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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),…

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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…

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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)…

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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…

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




With H
2SO 4-
However if more water is added to the product, an alcohol is formed and sulphuric
acid reforms, rendering it a catalyst for the reaction

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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…

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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. PET makes fleeces

Only works for thermoplastics like HDPEand Poly(propene)



Feedstock Recycling

Plastics heated to a temperature that breaks bonds to produce monomers…

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