been swapped for NO₂, ring intact.
- The same pattern (make electrophile → attack ring → lose H⁺) covers halogenation and Friedel–Crafts.
You've got it
- electrophilic substitution: an electrophile replaces an H on the benzene ring (how benzene reacts)
- it keeps the stable delocalised ring — benzene substitutes rather than adds
- worked: nitration → NO₂⁺ attacks, loses H⁺, gives nitrobenzene
- addition–elimination: a molecule adds on, then a small molecule is removed (2,4-DNPH, acyl chlorides)