Polymerisation and the addition mechanism
Introduced| English | Meaning |
|---|---|
| polymer/ˈpɒlɪmə/ | A large molecule made from many linked repeating units derived from smaller molecules. |
| carbocation/ˌkɑːbəˈkeɪʃn/ | An organic ion in which a carbon atom carries a positive charge. |
| Markovnikov's rule/ˈmɑːkəvnɪkɒvz ruːl/ | In adding HX to an unsymmetrical alkene, hydrogen joins the carbon already bearing more hydrogens. |
| addition polymerisation/əˈdɪʃn ˌpɒlɪməraɪˈzeɪʃn/ | The joining of unsaturated monomers into a polymer without forming a separate small-molecule product. |
| inductive effect/ɪnˈdʌktɪv ɪˈfekt/ | The shift of electron density through sigma bonds caused by differences in electronegativity or charge. |
Tiny units, giant chains
- Many alkenes can join into one long polymer 聚合物.
- The addition mechanism goes through a carbocation 碳正离子.
- Its stability explains Markovnikov's rule 马氏规则.
Many ethene molecules join to form the polymer poly(______).
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Addition polymerisation of the alkene.
Addition polymerisation 加成聚合
- In addition polymerisation, many alkene molecules open their double bonds and join into one long chain — with no other product.
- Ethene → poly(ethene); the long-chain product is a polymer.
Many ethene molecules join into poly(ethene)
Addition polymerisation
Many alkene monomers open their double bonds and join into one long chain.
In addition polymerisation:
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Alkenes open their double bonds and join into a polymer such as poly(ethene); nothing else is formed.
Match each addition idea.
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Each item links the term to its correct meaning.
The electrophilic addition mechanism
- The electron-rich C=C attracts the electrophile (e.g. $\text{Br}^{\delta+}$).
- A positive intermediate, a carbocation, forms.
- The negative part (e.g. $\text{Br}^-$) then attacks the carbocation.
Electrophilic addition of bromine to ethene via a carbocation
During electrophilic addition, the positive intermediate formed is called a:
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The C=C attacks the electrophile, forming a positively charged carbocation, which the negative ion then attacks.
Carbocation stability and Markovnikov
- Alkyl groups push electrons towards the positive carbon — the inductive effect 诱导效应.
- So tertiary > secondary > primary in stability.
- When HBr adds to propene, the more stable carbocation forms — so H adds to the carbon that already has more H atoms. This is Markovnikov's rule.
Alkyl groups stabilise the positive carbon, so tertiary > secondary > primary — the basis of Markovnikov's rule
Carbocation stability increases in the order:
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More alkyl groups push electrons towards the positive carbon (inductive effect), so tertiary is most stable.
Markovnikov's rule predicts that when HBr adds to propene:
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The more stable carbocation forms, so the major product follows Markovnikov's rule.
You've got it
- addition polymerisation: many alkenes join into one long polymer (no other product), e.g. poly(ethene)
- electrophilic addition: C=C attacks the electrophile → carbocation → negative ion attacks
- carbocation stability tertiary > secondary > primary (inductive effect) → Markovnikov's rule