Alkenes (Reactions)

Why are alkenes more reactive than alkanes?

Because of the presence of the pi bond

• The pi electrons are located on the outside of the double bond, making it easier to break and thus possible to add atoms/groups to the alkene

What do alkenes undergo addition reactions with? List them.

• Hydrogen

• Halogens (e.g. Cl2)

• Hydrogen halides (eg. HBr or HCl)

• Steam (i.e. H2O as a gas)

What is the reaction between alkenes and hydrogen referred to as?

Hydrogenation

What are the conditions needed for hydrogenation to occur?

• Presence of Nickel catalyst (Ni)

• Temp of 150 deg Celsius

What occurs in a hydrogenation reaction?

• Pi bond breaks

• Two hydrogen atoms add across the C=C double bond.

What is the product of a hydrogenation reaction?

• Alkanes

1) Write down the reaction of penta-1,4-diene with excess H2 (skeletal).

2) Include the conditions needed for this reaction.

3) Name what kind of reaction this is

• Nickel catalyst and temperature of 150 deg C needed.

• C5H8 + 2H2 = (Ni, 150 deg) C5H12

• Hydrogenation

What temperature do alkenes react with halogens at?

Room temperature

What occurs in halogenation reactions?

The pi bond breaks and the 2 halogen atoms add across the C=C double bond.

What is the product of halogenation reactions?

A dihaloalkane

How can we test for alkenes/unsaturation?

• Place alkene in bromine

• If it tests positive the bromine is decolourised

1) Write down the reaction of cyclohexa-1,4-diene with excess chlorine (skeletal).

2) Name what kind of reaction this is

1) C6H8 + Cl2 = C6H8Cl4

2) Halogenation

What temperature do alkenes react with hydrogen halides at?

Room temp

What occurs in hydrogen halide addition reactions?

• Pi bond breaks

• the hydrogen and halogen atoms add across the C=C double bond

What is the product of the halogen halide addition reactions?

Haloalkane

1) Write down the reaction of propene with hydrogen chloride (skeletal). Include the 2 structural isomers

2) Name what kind of reaction this is

1) C3H6 + HCl = C3H7Cl (chloropropane or 2-chloropropane)

2) Hydrogen halide addition

What are the conditions needed for hydration to occur?

• Strong acid catalyst (e.g. H3PO4 or conc. H2SO4)

• Temperature greater than 100 deg

What is the reaction between alkenes and steam referred to as?

Hydration

What occurs in hydration reactions?

• Pi bond breaks

• the hydrogen and OH- group add across the C=C double bond

What is the product formed via hydration reaction?

Alcohol

1) Write down the equation for the hydration of pent-2-ene

1) C5H10 + H2O = (Strong acid catalyst, >100 deg) C5H11OH (pentan-2-ol or pentan-3-ol)