alkenes (3.3.4)
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what are alkenes?
unsaturated hydrocarbons
what is the bonding like in alkenes?
they contain a double covalent bond which is a centre of high electron density
how do you test for unsaturation in alkenes?
using bromine, which is decolourised
what type of reaction mechanism do alkenes undergo?
electrophilic addition
which substances may alkenes react with in electrophilic addition?
HBr, H2SO4, Br
what is the name for the intermediate formed in the second stage of an electrophilic addition reaction?
carbocation intermediate
what are the possible carbocations that can form in addition reactions of unsymmetrical alkenes?
major (more stable) or minor (less stable)
how do you determine the classification of a carbocation?
by looking at how many alkyl groups are bonded to the positive carbon atom (1=primary, 2=secondary, 3=tertiary) **H atoms are not alkyl groups **
what are the relative stabilities of primary, secondary and tertiary carbocation intermediates?
primary - less stable secondary - more stable tertiary - most stable because the 2 or 3 alkyl groups bonded to the positive carbon are electron donating, so they release electrons towards the positive carbon, which stabilises the charge on the carbocation intermediate
what is the positive inductive effect?
when alkyl groups bonded to a positive carbon in a carbocation intermediate release electrons towards it
which product is more likely to form in an electrophilic addition reaction?
the major product
why can a molecule of bromine act as an electrophile?
the electrons in the C=C bond repel the pair of electrons in the Br-Br bond the electrons in the Br-Br bond are pushed closer to the Br atom furthest from the C=C bond this creates an induced dipole with one Br atom becoming partially positive and the other becoming partially negative the partially positive Br atom attracts a a pair of electrons in the C=C bond
what are the two steps when an alkene eg. ethene reacts with aqueous sulfuric acid to form an alcohol?
1 - ethene react with sulfuric acid forming ethyl hydrogensulfate (CH3CH2OSO3H) 2 - ethyl hydrogensulfate reacts with water forming ethanol and sulfuric acid (catalyst)
what are addition polymers formed from?
alkenes and substituted alkenes
what conditions are required for polymerisation?
high temperature, high pressure, catalyst
how do monomers join together to form an addition polymer?
the double bond in each alkenes molecule breaks and the electrons in the broken bond are used to attach one molecule to the next
why are addition polymers unreactive?
they contain a large number of C-H and C-C bonds which are non-polar and difficult to break, making them relatively strong
how do you identify the repeating unit of an addition polymer?
take any two adjacent carbon atoms on the main polymer chain and draw those two atoms alongside any atoms above or below
how do you identify the monomer of an addition polymer?
take any two adjacent carbon atoms on the main polymer chain and draw those two atoms alongside any atoms above or below, and replace the single C-C bond with a double C=C bond
what is poly(chloroethene)/PVC?
a rigid polymer typically used to make plastic pipes
what is flexible PVC used to make?
flooring and insulation for electrical cables
how can you modify PVC to make it flexible?
using a plasticiser (small molecule which fits between the polymer chains) - the chains move further apart so the van der Waals forces between them are weaker, allowing the chains to slide over each other