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Properties of Alkenes

Key Concepts

  • Alkenes were once known as olefins.

  • Alkenes are hydrocarbons containing a double bond between two carbon atoms (C=C).
    Alkenes are therefore unsaturated hydrocarbons.

  • Alkenes can be produced, synthesised, by elimination reactions involving alkanols or haloalkanes.
Physical Properties of Alkenes:

  • colourless

  • low boiling points

  • insoluble in water

Chemical Properties of Alkenes:

Laboratory Tests for Alkenes:

  • alkenes decolourise bromine solutions

  • alkenes decolourise potassium permanganate solutions

Synthesis of Alkenes

Alkenes can be prepared by elimination reactions in which a small molecule such as water is eliminated from a reactant molecule such as an alkanol or haloalkane.
Two elimination reactions commonly used to synthesise alkenes are

(i) Acid-catalysed Dehydration of Alkanols (alcohols)

alkanol H+

Δ
alkene + water
general equation
H
|
OH
|
R-C-C-R'
|
H
|
H
H+

Δ
R-C=C-R'
|
H
|
H
+ H2O
example ethanol conc H2SO4

Δ
ethene + water
H
|
OH
|
H-C-C-H
|
H
|
H
conc H2SO4

Δ
H-C=C-H
|
H
|
H
+ H2O
example butan-2-ol
(2-butanol)
conc H2SO4

Δ
but-2-ene
(2-butene)
(major product)*
+ but-1-ene
(1-butene)
(minor product)*
+ water
H
|
OH
|
H
|
H
|
H-C-C-C-C-H
|
H
|
H
|
H
|
H
conc H2SO4

Δ
H
|
H
|
H-C-C=C-C-H
|
H
|
H
|
H
|
H
+
H
|
H
|
H-C=C-C-C-H
|
H
|
H
|
H
|
H
+ H2O

(ii) Dehydrohalegnation of Haloalkanes

The conditions of the reaction are extremely important!

In order to eliminate water, a solution of potassium hydroxide in alcohol is used (referred to as alcoholic KOH).
If aqueous KOH were used, the result would be a substitution reaction producing an alcohol and a salt!

general
equation:
haloalkane alcoholic
KOH

Δ
alkene + water + salt
H
|
X
|
R-C-C-R'
|
H
|
H
alcoholic
KOH

Δ
R-C=C-R'
|
H
|
H
+ H2O + K+X-
example: chloroethane alcoholic
KOH

Δ
ethene + water + potassium chloride
H
|
Cl
|
H-C-C-H
|
H
|
H
alcoholic
KOH

Δ
H-C=C-H
|
H
|
H
+ H2O + K+Cl-
example: 2-bromobutane alcoholic
KOH

Δ
but-2-ene
(2-butene)
(major product)*
+ but-1-ene
(1-butene)
(minor product)*
+ water + potassium bromide
H
|
Br
|
H
|
H
|
H-C-C-C-C-H
|
H
|
H
|
H
|
H
alcoholic
KOH

Δ
H
|
H
|
H-C-C=C-C-H
|
H
|
H
|
H
|
H
+
H
|
H
|
H-C=C-C-C-H
|
H
|
H
|
H
|
H
+ H2O + K+Br-

Physical Properties of Alkenes

(i) Boiling Points

Alkenes are non-polar molecules.
Only weak intermolecular forces (dispersion or London forces) act between the molecules.
Since little energy is required to disrupt these weak intermolecular forces, alkenes are expected to have low melting and boiling points.

number of
carbon atoms
in carbon chain
IUPAC
Name
boiling
point (oC)
state
(25oC, 1 atm)
2 ethene -102 gas
3 propene -48 gas
4 1-butene -6 gas
5 1-pentene 30 liquid

Boiling points of alkenes are low.
Alkene boiling points increase with increasing molecular mass.
As the number of carbon atoms in the carbon chains increases, the long carbon chains are increasingly attracted to each other by weak intermolecular forces (dispersion or london forces) so more energy is required to separate the molecules and the boiling points of the alkenes increase.
Members should log-in to use the graph.

(ii) Solubility

  • Alkenes are soluble in non-polar solvents.
    Non-polar alkene molecules are attracted to other non-polar molecules by weak intermolecular forces (Van der Waals Forces, Dispersion Forces, London Forces), so non-polar alkene molecules will dissolve in non-polar solvents.

  • Alkenes are insoluble in polar solvents like water.
    The molecules in a polar solvent such as water are strongly attracted to each other as a result of the attraction of partial negative and partial charges within each molecule:
    δ+H-Oδ--Hδ+
    .
    .
    .
    .
    .
    Red dotted lines (...) represent the intermolecular attraction between the partial negative charge on the oxygen atom of one water molecule and the partial positive charge on the hydrogen atom of a different water molecule. This type of intermolecular attraction is known as a hydrogen bond.
    δ+H-Oδ--Hδ+

    When a non-polar alkene is added to a polar solvent:
        (a) the alkene molecules are attracted to each other but are not attracted to the water molecules
        (b) the water molecules are attracted to each other but are not attracted to the alkene molecules
    so the alkene does not dissolve in the polar solvent.

Chemical Reactions of Alkenes

The reactive site in alkene molecules is the carbon-carbon double bond (C=C).
In chemical reactions this double bond either opens out to leave a carbon-carbon single bond (C-C) or it breaks completely to separate the molecule into two smaller fragments.

(i) Combustion of Alkenes

Complete combustion of alkenes (combustion in exess oxygen) produces carbon dioxide and water.

general equation: alkene + oxygen carbon dioxide + water
example: ethene + oxygen carbon dioxide + water
C2H4 + 3O2 2CO2 + 2H2O

Incomplete combustion of alkenes (combustion in insufficient oxygen) produces water, and, carbon monoxide and/or carbon.

sample equation: alkene + oxygen carbon + water
example: ethene + oxygen carbon + water
C2H4 + O2 2C + 2H2O

(ii) Addition Reactions of Alkenes

In addition reactions, atoms are added across the carbon-carbon double bond (C=C) of the alkene to produce an alkane or a substituted alkane.

Addition of General Equations   Example
hydrogen
(hydrogenation)
alkene + hydrogen alkane   ethene + hydrogen Pt

catalyst
ethane
R-C=C-R'
|
H
|
H
+ H2 metal

catalyst
H
|
H
|
R-C-C-R'
|
H
|
H
 
H-C=C-H
|
H
|
H
+ H2 Pt

catalyst
H
|
H
|
H-C-C-H
|
H
|
H
halogen
(halogenation)
alkene + halogen dihaloalkane   ethene + bromine 1,2-dibromoethane
R-C=C-R'
|
H
|
H
+ X2
X
|
X
|
R-C-C-R'
|
H
|
H
 
H-C=C-H
|
H
|
H
+ Br2
Br
|
Br
|
H-C-C-H
|
H
|
H
hydrogen
halide
(hydrohalogenation)
alkene + hydrogen
halide
haloalkane   ethene + hydrogen
bromide
bromoethane
R-C=C-R'
|
H
|
H
+ HX
H
|
X
|
R-C-C-R'
|
H
|
H
 
H-C=C-H
|
H
|
H
+ HBr
H
|
Br
|
H-C-C-H
|
H
|
H
water
(hydration)
alkene + water alkanol   ethene + water ethanol
R-C=C-R'
|
H
|
H
+ H2O heat

pressure
H
|
OH
|
R-C-C-R'
|
H
|
H
 
H-C=C-H
|
H
|
H
+ H2O 300oC

100 atm
H
|
OH
|
H-C-C-H
|
H
|
H

(iii) Oxidation of Alkenes

Under mild oxidising conditions such as cold, dilute aqueous potassium permanganate, alkenes are oxidised to alkanediols (diols).
These reactions are also known as hydroxylation reactions because hydroxyl (OH) groups add across the alkene's carbon-carbon double bond (C=C).

general equation: alkene cold dil. KMnO4(aq)
alkanediol  
R-C=C-R'
|
H
|
H
cold dil. KMnO4(aq)
OH
|
OH
|
R-C-C-R'
|
H
|
H
example: ethene cold dil. KMnO4(aq)
ethane-1,2-diol
(1,2-ethanediol)
 
H-C=C-H
|
H
|
H
cold dil. KMnO4(aq)
OH
|
OH
|
H-C-C-H
|
H
|
H

Under strong oxidising conditions, such as hot, concentrated potassium permanganate solution, the double bond in the alkene breaks completely.
The products of the reaction will depend on the location of the carbon-carbon double bond (C=C).

straight-chain 1-alkene alke-1-ne hot conc. KMnO4(aq)
alkanoic acid
(carboxylic acid)
+ carbon dioxide + water
R-C=C-H
|
H
|
H
hot conc. KMnO4(aq)
R-C=O
|
OH
+ CO2(g) + H2O
but-1-ene
(1-butene)
hot conc. KMnO4(aq)
propanoic acid + carbon dioxide + water
H
|
H
|
H-C-C-C=C-H
|
H
|
H
|
H
|
H
hot conc. KMnO4(aq)
H
|
H
|
 
H-C-C-C=O
|
H
|
H
  |
OH
+ CO2(g) + H2O

straight-chain n-alkene alk-n-ene
(n-alkene)
hot conc. KMnO4(aq)
alkanoic acid
(carboxylic acid)
+ alkanoic acid
(carboxylic acid)
R-C=C-R'
|
H
|
H
hot conc. KMnO4(aq)
R-C=O
|
OH
+
O=C-R'
|
OH
but-2-ene
(2-butene)
hot conc. KMnO4(aq)
acetic acid
(ethanoic acid)
+ acetic acid
(ethanoic acid)
H
|
H
|
H-C-C=C-C-H
|
H
|
H
|
H
|
H
hot conc. KMnO4(aq)
H
|
H -C-C=O
|
H
|
OH
+
H
|
O=C-C- H
|
HO
|
H

single branched-chain alkene alkylalkene hot conc. KMnO4(aq)
alkanoic acid
(carboxylic acid)
+ alkanone
(ketone)
R-C=C-R'
|
H
|
R"
hot conc. KMnO4(aq)
R-C=O
|
OH
+
O=C-R'
|
R"
2-methylbut-2-ene
(2-methyl-2-butene)
hot conc. KMnO4(aq)
acetic acid
(ethanoic acid)
+ acetone
(propan-2-one)
H
|
H
|
H-C-C=C-C-H
|
H
|
H
|
H-C-H
|
H
|
H
hot conc. KMnO4(aq)
H
|
H -C-C=O
|
H
|
OH
+
H
|
O=C - C- H
|
H-C-H
|
H
|
H

double branched-chain alkene alkylalkene hot conc. KMnO4(aq)
alkanone
(ketone)
+ alkanone
(ketone)
R-C=C-R'
|
R'"
|
R"
hot conc. KMnO4(aq)
R-C=O
|
R'"
+
O=C-R'
|
R"
2,3-dimethylbut-2-ene
(2,3-dimethyl-2-butene)
hot conc. KMnO4(aq)
acetone
(propan-2-one)
+ acetone
(propan-2-one)
H
|
H
|
H-C - C = C -C-H
|
H
|
H-C-H
|
H-C-H
|
H
|
H
|
H
hot conc. KMnO4(aq)
H
|
H-C - C = O
|
H
|
H-C-H
|
H
+
H
|
O=C - C- H
|
H-C-H
|
H
|
H

(iv) Polymerisation of Alkenes

Using an alkene as the monomer, polymerisation occurs when the carbon-carbon double bond (C=C) in the alkene opens out to form new single bonds with the neighbouring alkene monomers.
This type of polymerisation is known as addition polymerisation.

monomer
(alkene)
catalyst
polymer
(polyalkene)
General Equation
  R
|
  R'
|
nC=C
  |
R"
  |
R'"
catalyst
-(-
R
|
  R'
|
C-C
|
R"
  |
R'"
-)n-
ethene → polythene
(ethylene → polyethylene)
  H
|
  H
|
nC=C
  |
H
  |
H
catalyst
-(-
H
|
  H
|
C-C
|
H
  |
H
-)n-
propene → polypropene
(propylene → polypropylene)
  H
|
  CH3
|
nC=C
  |
H
  |
H
catalyst
-(-
H
|
  CH3
|
C-C
|
H
  |
H
-)n-


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*Saytzeff orientation rule: In general, if two or more products are possible, the more highly substituted product, that is the alkene with the larger number of alkyl groups, is favoured.

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