Chapter 13 Flashcards
What are alkenes?
They are unsaturated hydrocarbons. (When naming, give the double bond the lowest number)
What is a pi-bond and briefly explain the formation?
Two regions of electron density above and below a sigma bond. Each C has 4 electrons, so when 3 bond, 1 is left free in a p-orbital. The sideways overlap of p-orbitals creates a pi-bond.
What does a pi-bond restrict?
Rotation around carbon atoms in double bond. Alkanes do not have this restriction as of all single (sigma bonds)
What is E/Z isomerism?
Type of stereoisomerism which only occurs with a C=C bond as pi-bond restricts rotation + when there are 2 different groups attached to each C of the C=C bond. E is for diff. sides, Z is for the same side.
With E/Z isomerism, if there are different groups on the same side of the C=C, what gets priority?
The atoms with the highest atomic number get priority. If the high priorities are on the top for both sides it’s a Z. Visa versa.
What is cis/trans isomerism?
When there is a C=C bond, two diff. groups on each C, but one of the attached groups on each C must be the same. Cis is same side, trans is different side.
Why are alkenes more reactive than alkanes?
The pi-electrons are more exposed than the σ-electrons as they’re on the outside, so the pi-bond requires less energy to break.
What is hydrogenation?
Alkene + Hydrogen in presence of nickel catalyst –> alkanes. e.g to turn oils into spreadables.
What is addition with hydrogen halides?
Alkene + gaseous hydrogen halides at room temp –> haloalkanes. e.g ethene +HBr -> bromoethane.
What is hydration?
Alkene + steam in presence of phosphoric acid catalyst –> alcohols.
What is an electrophile?
An atom/group of atoms attracted to an electron-rich centre (e.g a pi-bond).
Describe the process of electrophilic addition with HBr.
HBr is polar as Br is more electronegative, so the electron pair in pi-bond is attracted to the δ+ H. H-Br bond breaks by heterolytic fission and electron pair goes to δ- Br atom. A bromide ion and carbocation intermediate form, which react together to form the addition product. (use curly arrows).
What is Markownikoff’s rule?
If it is an unsymmetrical alkene, the major product of electrophilic addition forms via the most stable carbocation intermediate. Stability increases with the more alkyl groups added to the +ive C atom. (Primary=1, secondary=2 etc)
Describe the halogenation reaction mechanism with Br-Br.
Bromine is non-polar, so when it approaches the alkene, the pi-electrons induce a dipole in the Br molecule, breaking the Br-Br bond by heterolytic fission with the electron pair going to the δ- Br atom. A bromide ion and carbocation intermediate form, which react to form the addition product.
What is halogenation?
Alkenes + halogens at room temp –> dihaloalkenes.
What is a polymer and addition polymerisation?
Polymer = many monomers joined together to form a long chain.
Addition polymerisation = unsaturated molecules become a polymer with single bonds.
As many alkene-based polymers are non-biodegradable due to lack of reactivity, what are the 3 methods of recycling?
- Recycling - conserves fossil fuels, decreases landfill waste, but costly to sort by hand and can contain mixtures.
- PVC recycling - releases toxic HCl + dioxins when burnt. Solvents used to dissolve it. PVC is then recovered by precipitation and solvent is reused.
- Feedstock recycling - reclaiming monomers from waste polymers to use them in production of new polymers.
Other than recycling, how else can we remove alkene-based polymers?
Can be incinerated to produce heat, generating steam to power turbines producing electricity.
What are bioplastics?
Plastics made from plant based materials. Renewable and sustainable.
What does bio/photodegradable mean?
Biodegradable - broken down by bacteria into water, CO2 and biological compounds.
Photodegradable - contain bonds which are weakened by absorbing UV (light).
