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
