Alkenes Flashcards
are alkenes saturated or unsaturated hydrocarbons? why?
- unsaturated
- have at least 1 double covalent bond and can undergo addition reactions
what makes alkenes reactive?
the double bonds have a high electron density
what is an electrophile? give some examples.
- electron pair acceptor
- they are deficient in electrons and are attracted to the double bond
- ex: pos charge ions (e.g. H+) or polar (delta pos) molecules (e.g. H2SO4)
how can you test for alkenes? what makes this possible?
- adding bromine water to an alkene causes a colour change from brown-orange to colourless
- bromine (brown-orange) is the electrophile and adds to the alkene forming a dibromoalkane (colourless)
state the name and describe the mechanism of the reaction that takes place when adding bromine water to ethene
- electrophilic addition
1. Br2 is polarised as the electrons in the double bond repels the electrons in the Br2
2. an electron pair in the double bond is attracted to the partially positive bromine and forms a bond. this breaks the Br-Br bond.
3. a carbocation intermediate is formed and Br- is attracted to C+
4. colourless 1,2-dibromoethane is formed
what is formed when alkenes react with hydrogen halides?
halogenoalkanes
outline the mechanism of H-Br reacting with ethene
- HBr is polarised
- an electron pair in the double bond is attracted to the partially positive hydrogen and forms a bond. this breaks the H-Br bond
- a carbocation intermediate is formed and Br- is attracted to C+
- bromoethene is formed
what may happen if an unsymmetrical alkene is reacted with a hydrogen halide?
- 2 different products formed
- amount of each product is determined by the stability of the carbocation intermediate
- more alkyl groups bonded to the carbocation = more stable intermediate is = more likely product will form
- this is because alkyl groups push electrons towards the positive carbocation, stabilising it
how do alkenes behave when added to sulfuric acid?
- react with cold, concentrated sulfuric acid to form alkyl hydrogen sulfates
- H2SO4 is used as a catalyst in making an alcohol from an alkene
what can be added to ethyl hydrogen sulfate to form ethanol? what kind of reaction is this?
- adding cold water to warm ethyl hydrogen sulfate will form ethanol
- hydrolysis reaction
give the overall reactions of the addition of sulfuric acid to ethene (step 1) and the formation of an alcohol (step 2) using the products formed in step 1
step 1: H2C=CH2 + H2SO4 –> CH3CH2OSO2OH
step 2: CH3CH2OSO2OH + H2O –> CH3CH2OH + H2SO4
what are the 2 types polymers? give some examples.
- natural (e.g. proteins + natural rubber)
- synthetic (e.g. poly(ethane) and poly(propene))
are poly(alkenes) saturated or unsaturated? give some properties.
- saturated molecules
- normally non-polar + therefore are unreactive
- do not degrade well in landfill
describe the properies of poly(alkenes)
- most polyalkene chains are non-polar so only have vdw forces beteen chains
- longer chain and closer they are to each other = more vdw
- shorter polymer chains + those with lots of branching tend to be more flexible and weaker
- polymers with little/no branching = more rigid + stronger
- some halogens (e.g. PVC) can form stronger permanent dipole-dipole forces so will have different properties to other non-polar polyalkenes
what are plasticisers? why are they useful?
- added to polymers to change their properties
- makes polymers more flexible
