Alkenes (without addition polymers e.g monomers, repeating unit)

What type of hydrocarbon is an alkene?

Unsaturated

What is produced when ethene reacts with bromine?

1,2 dibromoethane

What happens during the reaction of bromine with an alkene?

The solution changes colour from orange to colourless

Electrophile definition

A positively charged species which accepts a lone pair of electrons to form a covalent bond.

How to test for unsaturation

Bromine water

What type of mechanism do Alkenes undergo?

Electrophilic addition

Why is the C=C bond attracted by electrophiles?

The C=C is a centre of high electron density

Which molecules do Alkenes undergo Electrophilic addition with?

HBr

Br2

H2SO4

what are the steps of Electrophilic addition?

• the electron rich C=C bond induces à Dione in the bromine molecule and it becomes polarised.

• The d+ Br acts as an electrophile and anew bond is formed with one of the carbon atoms

• The bromide ion also produced reacts with the other carbon atom on the carbocation, donating an electron

• Another carbon-bromine covalent bond is formed

Reaction of an alkene with Br2

Reaction of an alkene with H2SO4

Why can you get multiple different products from Electrophilic addition reactions?

The alkene is unsymmetrical

What determines whether a product is major or minor?

The more stable intermediate carbocation is the major product.

What is a primary carbocation?

Where the positive carbon atom is attached to one other carbon atom.

What is a secondary carbocation?

Where the positive carbon atom is attached to two other carbon atoms.

What is a tertiary carbocation?

Where the positive carbon atom is attached to three other carbon atoms.

Reaction of an alkene with HBr

What is the order of stability of the carbocation intermediates?

tertiary>secondary>primary

Explain why some carbocations are more stable than others.

• electron releasing alkyl groups have a positive inductive effect towards the carbocation.

• This minimises the positive charge on the carbocation, making it more stable.

• The more alkyl groups around the carbocation the grater the positive inductive effect.

Model answer what is the major and a minor product?

The major product is product as it is formed via the carbocation type which is more stable that the lower carbocation type by which the minor product, product, is formed.

Why is sulphuric acid used in Electrophilic addition?

• Sulphuric acid dissociates to release hydrogen ions

• Hydrogen ions can act as electrophiles

What possible reactions can happen after an alkene reacts with sulphuric acid?

• Hydrolysis by the addition of water

• Ethanol and sulphuric acid are produced

What does sulphuric acid act as after hydrolysis after it has reacted with an alkene?

• a catalyst

What is the equation of hydrolysis with the product of a reaction between sulphuric acid and an alkene?

What type of of polymerisation do alkenes undergo?

Addition polymerisation

Why are addition polymers chemically unreactive?

• very strong C-C and C-H bonds

• The lack of polarisation of the C-C and C-H bonds.

Why are addition polymers non biodegradable?

The non-polar carbon-carbon bond in the chain cannot be broken down by nucleophiles or hydrolysed

What is a property of addition polymers?

Non biodegradable

What is the strongest intermolecular force between polymer chains in hydrocarbon addition polymers?

Van der Waals

What is the strongest intermolecular force in PVC and how does it come about?

• dipole-dipole

• the C-Cl bond is strongly polar

What is PVC used for and why?

• drainpipes

• It is strong and rigid

• Due to dipole dipole intermolecular forces between chains

• due to strongly polar C-Cl bonds

What does PVC stand for?

polychloroethene

How can PVC be made more flexible?

• By adding a plasticiser

• Plasticiser molecules penetrate polymer and increase distance between polymer chains

• So polymer chains can slide over each other