Alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons meaning they are made from hydrogen and carbon only and contain at least one C-C double bond. They have a general formula of Cn H2n.
Why can’t a double bond rotate?
As well as a normal C-C bond, there is a p orbital, containing one electron, on each carbon. These overlap to form a pi orbital which is a cloud of electron density above and below the single bond. It’s presence means the bond cannot rotate.
Geometrical isomers
Where the two molecules have the same structural formula but the bonds are arranged differently in space.
Why are alkenes more reactive than alkanes?
The C=C bond forms an electron rich area in the molecule which can easily be attacked by electrophiles (electron pair acceptors).
Define electrophile
An electron deficient atom, ion or molecule that attacks areas of high electron density in another reactant. They are electron pair acceptors.
Electrophilic addition with hydrogen halides
X is more electronegative so is delta- and H is delta+ so is the electrophile. One of the pairs of electrons in the double bond forms a bond with the H to form a positive ion (carbocation). Both the electrons in the H-X bond go to X (bond breaks heterolytically) leaving a X- ion which attaches to the positive carbon in the carbocation.
Trend in reaction speed of alkenes with hydrogen halides
As you go down group 7, the reactivity increases because H-X bond gets weaker.
Order of stability of carbocations
Primary < secaondary < tertiary
Explain the trend in the stability of carbocations
Alkyl groups have a tendency to release electrons (positive inductive effect) which stabilises the positive charge of the intermediate carbocation. Therefore the more alkyl groups, the more stable the carbocation is. A more stable on is more likely to be formed in electrophilic addition.
Electrophilic addition with sulfuric acid
Same as hydrogen halide reaction but using an OH. It forms an alkyl hydrogensulfate. When water is added to the product, an alcohol and sulfuric acid are formed.
Polymer
A very large molecule built up from small molecules called monomers.
What are addition polymers?
Polymers made from monomers with a carbon carbon double bond (alkenes).
What happens in polymerisation?
The double bond opens up so the monomers can bond together
What do plasticisers do?
They are small molecules that get between the polymer chains forcing them apart and allowing them to slide over each other.
How is low density polythene made?
Polymerising ethene at a high pressure and temperature via free radical mechanism. The polymer has a certain amount of chain branching. The branched chains don’t pack together well make the product flexible and of low density.
