Alkenes

IB Chemistry Topic 10.2

What is the difference between alkanes and alkenes?

Which do you think would be more reactive?

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 ● the distance between

the atoms shorter

Lengthnm

Strength(kj mol-1)

C-O 0.143 356C-C 0.154 348C=O 0.121 736C=C 0.134 657C C 0.120 908

Then why are alkenes MORE reactive than alkanes???

Order of ReactivityAlkynes>Alkenes>Alkanes

It requires less energy to break ONE of the bonds in a C=C double bond than to break a C-C single Bond

Ex. Energy to break ONE bond from a C=C:

● 657/2 = 328.5 kJmol-1 vs 348 kJmol-1 of a C-C bond

Lengthnm

Strength(kj mol-1)

C-O 0.143 356C-C 0.154 348C=O 0.121 736C=C 0.134 657C C 0.120 908

Ex. Energy to break ONE bond from a C≡C

● 908/3 = 302.7 kJmol-1

Reactions of Alkenes:The general overview● 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=oPdOt8Z8oKU

Reactions of Alkenes: with hydrogen● Alkenes react hydrogen gas to create an

alkane, using nickel as a catalyst at 150ºC:

C2H4 + H2 → CH3CH3

Reactions of Alkenes: with halogens● Alkenes react readily with chlorine or

bromine to create a di-halogeno-alkane:C2H4 + 2 Cl2 → CH2Cl CH2Cl

Reactions of Alkenes: with hydrogen halogens● Alkenes react readily with hydrogen halides

to create a halogeno-alkane:C2H4 + HBr → CH3CH2Br

Reactions of Alkenes: with water● Alkenes do not react readily with water. ● If concentrated sulfuric acid is used as a

catalyst, an alcohol will be created :C2H4 + H2O → CH3CH2OH

● Remember that H2O is the same as HOH, which can be dissociated into H+ and OH-

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

Distinguish between alkanes and alkenes using bromine water.

• The Fuse School: https://www.youtube.com/watch?annotation_id=annotation_2341911331&feature=iv&src_vid=1r9aYS4Ndac&v=qEm-CaqhcOs

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

Reactions of Alkenes: Polymerization ● Mono- = one● -mer = part● Poly- = many ● So if several monomers are connected (think

Lego’s), you get a … ???

Reactions of Alkenes: Polymerization● Naming polymers: put “poly-” in front of the

name of the monomer● There are 3 polymerization reactions that

we will look at as examples:1. Polyethene2. Polychloroethene3. Polypropene

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

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

Reactions of Alkenes: Polymerization: Polypropene

+ =

10.3.4 Reactions of Alkenes: Polymerization: TypesAddition: 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=PLW

0gavSzhMlReKGMVfUt6YuNQsO0bqSMV&index=3

10.3.4 Reactions of Alkenes: A HintDraw 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:

= +

Reactions of Alkenes: Practice

Draw the structure for 7 units of Poly-2-chloropropene.

Uses of alkene productsEthene 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

Economic importance of the reactions of alkenes● Making margarine

● Hydrogenation (addition of H) of vegetable oils (alkenes)

● Making ethanol● Ethene + water

● Alcoholic beverages or use as fuel

● Making plastics● Teflon

● Tetrafluoroethene

● PVC● Polychloroethene

Teflon non-stick pans