Alkenes

IB Chemistry Topic 10.3

What is the difference between alkanes and alkenes?

Which do you think would be more reactive?

4.2.4 State and explain the relationship between the number of bonds, bond length and bond strength.

The more pairs of Electrons that are

shared between two atoms (bonds) in a molecule will

make the attraction between the atoms

stronger and shorter

Lengthnm

Strength(kj mol-1)

C-O 0.143 356

C-C 0.154 348

C=O 0.121 736

C=C 0.134 657

C C 0.120 908

Then why are alkenes MORE reactive than alkanes???

10.3 Alkenes Asmt. Stmts 10.3.1 Describe, using equations, the

reactions of alkenes with hydrogen and halogens.

10.3.2 Describe, using equations, the reactions of symmetrical alkenes with hydrogen halides and water.

10.3.3 Distinguish between alkanes and alkenes using bromine water.

10.3.4 Outline the polymerization of alkenes. 10.3.5 Outline the economic importance of

the reactions of alkenes.

10.3.1

Describe, using equations, the reactions of alkenes with hydrogen and halogens.

10.3.1 Reactions of Alkenes:The general mechanism

Alkenes react with many substances to form a new substance. Catalysts, acids or other substances may be required to complete the reaction:

C2H4 + XY → CH2X CH2Y Process occurs by breaking the double bond.

Addition Reactions

An addition reaction occurs when atoms are added to a molecule

There are different types of addition reactions: Hydrogenation Bromination / chlorination Hydration Polymerization

http://www.youtube.com/watch?v=8G1NXYZKGL0

10.3.1 Reactions of Alkenes: with hydrogen

Alkenes react hydrogen gas to create an alkane, using nickel as a catalyst at 150ºC:

C2H4 + H2 → CH3CH3

10.3.1 Reactions of Alkenes: with halogens

Alkenes react readily with chlorine or bromine to create a di-halo-alkane (what is this?):

C2H4 + 2 Cl2 → CH2Cl CH2Cl

H

H H

H

+HH

Cl

HH

ClCl Cl

10.3.2

Describe, using equations, the reactions of symmetrical alkenes with hydrogen halides and water.

10.3.2 Reactions of Alkenes: with hydrogen halogens

Alkenes react readily with hydrogen halides to create a halo-alkane:

C2H4 + HBr → CH3CH2Br

H

H H

H

+HH

H

HH

ClH Cl

10.3.2 Reactions of Alkenes: with water Alkenes do not react readily with water. If sulfuric acid is used (an intermediate

substance will be made), an alcohol will be created :

C2H4 + H2O → CH3CH2OH

Remember that H2O is the same as HOH,

which can be dissociated into H+ and OH-H

H H

H

+ OHH

a c idO

H

H

H H

H

H

10.3.3 Distinguish between alkanes and alkenes using bromine water.

Bromine water (a red liquid) tests for unsaturated hydrocarbons (alkenes) Alkanes (cyclohexane) → stay yellow/orange

no reaction Alkenes (cyclohexene) → turn clear /

colourless because of reaction with unsaturated hydrocarbon

10.3.3

Distinguish between alkanes and alkenes using bromine water.

• The Fuse School: https://www.youtube.com/watch?v=1r9aYS4Ndac&list=PLW0gavSzhMlReKGMVfUt6YuNQsO0bqSMV&index=84

• http://www.youtube.com/watch?v=NjIuBvod2eM

10.3.4

Outline the polymerization of alkenes.

10.3.4 Reactions of Alkenes: Polymerization

Mono- = one -mer = part Poly- = many So if several monomers are connected (think

Lego’s), you get a … ???

10.3.4 Reactions of Alkenes: Polymerization

Naming polymers: put “poly-” in front of the name of the monomer

There are 3 polymerization mechanisms that you need to be familiar with:

1. Polyethene

2. Polychloroethene

3. Polypropene

10.3.4 Reactions of Alkenes: Polymerization: Polyethene

Monomer: ethene (aka ethylene), CH2=CH2

Conditions: Temp: 200º C Pressure: 2000 atm Initiator: O2

Polyethene

Undergoes additions reactions with itself to make a chain

n CH2=CH2 [-CH2-CH2-]n

10.3.4 Reactions of Alkenes: Polymerization: Polychloroethene

Each chloroethene contains 1 chlorine Therefore when the chloroethene

molecules polymerize, every other carbon will bond to 1 chlorine

This is PVC

10.3.4 Reactions of Alkenes: Polymerization: Polypropene

+ =

10.3.4 Reactions of Alkenes: Polymerization: Types

Addition:

All of the examples we’ve seen are classified as “Addition Polymerization”

vs.

Condensation

The Fuse School: https://www.youtube.com/watch?v=nz1ucI6gCIg&list=PLW0gavSzhMlReKGMVfUt6YuNQsO0bqSMV&index=3

10.3.4 Reactions of Alkenes: A Hint

Draw the structure of your alkene so that the C=C is in the middle and the other parts bond above

and below the carbons.

For example. A polymer of but-2-ene should look like:

=

+

10.3.4 Reactions of Alkenes: Practice

Draw the structure for 7 units of

Poly-2-chloropropene.

10.3.4 Reactions of Alkenes: Reaction Type Definitions

Addition:

Bromination:

Dehydration:

Hydration:

Hydrogenation:

10.3.5

Outline the economic importance of the reactions of alkenes.

10.3.5 Uses of alkene products

Ethene can be used for a variety of purposes: Steam → ethanol Polymerize → polyethene React with chlorine → chloroethene… which

interacts with ethanol → PVC React with benzene → phenylethene

(styrene)… which interacts with ethanol → polystyrene

Outline the economic importance of the reactions of alkenes (10.3.5)

Making margarine Hydrogenation (addition of H) of vegetable oils

(alkenes)

Making ethanol Ethene + water

Alcoholic beverages or use as fuel

H

H H

H

+ OHH

a c idO

H

H

H H

H

H

Making plastics Teflon

Tetrafluoroethene

PVCPolychloroethene

Teflon non-stick pans