Module 4: Chapter 13 Alkenes

Alkenes are unsaturated aliphatic hydrocarbons with double carbon-carbon bonds (C=C is the functional group)

Start of alkene name (+a if there is more than one double bond) - at what position is the double bond - ENE (-di, -tri, -tetra etc added if more than one double bond is there) e.g. buta-1,3-diene • if there are any alkyl groups they go at the beginnings of the alkene name with regular rules

• three regions of electron density around each carbon atom • 3 regions of electron density repel to be far apart as possible • trigonal planar shape formed with a bond angle of 120°

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

an example of stereoisomerism that needs the following conditions: • no free rotations around the C=C bond • Different groups attached to each carbon atom of the C=C bond · a E isomer has the the non-hydrogen groups diagonal and Z isomers have them both on the up/down side of the C=C bond

• a sigma bonds is formed between the carbon atoms • a pi bonds is formed between the carbon atoms from the overlap of p-orbitals (the pi bond is above and below the plane but is only a single entity)

a special case of E/Z isomerism in which two of the substituent groups attached to each carbon atom of the C=C group are the same. • cis isomers (Z isomers) have the hydrogen atoms on each carbon in this double bond on the same side of the molecule • trans isomers (E isomers) have hydrogen atoms diagonally opposite each other

• priority rule to work out wether a molecule is an E/Z isomer • take each carbon apart of the C=C bonds in turn and the atom with the biggest atomic number is the high priority • if the high priority atoms are on the same side then it’s a Z isomer, if they are diagonal it’s a E isomer

• alkenes tend to undergo addition reactions - part of the C=C bond breaks and an unsaturated substance is normally converted into a saturated one • the pi bond is weaker than the sigma bond so breaks more easily

An electrophile is a species that is attracted to areas of negative charge. Electrophiles are able to accept a pair of electrons

Hydrogenation alkene + H-H -> alkane nickel catalyst used unsaturation to saturation

• for example could form a 1,2-dichloroethane • conditions: room temperature and pressure • this reaction can be used to test for unsaturation (bromine water, Br2 in H2O, will change from orange to colourless on shaking)

• produces a halogenalkane • conditions: room temperature and pressure and inert solvent • H-Cl or H-Br

alkene + H-OH -> alcohol - conditions: (330°c, 600MPa) gaseous alkene and steam passed over a concentrated phosphoric acid catalyst

An addition reaction has a single product - the reactants have ‘added’ together

• asymmetrical alkenes produce 2 addition reaction products • the major product is always the product that is produced from the most stable carbocation • the more alkyl groups (carbons) attached to the carbocation the more stable the carbocation, tertiary carbocations are the most stable • the attached alkyl group has an electron-donating ability, the more alkyl groups attached the more the donation and the greater the spreading the charge and the greater the stability

• a test for alkenes is shaking with bromine water (the colour will change from orange to colourless) e. g. ethene + bromine -> 1,2-dibromoethane • this is an electrophilic addition reaction • bromine molecule becomes polarised as it approaches an alkene • partially positively charged bromine behaves as an electrophile and reacts with the double bond • one of the carbon atoms now has a positive charge (carbocation) • carbocations react very rapidly with anything that has electrons to share such as the bromide ion also formed in this reaction • as bromine water is used a lone pair of electrons on the oxygen of a water molecule can also react with carbocation forming bromoalcohol and a H+ ion (mixtures of products)

• alkenes tend to undergo addition reactions where monomer alkenes are joined together to form larger molecules • conditions: high temp, high pressure and with a catalyst • addition polymers have a high molecular mass, they are sometimes called macromolecules • a repeat unit is used to show a section of the polymer

• low density polyethene (LDPE) are branched molecules in polymer, flexible and light (e.g. used in plastic bags) • high density polyethene (HDPE) linear chain in polymer, strong and rigid (e.g. used to make plastic bottles)

• also known as PVC or polyvinyl chloride • used a chloroethene monomer (C2H3Cl) • used in window frames and plastic bottles

• Recycling polymers - washed and melted for reuse, shredded for insulation/filling use or can be made into new products • PVC is dissolved in a solvent as it can’t be recycled (must involve as pure starting products as possible) • Incineration - polymers are made from the petroleum fraction of crude oil, so they readily burn • Feedstock - some polymers are thermally/chemically broken into monomers and become raw materials for new products (no washing or sorting for this)

• some new polymers are biodegradable, microbes breakdown down the polymers into water, CO2 and biological compounds, they use starch or cellulose to link/make polymer chains • compostable bags are made this way • disposable cups cane be made of sugar cane fibre replacing conventional polystyrene cups

Oil based polymer that break down when exposed to sunlight