Unit 3 Chemistry – Structure & Naming of Alkenes [to C10]
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alkenes are hydrocarbons that contain a C=C double bond.
alkenes are unsaturated hydrocarbons as they can react with hydrogen to
become saturated
the alkene homologous series starts with ethene and has the general formula CnH2n
The alkenes = unsaturated homologous series CnH2n
C2H4
C3H6 = propene
C7H14 = heptene
C6H12 = hexene
C10H20 = decene
C4H8 = butene
C8H16 = octene
C5H10 = pentene
C9H18 = nonene
= ethene
Note: the alkene name ends in “ene” and the prefix gives the number of carbons in
the alkene  prop = 3 carbons, hex = 6 carbons, oct = 8 carbons etc.
3 different structural formula representations of propene – C3H6
double bond
planar
shape

tetrahedral
shape
the double bond part of the alkene molecule has a planar shape, but other carbons
with single bonds still have a tetrahedral arrangement of atoms around them
Naming Alkene Isomers
 the double bond in alkenes larger than propene can be between different carbons,
giving rise to a number of isomers
 the longest carbon chain containing the double bond is identified, numbered from
the 1st carbon in the double bond [closest to the end of the molecule]and named
eg. CH3CH=CHCH3 has 4 carbons, the double bond starts on the second carbon,
there are no side chains, so the molecule is but-2-ene
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5 carbons in longest chain with double
bond = pentene
 double bond starts on 2nd carbon =
pent-2-ene
 methyl side chain on 3rd carbon =
3-methylpent-2-ene
Benzene Rings
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Benzene [C6H6] is a 6 carbon hydrocarbon arranged in a ring structure, with
alternating single and double bonds between the carbons in the ring
Equal C–C bond lengths suggest that, rather than alternating single and double
bonds, each carbon in the ring is joined to others by single covalent bonds, but also
has a delocalised electron that it shares with the others …
Different representations of the benzene ring [C6H6]
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Hydrocarbons containing rings with delocalized electrons are known as aromatic
hydrocarbons or arenes.
Some important arenes …
phenol
phenylalanine is an amino acid
styrene is the monomer
from which polystyrene is
made
salicylic acid is a
precursor in the making of
aspirin
terephthalic acid is the
di-acid used for making
polyester
acetosalicylic acid
[aspirin]
Examples involving the Structure and Naming of Alkenes
---------------------------------------------------------------------------------1. Which of the following organic compounds is an alkene?
[i] C3H8
[ii] C7H14
[iii] CH3CH=CH2CH3
[iv] C2H4
[v] C2H2
[vi] C5H10
[vii] CH3CH2OH
[viii] C10H22
---------------------------------------------------------------------------------2. Copy and complete the following table…
Name
molecular semi-structural
formula
formula
structural formula
ethene
propene
pent-1-ene
2-methylpropene
3-ethylhex-2-ene
---------------------------------------------------------------------------------3. Draw structural formula and name the 3 structural isomers of butene.
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