3.4 ALKENES

what is an alkene

unsaturated hydrocarbon

C_nH_2n

what does C=C consist of

one sigma bond

one pi bond

→ pi bonds are exposed. high electron density.

→ → they’re therefore vulnerable to attack by electrophiles

what reagents are used for electrophilic addition of alkenes

• Halogen

• hydrogen halide

• sulfuric acid (electrophilic addition for stage 1, hydrolysis for stage 2)

• (condition for all: room temperature)

how to test for alkenes

Bromine water.

positive result: orange → colourless

Hydration

alkenes are industrially converted to alcohols in one step, they’re reacted with in the presence of an acid catalyst.

conditions:

300-600C

70atm (high P, can’t be done in lab, but high atom economy + easier and cheaper)

catalyst: H₃PO₄

how are addition polymers formed from alkenes

addition polymerisation

with high pressure catalyst

poly(chloroethene) properties

waterproof, electric insulator, doesn’t react w/ acids.

• in pure form, a rigid plastic (bc the strong IMF between polymer chains prevents them from moving over each other.)

what is poly(chloroethene) used for (unplasticised)

• in unplasticised form, used to make uPVC window frame coverings and guttering.

what is poly(chloroethene) used for (plasticised)

• if plasticiser is added, IMFs are weakened

• → this allows chain s to move easily, so the polymer is more flexible.

• in this form, PVC is used to make insulation on electrical wires and waterproof clothes.