Chemistry - 3.3.4: Alkenes

Alkenes

Unsaturated hydrocarbons

Why are alkenes more reactive than alkanes?

The double bond is an area of high electron density - can be attacked by electrophiles (positively charged reagents)

General formula for alkenes

CnH2n

Why can't double bonds rotate?

There is a p-orbital (has single electron) on each carbon - these overlap to form cloud of electron density above and below single bond: a pi orbital (pi orbitals do not allow rotation)

What alkenes can form isomers?

Alkenes with more than three carbons

3 types of isomerism in alkenes

Positional isomerism, chain isomerism and stereoisomerism (geometrical isomers)

Physical properties of alkenes vs alkanes (3)

- Have similar physical properties (double bond does not greatly affect BP/MP - only VdWs forces are present)
- BP/MP increases with number of carbon atoms present
- Insoluble in water

Electrophile

Lone pair acceptor - positively charged ion or has a positively charged area

Why are alkenes not used as fuels?

Their reactivity makes them very useful for other purposes

3 steps of electrophilic addition reactions with hydrogen halides

1. H+ of a molecule is attracted to double bond (curly arrow from C=C double bond to H+) [H-X bond is polar due to difference in electronegativity]
2. The bond between the H+ and X- (halide) breaks heterolytically and the lone pair of electrons goes to X (arrow from single bond to X)

3. Step 1 forms a carbocation, which attracts X- (arrow from lone pair on X- to carbocation) - forms halogenoalkane

Carbocation

An organic ion in which a carbon atom has a positive charge

When are there two products in addition reactions?

When the alkene is asymmetrical - double bond not exactly in middle of chain

What are the two possible products in addition reactions with hydrogen halides?

- Major product: forms when added halide is bonded to carbon bonded to most alkyl groups (more stable carbocation)
- Minor product: forms when added halide is bonded to least alkyl groups

In electrophilic addition, why is more of the major product produced? (3)

Major product is formed from a secondary/tertiary carbocation, while the minor product is formed from a primary/secondary* carbocation - the secondary/tertiary carbocation is more stable

* secondary only applies as minor product if major product is tertiary. primary could be for either.

Tertiary carbocation

A positively charged carbon atom which has three alkyl groups

Secondary carbocation

A positively charged carbon atom which has two alkyl groups

Primary carbocation

A positively charged carbon atom which has one alkyl group

3 steps of electrophilic addition reactions with halogens

1. Halogen (X2) experiences instantaneous dipole - Xδ+ is attracted to double bond (curly arrow from C=C double bond to Xδ+)
2. The bond between the δX+ and Xδ- breaks heterolytically and the lone pair of electrons goes to Xδ- (arrow from single bond to Xδ-)

3. Step 1 forms a carbocation, which attracts Xδ- (arrow from lone pair on Xδ- to carbocation) - forms halogenoalkane

How to test for unsaturated hydrocarbons?

Add bromine water - colour change from brown to colourless

3 steps of electrophilic reactions with sulfuric acid

H2SO4 = Hδ+ -- Oδ- -- SO3H
1. Hδ+ of a molecule is attracted to double bond (curly arrow from C=C double bond to H+)
2. The bond between the Hδ+ and Oδ- breaks heterolytically and the lone pair of electrons goes to O (arrow from single bond to Oδ-)

3. Step 1 forms a carbocation, which attracts Oδ- (arrow from lone pair on Oδ- to carbocation) - alkyl hydrogensulfate

Polymer

A very large molecule that is built up from small molecules, called monomers

Monomers

The small molecules that make up polymers

Addition polymers

Polymers made from a monomer/monomers with a carbon-carbon double bond (alkenes) - made from monomers based on ethene

General formula for the monomers of addition polymers

H H
| |
n C = C
| |
H R

What happens when monomers polymerise to form an addition polymer?

The double bond opens and the monomers bond together

General formula for an addition polymer

-[CH2-CHR]- n

What does R represent in addition polymers?

An alkyl or aryl group

PVC

Poly(chloroethene) - PVC = polyvinyl chloride

What is PVC used for?

Aprons, 'vinyl' records, drainpipes

Are addition polymers reactive?

No - they are unreactive due to their strong, non-polar C-C and C-H bonds

How can you modify polymers?

Adding plasticisers to make them more flexible

Plasticiser

A small molecule that gets between polymer chains, forcing them apart and allowing them to slide across each other

Are polyalkenes biodegradeable?

No - they are unreactive, so are not attacked by biological agents

2 ways to recycle plastics

1. Mechanical recycling: plastics separated and washed, then ground into small pellets - these are melted and remoulded
2. Feedstock recycling: plastics heated to very high temperature so polymer bonds break - this forms monomers, which are then used to make new plastics

Addition reaction

A reaction which increases the number of substituents or converts a double bond to single bond

Why might an alkene not have E-Z isomerism?

It has 2 H atoms bonded to one carbon of the double bond

Why is there an attraction between C=C and Br2?

C=C is an area of high electron density - the Br-Br becomes polarised, so the Br δ+ is attracted to the C=C

Name the three processes involved in the formation of poly(propene) and explain the importance of each step.

1. Fractional distillation: To separate into mixtures of compounds with similar boiling points
2. Cracking: to make alkenes / shorter molecules
3. Addition polymerisation: to produce a long chain molecule by joining the molecules together

Why are addition polymers unreactive?

The bonds are non-polar