4.1.3 Alkenes: Module 4: Core Organic Chemistry: Chemistry OCR A A Level

What are alkenes?

Unsaturated hydrocarbons that contain at least one C=C bond made up of a 𝜋 bond and a 𝜎 bond.

What is the general formula of alkenes?

CnH2n

How is a 𝜋 bond formed?

Electrons in the adjacent p orbitals overlap above and below the carbon atoms. They can only be made after a 𝜎 bond is formed.

What bond restricts the rotation of carbon atoms?

𝜋 bond

What is the angle and shape of a double bond?

Trigonal planar: 120°

Are they more or less reactive than alkanes? Why?

More reactive due to high electron density of double bond and the fact the pi-bond is slightly easier to break.

What intermolecular forces of attraction do they have?

Only London forces due to non-polar bonds.

Are they soluble in water? Why?

No, non-polar bonds.

What are the types of isomers that can be formed using alkenes?

E/Z isomers - due to the restricted rotation.

Cis-trans isomers - if two of the same substituents are attached to each carbon.

What is an electrophile?

Species that are electron pair acceptors.

What is the most stable type of carbocation intermediate? Why?

Alkyl groups have a positive inductive effect, so the most stable carbocation is the one bonded to the most other carbon atoms. i.e. A tertiary carbocation.

Major products will be formed from which kinds of carbocations?

Tertiary (or the most stable available)

What conditions are needed for the electrophilic addition of H2O to an alkene? What is this type of reaction called?

Steam in the presence of an acid catalyst, usually phosphoric acid. Reaction is called hydration.

What are the product(s) of the hydration reaction?

An alcohol.

What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene?

Hydrogen halide gases must be at room temperature.

What is the reaction called when a halogen is added to alkene?

Halogenation

How does a molecule with a non-polar bond react as if it is an electrophile?

C=C double bond with a high electron density induces a temporary dipole in the halogen molecule → δ+ atom attracted to double bond.

How can an alkene be converted into alkane? What is the reaction called and what are the required conditions?

Alkene + hydrogen = Alkane

Hydrogenation

Conditions → 150°C, nickel catalyst

What is an addition polymer?

Many monomers bonded together via rearrangement of bonds without the loss of any atom or molecule.

What are monomers? What form do they usually take?

Molecules which combine to form a polymer. Usually have a C=C bond which breaks to leave a repeating pattern.

What are the ways in which plastics can be disposed?

● Landfill

● Combustion

● Electricity generation

● Reuse

● Recycle

● Organic feedstock

What are the disadvantages of recycling?

● Plastics must be sorted into different types

● Expensive

● Labour intensive

● Requires high technology

How does photodegradable polymers break down?

They are broken down chemically using energy with wavelengths similar to light. Once the break down begins it is not possible to stop the process.

Explain what happens in organic feedstock.

Plastics are separated and broken down into small organic molecules through a series of reaction. The molecules can then be used produce plastics and in other industries.

Give a disadvantage of photodegradable polymers.

May not be exposed to sufficient light.