Flashcards in Reactions of Alkenes Deck (5)
Reactivity of alkenes
- Alkenes are much more reactive than alkanes because of pi-bonds.
- As electrons are concentrated on the outside of the double bond, they are more exposed than electrons in sigma bond.
- So pi bond has relatively low bond enthalpy.
- Pi bond readily breaks and alkenes undergo addition reactions easily.
Hydrogenation of alkenes
- Catalyst: Nickel
- Temp: 423 K
- Hydrogen is added across a double bond, making it single. This is known as hydrogenation.
Halogenation of alkenes
- With halogens chlorine or bromine.
- Temp: Room temp
- Halogen is added across a double bond.
- Make Dihaloalkanes
- Reaction of alkenes w/ bromine can b used to identify if there is C=C bond present and the organic compound is unsaturated.
- Add bromine water (orange), this makes the solution decolourise, indicating presence of double bond.
- Bromine in bromine water adds across the double bond.
- Test carried out on saturated compound will show no addition reaction and no colour change.
Addition reaction with Hydrogen Halides
- Temp: Room temp.
- React w/ gaseous hydrogen halides. If alkene is liquid, hydrogen halide is bubbles through it.
- Form haloalkanes.
- Often 2 possible products cause the halide can join either carbon atoms in c=c