OCR A-Level Chemistry: Chapter 13 - Alkenes

Unsaturated

CnH2n

Three sigma bonds formed.
One electron left in a p-orbital.
A pi bond forms from the sideways overlap of two p-orbitals.

The pi bond as it locks the two carbon atoms in position.

Trigonal planar

Three regions of electron density

Repel each other equally

In the same plane

120 degree bond angle

The same structural formula but a different arrangement of the atoms in space.

E/Z isomerism

Optical isomerism

Molecules have a C=C double bond

Each carbon in the double bond is attached to two different groups.

Hydrogen atoms on each carbon on the same side.

Hydrogen atoms diagonally opposite each other.

Higher atomic number is higher priority.

Groups of higher priority on the same side of the double bond, then Z isomer.

An alkene is mixed with hydrogen and passed over a nickel catalyst at 423K forming an alkane.

Rapid addition reaction between an alkene and chlorine or bromine at room temperature to form a halogenoalkane.

When bromine water (orange) is added to the sample of an alkene, the bromine reacts with the alkene and the colour disappears.

An alkene is reacted with steam in the presence of a phosphoric acid catalyst forming an alcohol (two products can be formed).

Electrophilic addition

H-Br is a polar molecule

Electron pair in pi bond attracted to H

Double bond breaks

Bond forms H and C atoms

H-Br bond breaks by heterolytic fission

Bromide ion and carbocation formed

Br- reacts with carbocation

When the molecule approaches the pi bond, the electrons in the bond are repelled and this induces polarity.

In electrophilic addition reactions, when the carbocation is formed it can be primary, secondary or tertiary.
The tertiary is the most stable so is the most likely product.

The connecting alkyl groups donate and push electrons towards the positive charge of the carbocation.
Therefore more alkyl groups means a more stable ion.

Addition polymerisation

Displayed formula with an n before it to signify many units.

Displayed formula
In square brackets
An n placed after to show a large number of repeats

High temperature and pressure

Childrens toys, packing crates, guttering, uPVC windows, rope fibre.

Packaging material, food trays and cups.

Coating for non stick pans, permeable membrane for clothing and shoes, cable insulation.

Very unreactive so are non-biodegradable, with serious environmental effects.

Reduces environmental impact by conserving fossil fuels and decreasing waste.

Polymers have to be sorted by type.

| Process is undermined if polymers are mixed as renders product unusable.

Once sorted, they are chopped into flakes, washed, dried, melted and then cut into pellets for use.

High chlorine content and range of additives present make it hazardous.
When burnt it releases the corrosive hydrogen chloride gas and toxic dioxins.

Solvents used to dissolve the polymer and the high grade PVC is recovered by precipitation and the solvent can be used again.
Previous recycling, PVC was ground down and then reused.

Waste polymers can be incinerated to produce heat for power plants.

Chemical and thermal processes that can reclaim monomers, gases and oils from waste polymers.

Able to handle unsorted and unwashed polymers

Produced from renewable sources and can be broken down by microrganisms. Made from starch or cellulose.

Contain bonds that are weakened by absorbing light to start degradation or light-absorbing additives are used.