3.3.4 Alkenes Flashcards
What does it mean by alkenes being unsaturated hydrocarbons?
they have 1 or more c=c bond
why are alkenes fairly reactive?
they have high electron density
How many fewer hydrogens do cycloalkanes have than branched alkenes?
2
What kind of mechanisms do alkenes undergo and why?
electrophilic addition due to their high electron density (so they get attacked by electrophiles)
Whats an electrophile
electron pair acceptor
Give examples of 2 TYPES of electrophiles
positive charge ions : H+, NO2+
polar molecules : H-BR, H2SO4
How do you test for alkenes? how does this work?
addition of bromine (brown-orange to colourless)
- high electron density of c=c bond induces dipole in Br2 molecule as it gets close
- delta positive side Br molecule attaches
- carbocation of the other carbon atom on the c=c forms
-delta negative Br donates its electrons to carbonated C+
- colourless alkane formed
Reacting hydrogen halides with unsymmetrical alkenes produces 2 different products, how?
the delta positive molecule can join onto the compound to allow for the most stable carbocation in which the delta negative molecule can join onto. (this forms the major product, vice versa produces the minor product)
the more stable the carbocation the more likely it’ll form,
The more alkyl groups attached to the carbocation, the more stable the intermediate becomes.
Why is a carbocation with more alkyl groups attached considered more stable?
Alkyl groups push electrons towards the positive carbocation hence stabilising it.
What does the addition of sulphuric acid to an alkene produce?
alkyl hydrogen sulfate
How do you form an alcohol from alkyl hydrogen sulfate?
- via hydrolysis
- add water, this provides the OH for alcohol and H to reform sulphuric acid
why is sulfuric acid considered a catalyst when it reacts with an alkene?
- the first time it reacts with alkene to for alkyl hydrogen sulfate
-the intermediate (alkyl hydrogen sulphate) is hydrolysed (+h2o) , to form an alcohol and the sulfuric acid is re-formed
-because sulfuric acid is reformed, its considered as a catalyst
What are 2 types of polymers?
Natural - proteins and natural rubber
Synthetic - poly(ethane) and poly(propene)
How are alkenes used as monomers to form addition polymers?
- double bond of alkene opens up and trailing bonds extend beyond the brackets
- n is placed outside bracket to show repeat units
Why are poly(alkenes) unreactive and why is this bad?
Poly(alkenes) are saturated molecules meaning they are normally NON-POLAR and hence UNREACTIVE so they dont degrade well in landfill
