Alkenes

Describe the structure of alkenes

• Unsaturated hydrocarbons

• Contains a C=C bond comprising a Pi-bond and a sigma bond

• Restricted rotation of the Pi bond

What is a Pi bond?

Sideways overlap of adjacent p-orbitals above and below the bonding C atoms

Explain the trigonal planar shape of an alkene C=C bond

• 3 bonding regions & 0 lone pairs

• Electron pairs repel as far as possible

• Creating trigonal planar shape with bond angle 120

Define the term “stereoisomers”

Same structural formula, different arrangement of atoms in space

Define the term “E/Z isomerism”

An example of stereoisomerism. Molecules must have

• A C=C bond

• Different groups must be attached to each carbon of the double bond

Define the term “cis-trans isomerism”

A special case of E/Z isomerism, where 2 of the groups attached to each carbon atom of the C=C group are the same

Cahn-Ingold-Prelog rules

• If higher priority groups are on the same side = Z isomer

• If higher priority groups are diagonally places across the double bond = E isomer

Explain why alkenes are more reactive than alkanes

• Alkenes have Pi-bond present

• Pi-bond has a low bond enthalpy, so it’s broken more readily

• Less energy is needed to overcome the bond

Hydrogenation of alkenes

• Alkene + Hydrogen gas —> alkane

• Nickel catalyst at 150C

Halogenation of alkenes

Alkene + halogen —> Haloalkane

Test for C=C double bond

• Bromine water decolourises in presence of C=C double bond

• From orange to colourless

Alkene reaction with Hydrogen Halides

• Alkene + Gaseous Hydrogen Halides —> Haloalkanes

• If alkene is gas - 2 gases are mixed

• If alkene is liquid - hydrogen halide is bubbled through

Hydration of alkenes

• Alkene + steam —> alcohol

• Phosphoric acid catalyst >100C

Define the term “electrophile”

An electron pair acceptor

Type of mechanism between alkenes and hydrogen halide to form haloalkane

Electrophilic addition

Mechanism for but-2-ene & HBr to form 2-bromobutane

Using Markowinkoff’s rule to predict the major & minor product of electrophilic addition

• Tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations.

• The more stable product = major product

• The less stable product = minor product

Describe the benefits of processing waste polymers by combustion for energy production

• Can be incinerated to produce heat, generating steam to drive a turbine producing electricity - renewable energy source

Describe the benefits of using waste polymers as organic feedstock for production of plastics & other organic chemicals

• Able to handle unsorted & unwashed polymers

• Products of feedstock recycling can be used as raw materials for production of new polymers

Describe the benefits of using waste polymers to remove toxic waste products during the disposal by combustion of PVC

• Combustion of PVC produces HCl, a corrosive gas

• Solvents are used to dissolve the polymer.

• High grade PVC is recovered by precipitation from the solvent

• Solvent is used again

Benefits of biodegradable polymers on sustainability

• Polymers made of starch or cellulose, which are broken down by microorganisms

• They degrade and leave no visible or toxic residue

Benefits of photodegradable polymers on sustainability

• Oil-based polymers

• Contain bonds which weaken when absorbing light to start degradation