Addition reactions of alkenes

Question 1

What are the conditions needed for an alkene to react with hydrogen to form an alkane?

Answer 1

• A mixture of hydrogen gas and a gaseous alkene is passed over a catalyst of nickel at a temperature of 150*C. - Reaction sometimes known as hydrogenation.

Question 2

Write out the reaction of ethene with hydrogen and state the conditions needed.

Answer 2

Question 3

Describe the conditions needed for an alkene to react with the halogens.`

Answer 3

• Alkenes react rapidly with halogens such as chlorine,broine and iodine at room temperature.
• The reaction is known as halogenation.

Question 4

Explain the test for unsaturation using bromine.

Answer 4

When bromine is added to a sample containing an alkene, the colour chages from orange to colourless. The colour change indictes that the bromine has reacted with the double bond.

Note: ( The solution appears orange due to bromine appearing orange in solutions so when the bromine is used up to form a halogenoalkane the solution becomes colourless.)

Question 5

Draw the reacton for propene with bromine and state the conditions required.

Answer 5

• Reacts with bromine at room temperature.

Question 6

What are the conditions needed for an alkene to react with a hydroge halide?

Answer 6

• Hydrogen halides are gases at room temperature
• They are bubbled into liquid alkanes.

Question 7

Draw ethene reacting with HBr and state the conditions required.

Answer 7

-Hydrogen halides are gases at room temperature an are bubble into liquid alkanes.

Question 8

Explain the conditions required for alkenes to react with steam.

Answer 8

• Steam and gaseous alkene are heated to a high temperature and pressure in the presence of a phosphoric acid catalyst.
• The process is often referred to as the hydration of an alkene.

Question 9

Write the reaction of ethene reacting with steam and state the conditions.

Answer 9

• Steam and gaseous alkene are heated to a high temperature and pressure in the presence of a phosphoric acid catalyst.

Question 10

Draw the electrophilic addition reaction of HBr with ethene.(Draw the 1st stage).

Answer 10

Question 11

Draw the electrophlic additon reaction of HBr with ethene( 2nd stage).

Answer 11

Question 12

Draw the electrophilic addition reaction of Cl₂ reacting with ethene.

Answer 12

Question 13

Describe, with the aid of suitable equations, the mechanism for the reaction between methane and bromine in the presence of ultraviolet radiation.

Answer 13

1) Balanced equation: CH₄+ Br₂——–>CH₃Br + HBr
2) Initiation:
- Radicals are fomed by the presence of UV light which provides the energy to break the covalent bond in a bromine molecule, Br-Br. The bond breaks by homolytic fission

Br₂——> 2Br*

3) Propagation:
- The products of the reaction are made.

Br^* + CH₄ —–> CH₃^* + HBr

CH₃^* + Br₂ ——-> CH₃Br + Br^*

Note: Bromine radicals catalyse the propagation stage. Although they are involved in the reaction steps, they are not used up in the overall reaction.

3) Termination: In the final stage of the mechanism, any two radicals can combine.

CH₃^*+Br^*——> CH₃Br

Br^*+Br^*——> Br₂

CH₃^*+ CH₃^*——-> C₂H₆