Chapter 13.3 - Reactions of alkenes

Question 1

Why are alkenes more reactive than alkanes?

Answer 1

Due to the presence of the pi bond
- The pi electrons are more exposed than the electrons in the sigma bond as they are on the outside of the double bond
- So the pi bond readily breaks(lower bond enthalpy than sigma bond) and alkenes undergo addition reactions relatively easily

Question 2

What is the name of the reaction mechanism of alkenes?

Answer 2

Electrophilic addition

Question 3

What are the four addition reactions of alkenes?

Answer 3

1) Hydrogenation
2) Halogenation
3) Hydrohalogenation
4) Hydration

Question 4

What is hydrogenation?

Answer 4

When alkenes are mixed with hydrogen and passed over a nickel catalyst at 423K, forming alkanes

Question 5

What is halogenation?

Answer 5

When alkenes are mixed with halogens at room temperature, forming dihaloalkanes

Question 6

What is hydrohalogenation?

Answer 6

When alkenes react with gaseous hydrogen halides at room temperature, forming haloalkanes

Question 6

Describe the test for unsaturated hydrocarbons

Answer 6

• Add drops of bromine water, an orange solution, to a sample of hydrocarbon
• If the hydrocarbon is an alkene, the orange bromine water would turn colourless, indicating the presence of alkenes
• This is because the C=C double bond breaks and the bromine molecules are added across the double bond

Question 7

How many products are formed during the addition reaction of alkenes with hydrogen halides?

Answer 7

• Symmetrical molecule = 1 product
• Unsymmetrical molecule = 2 products, one with a high percentage and the other with a low percentage yield

Question 7

What is hydration?

Answer 7

When alkenes react with steam, in the presence of H₃PO₄ or H₂SO₄ catalyst, forming alcohols