3.2.9 All Alkenes Notes with Mechanisms

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  • Created on: 11-03-15 20:19
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3.2.9 Alkenes
General formula of CnH2n
Always have at least one carbon carbon double bond (because they are hydrocarbons) to make an
unsaturated compound.
Similar properties to alkanes due to the only intermolecular forces being VDW forces.
Despite the fact alkenes have a higher bond enthalpy (C=C) than alkanes (C-C), alkenes are more
reactive because the C=C bond gives and electron rich area, susceptible to attack from
electrophiles (electron pair acceptors) and can undergo electrophilic addition reactions which the
majority are.
Isomers
Position isomers of alkenes
Can happen with 4+ carbon atoms in a chain where the position of the double bond is moved e.g.
but-1-ene and but-2-ene.
Stereoisomers of alkenes
These are possible due to the restriction of rotation around the C=C bond which is planar due to
the restriction of movement of the groups attached to the carbons. The rotation around the C=C is
restricted because of the sigma and pie bonds. Stereoisomers can also form due to the restricted
movement in a ring of carbon atoms.
E/Z Isomerism
This means that geometrical stereoisomers can be formed ­ E (entgegen - opposite) or Z
(zusammen - together).
IUPAC rules changed as Cis and Trans prefixes could not differentiate between 2+ substituents
around the double bond. E/Z looks on each carbon to see which out of the two substituents on
each carbon has the greatest mass number, then these are used in the E/Z isomers nomenclature.
Definition
The IUPAC preferred method for naming stereoisomers of alkenes (over cis and trans)
See reactions of alkenes for the rest of the notes on this topic.

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Reaction Reagents and conditions Observations Products
Combustion Excess oxygen Burns with a blue flame, no CO 2
Complete Spark or high soot H2 O
energy
Limited oxygen Burns with a sooty, yellow CO (variab
Incomplete
Spark or high flame (more common in C (variable
energy alkenes so not used as CO 2 (variab
fuels) H2 O (alwa
With halogens Liquid bromine Colour change from brown Dihaloalka
Electrophilic addition Room to colourless ­colourles
Need to know with Br 2 temperature and (electroph
(Bromine liquid) exclude light…read more

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With concentrated Concentrated Highly vigorous Alkyl
sulphuric acid sulphuric acid Exothermic hydrogens
Electrophilic addition (so ­OH groups e
aren't added on)
Standard
conditions
Reaction with steam Steam Alcohols ­
Electrophilic addition Phosphoric acid for ethano
on aluminium industriall
oxide or silica Only 5% y
300°C, 6500kPa then react
(MUST KNOW) are recycl
Too high give overa
pressure means of 95%
that the alkene
will polymerised
Ideally high
pressure (but
this would lead
to
polymerisation)
High
temperature
Excess steam
Hydrogenation Hydrogen gas Alkane
Electrophilic addition Exclude…read more

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Mechanisms
The positive part of the reagent attracts the double bond due to the high electron density in the
double bond. Then the negatively charged electrophile is attracted to the positively charged
carbocation.
With HBr (Hydrogen halides). Same mechanism with Bromine water however there the bromide
ions are Br+ and the ­OH groups are negatively charged with two lone pairs to act as the
electrophile.…read more

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With Bromine water
­ The bromine is already in the form of Br+ ions.…read more

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Production of Poly(alkenes)
Reaction is addition polymerisation to form addition polymers however the products are
unreactive as they are actually saturated alkanes.
When asked to draw the repeating unit of a polymer, ONLY draw the unit, not the brackets and
"n" as this represents the whole polymer, not solely the repeating unit.
Polymers are usually used to make plastics.
LDPE is formed by free radicals and so has many branches which cannot pack together well.…read more

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Mechanical Recycling
This is the simplest form where:
Plastics are separated according to type
Plastics washed and ground into pellets
Pellets are then melted and remoulded e.g.…read more

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