Alkenes Flashcards
Alkenes and cycloalkenes are….
Unsaturated hydrocarbons
Forming a C=C double bond
- one of the bonds is a boring old sigma bond (2 s-orbitals overlapping, 1 electron donated by each carbon)
- the other bond is a pi bond. When the sigma bond has formed, the p-orbitals of each carbon atom overlap above and below the plane of the carbon atoms, forming a pi-bond, 1 electron donated by each carbon
Shape of alkenes around C=C double bond
- 3 regions of electron density
- electron pairs repel each other as far away as possible
- each region of electron density repels equally, giving a trigonal planar shape of bond angle 120deg.
Addition reaction of alkenes with hydrogen
- requires a catalyst, e.g. Ni (nickel) and 150degC temperature
- reaction called hydrogenation
- get an alkane
Addition reaction of alkenes with halogens
- reacts rapidly at room temperature
- Halogen added across double bond to give a dihalogenoalkane
- one halogen to each carbon on C=C double bond
- reaction sometimes called halogenation
Addition reaction of alkenes with hydrogen halides
- hydrogen halides are HCl, HBr, HI
- bubbled into liquid alkenes (HX are gases at room temp)
- results in a halogenoalkane - could get more than 1 product, depending on alkene length and where double bond is
Testing for unsaturation
- add bromine to the sample
- if a colour change is observed from orange to colourless, this indicates the bromine has reacted with the double bond
Addition reaction of alkenes with steam
- steam and gaseous alkene heated to high temp and pressure
- use of a phosphoric acid catalyst
- called hydration of an alkene
- an H goes to one carbon in C=C, an OH to the other carbon: get an alcohol
- could get a more than 1 product, depending on alkene length and where double bond is
Electrophile
An electron pair acceptor. Attracted to an electron-rich centre or atom
Mechanism for electrophilic addition
- curly arrow from C=C to slightly positive atom
- curly arrow from slightly+ve - slightly-ve bond to slightly negative atom (heterolytic fission)
- get a carbocation and an anion
- curly arrow from electron pair on anion to carbon in carbocation
- product
Addition polymerisation of alkenes
- alkenes are monomers
- addition polymer is a long, saturated chain
- monomer forms a repeating unit
Manufacture of margarine
- catalytic hydrogenation of unsaturated vegetable oils
- requires a nickel catalyst
- oil must be hardened, so it can be spread
- if reduce number of double bonds, it will harden
- add hydrogen molecules in an addition reaction
- can control spreadability of margarine
Separating polymers
- part of processing waste polymers
- polymers have to be separated into types before they can be reused
- used to happen by hand
- now have optical scanning techniques
- if one polymer is left in with another, it can render the product unsuitable for re-use/release toxins if it goes through the wrong process
- e.g. if PVC (polyvinylchloride) isn’t separated from PET, it undermimes recycling process. And, PVC releases poisonous dioxins when heated
Polymers as a fuel source
Polymers can be burnt in controlled conditions to produce heat energy, which can then be converted into electrical energy.
Polymers - feedstock recycling
Processes are being developed to convert polymers into a synthesis gas of hydrogen and carbon monoxide. This can be used as a chemical feedstock, alongside hydrocarbons for conversion into useful products/fuel at oil refineries
Burning PVC
As PVC contains lots of chlorine, when it is burnt, HCl is released. This is bad for the environment, so chemists have to find ways of removing it from the fumes/finding a new way to recycle it - e.g. dissolving it in solvents to separate it from other plastics, then reclaiming it for use
Biodegradable plastics
If plastics are biodegradable, they won’t clog up landfill so much. These can be made from lactic acid, maize and starch. They will biodegrade under bacterial activity to carbon dioxide and water.
