Alkenes and their reactions
Alkenes and their reactions
- Alkenes have a C=C double bond ($\text{C}_n\text{H}_{2n}$).
- That double bond is the functional group, so alkenes are reactive.
- Most reactions are electrophilic addition.
Practice
Alkenes are reactive because they have:
The electron-rich C=C double bond is the reactive functional group of alkenes (CₙH₂ₙ).
Making alkenes
- elimination of HX from a halogenoalkane (NaOH in ethanol, heat).
- dehydration of an alcohol (hot $\text{Al}_2\text{O}_3$ or conc. sulfuric acid).
- cracking of a longer alkane.
Practice
Which makes an alkene?
Removing water from an alcohol (dehydration), or HX from a halogenoalkane (elimination), makes an alkene.
Addition reactions
| Reagent | Product |
|---|---|
| $\text{H}_2$ (Pt/Ni, heat) | alkane |
| steam + $\text{H}_3\text{PO}_4$ | alcohol |
| $\text{HX}$ | halogenoalkane |
| $\text{X}_2$ | di-substituted alkane |
- Cold dilute $\text{KMnO}_4$ gives a diol; hot concentrated $\text{KMnO}_4$ breaks the C=C apart.
Practice
Adding steam (with an H₃PO₄ catalyst) to an alkene gives:
Electrophilic addition of water (steam) across the C=C makes an alcohol.
Test for C=C
- Shake with orange bromine water: an alkene decolourises it (electrophilic addition); an alkane does not.
Practice
When an alkene is shaken with orange bromine water, the bromine water:
Alkenes decolourise bromine water by electrophilic addition; alkanes leave it orange.
You've got it
Key idea
- alkenes = C=C double bond ($\text{C}_n\text{H}_{2n}$), reactive; made by elimination, dehydration or cracking
- most reactions are electrophilic addition (H₂ → alkane, steam → alcohol, HX → halogenoalkane)
- bromine water test: an alkene decolourises it