Polymerisation and the addition mechanism
Polymerisation and the addition mechanism
- Many alkenes can join into one long polymer.
- The addition mechanism goes through a carbocation.
- Its stability explains Markovnikov's rule.
Addition polymerisation
- 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.
Practice
In addition polymerisation:
Alkenes open their double bonds and join into a polymer such as poly(ethene); nothing else is formed.
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.
Practice
During electrophilic addition, the positive intermediate formed is called a:
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.
- 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.
Practice
Carbocation stability increases in the order:
More alkyl groups push electrons towards the positive carbon (inductive effect), so tertiary is most stable.
Practice
Markovnikov's rule predicts that when HBr adds to propene:
The more stable carbocation forms, so the major product follows Markovnikov's rule.
You've got it
Key idea
- 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