3.3.4 Alkenes
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why’s there an attraction between a C=C double bond and Br2?
• high electron density • Br-Br becomes polarised • partially + Br attracted to C=C
what would happen if bromine water was added to a polyalkene?
• no reaction • no double bonds present
why’s more of an isomer formed in electrophilic addition?
• secondary carbocation more stable than primary • the isomer will be more stable than the other one
what reagent could react with but-1-ene and similars?
H-Br
what’s an addition reaction?
a reaction converting a double bond to a single bond
why does but-1-ene not exhibit stereoisomerism?
has 2 hydrogen atoms on the same carbon atom of the c=c bond
why does stereoisomerism occur in but-2-ene?
• restricted rotation around the C=C bond • has E and Z isomers
what ion cannot act as an electrophile?
Cl-
out of alkanes, alkenes and cycloalkanes which one has the highest Mr?
alkanes
what’s the general formula for an alkene?
CnH2n
are alkenes hydrocarbons?
yessir
why are alkenes fairly reactive?
has high electron density due to double bonds
what’s the general formula for cycloalkenes?
CnH2n-2
what type of mechanism do alkenes go through and where does the curly arrow start from?
• electrophilic addition • starts from the C=C double bond
what’s the test for alkenes?
• add bromine water • changes from brown-orange to colourless
what causes two different products to be formed when reacting hydrogen halides to alkenes?
• reacting w unsymmetrical alkenes • amount of 2 products determined by stability of carbocation intermediate
why does the carbocation stability go like tertiary > secondary > primary
• alkyl groups push electrons towards the positive carbocation, making it stable • more stable carbocation = more likely it’ll form
how can alkenes become alcohols?
• react with cold concentrated H2SO4 to form alkyl hydrogen sulfate intermediate • react w water to form an alcohol
what type of polymerisation do alkenes go through?
addition polymers
how do you draw a repeat unit for a poly(alkene)?
—(—CH3-CHCH3—)— n
what are the properties of poly(alkenes)?
• saturated • non polar • unreactive • don’t degrade well in landfill
explain why non-polar polyalkenes are held together at all if they lack permanent dipoles
they have vdw between chains
how does the length of a polymer chain directly influence its melting point?
• longer chains have more points of contact • increases the total vdw forces so more energy needed to overcome the intermolecular attraction
why is PVC typically more rigid than standard polyethylene?
• contains chlorine (a halogen) • chlorine is electronegative, it creates permanent dipole-dipole forces, which are significantly stronger than the vdw forces in non-polar polyalkenes