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Alkenes
N Goalby
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The Alkenes
• General formula is CnH2n - for non-cyclic alkenes
• They contain a carbon-carbon double bond somewhere in their
structure
• Classed as Unsaturated hydrocarbons
Ethene
C2H4
Propene
C3H6
Butene
C4H8
Pentene
C5H10
Hexene
C6H12
Heptene
C7H14
Octene
C8H16
Nonene
C9H18
Decene
C10H20
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Naming alkenes
•
•
•
Name of molecule ends in “ene”
Number chain so as to give double bond lowest possible numbers
Place number of first carbon of double bond in front of “ene”
H
H
C
Ethene
C2H4
CH2=CH2
C
H
H
H
But-1-ene
C4H8
CH2=CHCH2CH3
H
H
C
C
C
C
H
H
H
H
H
H
H
But-2-ene
C4H8
CH3CH=CHCH3
C
H
C
C
H
H
H
Propene
C3H6
CH2=CHCH3
KEY
Naming alkenes
H
H
H
C
H
C
H
C
Cyclohexene
C6H10
H
C
C
C
In cyclic compounds, “cyclo” is put in front
of the name
H
H
H
H
H
H
C
H
C
C
C
C
H
H
H
H
H
Penta-1,3-diene
C5H8
CH2CH=CHCHCH3
This compound contains two C=C bonds: it is a
diene. The same numbering rules apply.
H
H
H
C
H
C
C
H
H
C
H
H
methylpropene
C4H8
CH2=C(CH3)2
Doesn’t need ‘2’ as
no other branched
alkene is possible
KEY
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Bonding in alkenes
C-C sigma bond
H
C
H
C-H
sigma
bond
H
Ethene contains a C=C double covalent
bond: this consists of one sigma (σ)
bond and one pi (π
π) bond.
H
The H-C-H bond angles are
approximately 120o (trigonal planar)
C
KEY
C-C pi bond
This part of an alkene is
always planar
Orbital hybridisation
The group state electronic
configuration of a carbon atom is
1s22s22p2
A more stable arrangement can be
made If a little energy is used to
promote one of the 2s electrons into
the empty p orbital. The
configuration is now 1s22s12p3
2p
2s
1s
2p
2s
1s
EXTRA
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sp2 Orbital hybridisation
Three orbitals (a 2s and two 2p’s) HYBRIDISE to give three new hybrid
orbitals.
2s1
2p3
3 x sp2
2p
One 2p orbital is
left unchanged.
It is called sp2
hybridisation
because one s and
two p orbitals are
hybridised together
The three sp2 orbitals repel
into a planar arrangement and
the 2p orbital is at 90o to them
2p
sp2
sp2
sp2
EXTRA
Sigma bond formation
In ethene 2 carbons
bond
C
C
One sp2 orbital from each
carbon overlap to form a
single C-C bond called a
sigma σ bond
sigma σ bond
EXTRA
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Pi bond formation
The 2p orbitals overlap with each other to form a second bond
called a PI bond above and below the plane of the sigma bonds
E-Z stereoisomerism in alkenes
E-Z stereoisomers arise when:
(a) There is restricted rotation around the C=C double bond.
(b)There are two different groups/atoms attached to both ends of
the double bond
H
H
C
H
C
H
H
H
H
H
C
H
H
H
H
C
C
H
H
C
C
H
H
C
H
C
H
H
H
H
H
C
C
H
C
H
E-Z stereoisomers: two different groups
attached either end of the restricted
double bond
E-Z stereoisomers : two different groups
attached either end of the restricted
double bond
Same structure NOT E-Z
stereoisomers : two identical groups
attached to one end of the restricted
double bond.
KEY
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Naming E-Z stereoisomers
Cl
Priority
group side 1
Cl
Cl
C
Priority
group side 2
C
H
H
C
C
H
H
Z-1,2-dichloroethene
Cl
E-1,2-dichloroethene
On both sides of the double bond determine the priority group
PRIORITY Group: The atom with the higher atomic number is classed as
the priority atom (e.g. Cl is the priority atom on both sides of the double bond
in the above example)
If the priority atom is on the same side of the double bond it is labelled Z
from the german zusammen (The Zame Zide!)
If the priority atom is on the opposite side of the double bond it is labelled E
from the german entgegen (The Epposite side!)
KEY
E-Z isomers
H
H
Cl
Cl
C
H
C
H
C
C
Br
Is this an E or Z isomer?
Br
H
C
Cl
Is this an E or Z isomer?
Cl is priority group on the left
Cl is priority group on the left
Br is the priority group on the right
Br is the priority group on the right
E isomer (opposite sides)
Z isomer (same sides)
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EZ isomerism but-2-ene.
H3C
Priority
group side 1
H3C
CH3
C
Priority
group side 2
C
H
H
C
C
H
H
CH3
E -but-2-ene
Z- but-2-ene
H
H
H
H
HH
C
C
C
H
C
H
C
C
H
C
H
H
H
H
C
H
H
H
KEY
Structural isomerism in alkenes
•
This can be a result of:
Positional Isomerism
CH2 CH
H3 C
CH2 CH
H 3C
CH2
CH2
CH
CH3
Pent-2-ene
Pent-1-ene
• Chain Isomerism (Branching)
CH
H 2C
CH3
CH
3-methylbut-1-ene
CH3
KEY
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Isomerism in butene
There are three STRUCTURAL isomers of C4H8, which are alkenes
But-1-ene
H
H
C
C
But-2-ene
H
H
C
H
H
H
H
H
H
H
H
C
C
C
C
methylpropene
H
H
H
C
C
H
H
H
C
H
C
H
Which of these can
also form E-Z
isomers?
H
H
H
C
H
H
H
C
H
H
H
C
H
C
H
H
H
C
C
C
C
H
H
H
E- but-2-ene
H
C
H
H
H
Z- but-2-ene
KEY
What type of isomers are these?
Cl
H
C
H
C
Cl
C
H
Cl
H
H
Br
C
C
H
Br
Cl
C
Br
H
Br
Cl
H
C
H
C
H
C
Cl
Cl
C
H
Geometric (E-Z)
Cl
Cl
C
F
Same molecule
C
H
C
Cl
Positional Structural
C
Cl
C
H
Cl
H
Positional Structural
C
F
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What type of isomers are these?
Br
H
C
Br
C
Br
C
H
Br
C
H
H
Br
CH3 Br
C
H
C
H 3C
H H 3C
Br
CH3 H
C
Same molecule
C
C
Geometric (E-Z)
C
CH3
CH3
C
Same molecule
C
H3C
H H3C
Br
H3C
H H3C
CH3
C
H3C
C
C
F
H
C
F
Structural (chain and
positional)
Reactivity of alkenes
• Alkenes are more reactive than alkanes, because of the
double bond.
• A common reaction of alkenes is the addition of small
molecules across the double bond.
E(C=C) = +610 kJ mol-1.
E(C-C) = +346 kJ mol-1.
It is energetically more favourable to form two single
bonds than one double bond. The C=C bond energy is less than
two times the value of most single bonds.
KEY
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Reactivity of alkenes
H
H
π
σ
σ
σ
σ
σ
π
H
H
π bonds are exposed and have high electron density.
They are therefore vulnerable to attack by species which are attracted to high
electron density. These species are called electrophiles.
Exam defintion
Electrophiles are defined as : electron pair acceptors
KEY
Reaction of bromine with alkenes
Change in functional group: alkene
dihaloalkane
Reagent:
Bromine
Conditions:
Room temperature (not in UV light)
Mechanism: Electrophilic Addition
Type of reagent: Electrophile, Brδ+
Equation:
CH2CH2 (g) + Br2 (l)
H
CH2BrCH2Br (l)
H
C
+ Br2
C
H
H
ethene
H
H
H
C
C
Br
Br
H
1,2-dibromoethane
KEY
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Electrophilic addition mechanism
Bromine + ethene
H
H
C
C
H
H
δ+
Br
Br
δ-
Br approaches the alkene, the pi bond electrons repel
As the non-polar Br2 molecule
the electron pair in the Br-Br bond. This INDUCES a DIPOLE. Br2 becomes
Br (Brδ+).
polar and ELECTROPHILIC
The electrons in the pi bond come out to form a covalent bond with the positive
end of the Br2. The Br-Br bond breaks HETEROLYTICALLY: bromine attaches
to one of the carbon atoms and a bromide ion is formed.
KEY
Electrophilic addition mechanism
Bromine + ethene
H
C
H
C
H
H
H
H
+
C
C
H
δ+
Br
Br
H
H
H
H
C
C
Br
Br
H
Br-
Br
δAn INTERMEDIATE has been formed, which has a positive charge residing on a
carbon atom: this is called a CARBOCATION.
The bromide ion contains a lone pair. It acts as a NUCLEOPHILE and
ATTACKS the CARBOCATION. Overall, there is ADDITION of Br2 to the
alkene.
KEY
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Reaction of hydrogen halides with alkenes
Change in functional group: alkene
haloalkane
Reagent:
HCl or HBr gas
Conditions:
Room temperature
Mechanism: Electrophilic Addition
Type of reagent: Electrophile, HBr or HCl
Equation:
CH2CHCHCH3 (g) + HBr (g)
H
H
H
CH2CH2CHBrCH3 (l)
C
C
C
C
H
H
H
H
But-2-ene
H
+ HBr
H
H
H
H
H
C
C
C
C
H
H
Br
H
H
2-bromobutane
KEY
Electrophilic addition mechanism
Hydrogen bromide + but-2-ene
H3C
CH3
C
C
H
H
H
δ+
Br
δ-
HBr is a polar molecule because Br is more electronegative than H. The Hδ+ is
attracted to the electron-rich
H pi bond.
The electrons in the pi bond come out to form a covalent bond with the positive
end of the HBr. The H-Br Br
bond breaks HETEROLYTICALLY: H attaches to one
of the carbon atoms and a bromide ion is formed.
KEY
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Electrophilic addition mechanism
Hydrogen bromide + but-2-ene
H3C
CH3
C
CH3 H
C
H
+
C
H
C
H
H
CH3 H
CH3
H
H
δ+
Br-
C
C
Br
H
CH3
Br
δAn INTERMEDIATE has been formed, which has a positive charge residing on a
carbon atom: this is called a CARBOCATION.
The bromide ion contains a lone pair. It acts as a NUCLEOPHILE and
ATTACKS the CARBOCATION. Overall, there is ADDITION of HBr to the
alkene.
KEY
Addition of HBr to unsymmetrical alkenes
KEY
If the alkene is unsymmetrical, addition of hydrogen bromide can lead to
two isomeric products.
H
H
H
H
Major
:Br H
δ+
H
H 2C
δBr
CH
CH2
+
C
H3C
CH3
C
C
C
H
H
Br
H
H
product
90%
H
H
+
C
H
C
2-bromopropane
:Br -
CH3
This doesn’t arise with
ethene because it is
symmetrical
H
CH2
C
H
CH2 CH3
CH2
CH2 CH2 CH3
Br
1-bromopropane
Minor
product
10%
IN MOST CASES, BROMINE WILL BE ADDED TO THE CARBON WITH THE
FEWEST HYDROGENS ATTACHED TO IT: ‘Markownikoff’s Rule’
[If UV light is used then 1-bromobutane will be more abundant:
the reaction will proceed via the free radical substitution mechanism.]
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Why does this happen?
When the HBr adds on there are two possible intermediates
H
H
C
H
H
C
+
H
C
H
…more
H stable
than…
H
H
H
H
C
C
C
+
H
H
H
The first carbonium ion intermediate is more stable because the
methyl groups on either side of the positive carbon are electron
releasing and reduce the charge on the ion which stabilises it.
The order of stability for carbonium ions is therefore
tertiary > secondary >primary
In electrophilic addition to alkenes, the major product is
formed via the more stable carbocation intermediate.
KEY
Explanation for the exam:
• Draw out both carbocations and identify as primary,
secondary and tertiary
• State which is the more stable carbocation e.g.
secondary more stable than primary
• State that the more stable carbocation is stabilised
because the methyl groups on either (or one) side of
the positive carbon are electron releasing and reduce
the charge on the ion.
• (If both carbocations are secondary then both will be
equally stable and a 50/50 split will be achieved)
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enthalpy
Stability of intermediates
Secondary
carbocation more
stable than primary
carbocation
CH3-CH2-CH2
+
CH3-CH-CH3
+
CH3CH2CH2Br
CH3CH=CH2 + H-Br
CH3CHBrCH3
2-Bromopropane will
be the major product
Reaction progress
Reaction of sulphuric acid with alkenes
Change in functional group: alkene
alcohol
Reagents:
Stage 1: concentrated H2SO4
Stage 2: water
Conditions:
Stage 1: room temperature
Stage 2: warm mixture
Mechanism(stage 1): Electrophilic Addition
Type of reagent:
Electrophile, H2SO4
Overall
role of
H2SO4 is
catalyst
Equation:
Stage 1: electrophilic addition of H2SO4
CH2=CH2 (g) + H2SO4 (conc)
CH3CH2OSO2OH (aq)
ethene
ethyl hydrogen sulphate
Stage 2: hydrolysis to form alcohol
CH3CH2OSO2OH (aq) + H2O (l)
CH3CH2OH (l) + H2SO4(aq)
ethanol
KEY
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Electrophilic addition mechanism
Cyclohexene + sulphuric acid
H
H
+
O
O
-O
+
O
S
+
H
S
O
O
H
O
O
Stage 1: electrophilic addition of H2SO4
H
H
+ H2O
H2SO4 +
OSO 2OH
OH
Cyclohexyl hydrogen sulphate
Cyclohexanol
Stage 2: hydrolysis of alkyl hydrogen sulphate to form alcohol
KEY
Electrophilic addition mechanism
propene + sulphuric acid
H
H3C
C
H
H
O
O
H
S
C
H
H
H
H
H
C
C
+
C
H
-O
H
O
H
S
O
O
H
O
O
Stage 1: electrophilic addition of H2SO4
H2SO4 + H
H
H
H
C
C
C
H
OH
H
H
+ H2O
H
H
H
H
C
C
C
H
H
H
OSO 2OH
Stage 2: hydrolysis of alkyl hydrogen sulphate to form alcohol
KEY
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Industrial Production of ethanol from ethene
Industrially alkenes are converted to alcohols in one step rather than the two in
the above sulphuric acid reaction. They are reacted with water in the presence of
an acid catalyst.
EQUATION
CH2=CH2 (g) + H2O (g)
CH3CH2OH (l)
Essential Conditions
This reaction can be
called hydration: a
reaction where water is
added to a molecule
high temperature 300 °C
high pressure 70 atm
strong acidic catalyst of conc H3PO4
KEY
Reduction of alkenes by hydrogen
Change in functional group: alkene
Reagent:
Conditions:
Type of reaction:
Type of reagent:
alkane
Hydrogen gas
Ni catalyst at high pressure (20 atmosphere)
Reduction (also addition or hydrogenation)
Reducing agent
Equation:
CH2CH2 (g) + H2 (g)
H
CH3CH3 (g)
H
C
C
H
+ H2
H
Ethene
H
H
H
C
C
O
H
O
H
H
Ethane
If there are two double bonds in the alkene, then 1 mole of
alkene will react with 2 moles of hydrogen.
KEY
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Oxidation of alkenes by potassium manganate(VII)
Change in functional group: alkene
diol
Reagent:
KMnO4 acidified with sulphuric acid
Conditions:
Room temperature
Type of reaction:
Oxidation
Type of reagent:
Oxidising agent
Observation:
Purple colour decolourises (can be used as a
functional group test for alkenes)
Equation:
CH2CH2 (g)
H
MnO4- / H+
CH2OHCH2OH (l)
H
C
MnO4- / H+
C
H
H
Ethene
H
H
H
C
C
O
O
H
H
H
Ethan-1,2-diol
EXTRA
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