4.1.3 Alkenes

Definitions

Term	Definition
Electrophile	An atom or group of atoms which is attracted to an electron-rich centre of atom, where it accepts a pair of electrons to form a new covalent bond, usually a cation or an atom or molecule with \(\delta+\) dipole
Electrophilic addition	An addition reaction in which the first step is attack by an electrophile on a region of high electron density
Addition polymerisation	Formation of a very long molecular chain, by repeated addition reactions of many unsaturated alkene molecules (monomers)

Comprised of
- A \(\sigma\)-bond: head on overlap of orbitals directly between the bonding atoms
- A \(\pi\)-bond: sideways overlap of adjacent p-orbitals above and below the bonding carbon atoms
- The \(\pi\)-bond locks the two carbon atoms in position and prevents them from rotating around the double bond (restrict rotation)
Trigonal planar shape around each carbon atom in the \(C=C\) bond (120° bond angle)
- 3 regions of electron density around each carbon atom (3 bonding regions)
- The 3 regions repel each other as far apart as possible

	\(\sigma\)-bond	\(\pi\)-bond
Position of electron density	Between bonding atoms	Above and below bonding atoms
Overlap of orbitals	Head on overlap of orbitals	Sideways overlap of orbitals
Bond enthalpy / strength	Higher	Lower
Size	Larger	Smaller

Compounds with the same structural formula but with a different arrangement in space

An type of stereoisomerism
Different groups attached to each carbon atom of a \(C=C\) double bond may be arranged differently in space because of the restricted rotation about the \(C=C\) bond
Rotation about a double bond is restricted (due to the \(\pi\)-bond) so the groups attached to each carbon atom are fixed relative to each other

A \(C=C\) double bond
Two different groups to be attached to each carbon atom of the double bond

A special case of E/Z isomerism
One of the attached groups on each carbon atom of the double bond must be the same
Same group on same side = cis, same group on different sides = trans (only works if there is a hydrogen atom bonded to both carbon atoms)

Assigning priority
- Examine the atomic number of the atoms directly attached to the carbon atoms of the double bond
- Higher atomic number = higher priority
- Two same atoms attached to the carbon atom
  - Find the first point of difference
  - Higher atomic number at first point of difference = higher priority
The groups of higher priority are on the same side = Z isomer
The groups of higher priority are diagonally placed across the double bond = E isomer

Much more reactive than alkanes
Relative low bond enthalpy of the \(\pi\)-bond so it is broken more readily
- It is on the outside of the \(\sigma\)-bond so its electrons are more exposed

Reaction	Condition	Detail
Hydrogenation	- Nickel catalyst - 423 K (150°C) - High pressure	- Alkene + hydrogen \(\rightarrow\) alkene / \(R-CH=CH_2 + H_2 \rightarrow R-CH_2-CH_3\) - Type: hydrogenation / addition
Halogenation	- RTP	- Alkene + halogen \(\rightarrow\) dihaloalkane e.g. \(R-CH=CH_2 + Br_2 \rightarrow R-CHBr-CH_2Br\) - Type: electrophilic addition (see below for mechanism) - Reaction of alkenes with bromine can be used to test if the organic compound is unsaturated - Bromine water added dropwise to alkene - Bromine adds across the double bond - The orange colour of bromine water disappears - Added to an saturated compound: no addition reaction so no colour change
Addition with (gaseous) halogen halides	- RTP	- Alkene + halogen halide \(\rightarrow\) haloalkane e.g. \(R-CH=CH_2 + HBr \rightarrow R-CHBr-CH_3\) - Type: electrophilic addition (see below for mechanism) - Alkene is a gas: reaction takes place when the two gases are mixed - Alkene is a liquid: hydrogen halide bubbled through it - Can also react with concentrated hydrochloric or hydrobromic acid - Two possible products
Hydration	- Steam - Phosphoric acid (\(H_3PO_4\)) catalyst	- Alkene + \(H_2O(g) \rightarrow\) alcohol - Type: hydration - \(R-CH=CH_2 + H_2O \rightarrow R-CH(OH)-CH_3\) - Two possible products

Electrophile = \(\delta+\) atom (accepts the \(\pi\)-electrons from the double bond)
Electron pair in the \(\pi\)-bond is attracted to the \(\delta+\) atom \(\rightarrow\) double bond breaks
A bond forms between the \(\delta+\) atom and a carbon atom from the double bond
The bond in the molecule breaks by heterolytic fission, electron pair goes to the \(\delta-\) atom
An anion and a carbocation (positively charged carbon atom) are formed
They react to form the addition product

Type	Definition
Primary	1 alkyl group attached to the positively charged carbon atom
Secondary	2 alkyl groups attached to the positively charged carbon atom
Tertiary	3 alkyl groups attached to the positively charged carbon atom

For unsymmetrical alkanes
Major product is formed from the most stable carbocation intermediate
Stability: tertiary carbocation > secondary carbocation > primary carbocation
Halide / \(OH^-\) ion attached to the carbon atom attached to the carbon atom with the least hydrogens attached / most alkyl groups attached
The hydrogen attaches itself to the carbon atom with the most hydrogens attached

Short chain monomers join together to form long chain polymers under high pressure
Double bond of the alkene is replaced by single bonds to form a repeating unit + bond with other monomers to form the polymer
Addition polymers as the short chains join together to form a single product

Benefits of cheap oil-derived plastics are counteracted by problems for the environment of landfill
They are unreactive so they are non-biodegradable and cannot be broken down by species in nature
Non-biodegradable waste polymers can become a threat to wildlife

Recycle
- High cost of collection and re-processing
- The different types of polymer have to be separated
Combustion to release heat energy for generating electricity
- Toxic fumes produced from burning halogenated polymers
  - HCl is removed during the combustion of chlorine containing haloalkanes
  - CO produced during incomplete combustion
  - Can be removed by scrubbing in the chimney
- Greenhouse gases can be released which causes global warming
Organic feedstock
- Use the waste for the production of useful organic compounds
- New technology can convert waste into hydrocarbons
- Hydrocarbons can then be turned back into polymers

Biodegradable polymers
- Broken down by microorganisms into water, \(CO_2\) and organic compounds
- Compostable polymer degrade and leave no visible or toxic residues
- e.g. can be used as bin liners for food waste
Photodegradable polymers
- Contain weak bonds that break when they absorb light energy
Benefits
- Conserve fossil fuel reserves
- Reduce pollution from disposing polymers