Reactions of Alkenes

Question 1

Reactivity of alkenes

Answer 1

• 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.

Question 2

Hydrogenation of alkenes

Answer 2

• Catalyst: Nickel
• Temp: 423 K
• Hydrogen is added across a double bond, making it single. This is known as hydrogenation.

Question 3

Halogenation of alkenes

Answer 3

• 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.

Question 4

Addition reaction with Hydrogen Halides

Answer 4

• 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

Question 5

Hydration reactions of alkenes

Answer 5

• Reacted with steam.
• Catalyst: Phosphoric acid (H3PO4).
• H and OH adds across the double bond.
• Forms alcohols
• 2 possible products.