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Alkenes
A.
Introduction
The table bellows shows the comparison between alkanes and alkenes.
alkanes
alkenes
general formula
CnH2n+2
CnH2n
carbon-to-hydrogen ratio
functional group
n
2n + 2
nil
n
2n
C=C
description
saturated
unsaturated
reactivity
low
high
Usually alkenes are much more reactive than alkanes although C=C double bond is
stronger than C-C.
Bond
C-C
C=C
bond enthalpy (kJ/mol)
+348
+612
Examples of reactions of alkenes
How do you account for this?
The most commonly encountered reaction of compounds containing a carbon-carbon
double bond is Addition reaction.
An addition reaction results in the conversion of one π bond and one σ bond into two σ
p.2
bonds.
The result of this change is usually energetically favorable. The heat evolved in making 2
σ bonds exceeds that needed to break one σ and one π bond, and addition reactions are
usually exothermic.
The electrons of the π bond are exposed because the π orbital has considerable p
character. Thus, a π bond is particularly susceptible to electron-seeking reagents. Such
electron-seeking reagents are said to be electrophilic and are called electrophiles.
The high electron density around the double bond can also account for the electrophilic
attack by other reagents.
B.
Addition of hydrogen halides
i.
General reaction
Hydrogen halide (HF, HCl, HBr, and HI) add readily to the double bond of
alkenes.
In carry out these reactions, the hydrogen halides may be dissolved in acetic acid
and mixed them with the alkenes or gaseous hydrogen halides may be bubbled
directly into the alkenes with the alkenes, itself, being used as the solvent.
The addition of HX to an unsymmetrical alkene could conceivably occur in two
ways. In practice, however, one product usually predominates.
Russian chemist, Vladimir Markovnikov (in 1869) stated that in the addition of
HX to an alkene, the hydrogen added to the carbon of the double bond with the
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greater number of hydrogens.
ii.
Mechanism of addition of HX
The mechanism for addition of a hydrogen halide to an alkene involves two steps
shown below:
The important step - because it is the rate-determinating step - is step _____. The
formation of carbocation is usually highly endothermic and has high energy
of activation.
According to the mechanism, Markovnikov rule can be explained in terms of the
stabilities of the carbocations formed.
For example,
These two carbocations are not of the same stability. The 2o carbocation is more
stable than the 1o. In 2o carbocation, the two methyl groups are of
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electron-releasing property that stablize the carbocation. The more stable
carbocation is formed preferentially because it is formed faster.
Class work:
Notice that Markovnikov's rule allows us to predict the outcome of the addition of
a reagent such as ICl.
(CH3)2C=CH2 + I-Cl Æ
iii.
Free radical addition of alkenes - Anti-Markovnikov additions
Alkenes contain peroxides reacted with hydrogen bromide, anti-Markovnikov
addition occured.
Explanation in terms of mechanism
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C.
Addition of sulphuric acid to alkenes
When alkenes are treated with cold concentrated sulphuric acid, they react by addition to
form alkyl hydrogen sulphate. The mechanism is similar to that of the addition of HX.
Alkyl hydrogen sulphate can be hydrolysed to alcohols.
Does the reaction follow Markovnikov's rule?
D.
Acid-catalysed hydration of alkenes
i.
General reaction
Because the reaction follows Markovnikov's rule, acid-catalysed hydration of
alkenes do not yield primary alcohols.
The mechanism for hydration of an alkene is shown below:
The rate-controlling step of the hydration mechanism is step 1 : the formation of
carbocation. By comparing the stability of carbocations:
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Classwork:
1.
The following order of reactivity is observed when the following alkenes
are subjected to acid-catalysed hydration. Explain the order of reactivity.
(CH3)2C=CH2 > CH3CH=CH2 > CH2=CH2
2.
E.
When 2-methylpropene is dissolved in methyl alcohol containing a strong
acid, a reaction takes place to produce CH3OC(CH3)3. Write a mechanism
that accounts for this.
Halogenation of alkenes
i.
General reaction
The behavior of alkenes toward bromine in carbon tetrachloride is a useful test for
carbon-carbon multiple bonds.
ii.
Mechanism of halogenation of alkenes
In step 1, the π bond electrons of the alkenes attack the halogen and produce
bromonium ion and bromide ion. Since polarization weakens the
bromine-bromine bond. In bromonium, a bromine atom is bonded to two carbon
atoms by two pairs of electrons. This is possible of the unshared electron pairs on
bromine.
In step 2, the bromide ion acts as nucleophile to attack the bromonium ion.
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In bromination of cyclopentene provides additional evidence for the above
mechanism.
F.
Hydrogenation of alkenes
i.
General reaction
Alkenes react with hydrogen in the presence of metal catalyst such as nickel and
platinum to produce alkanes. This converts the alkenes to alkanes.
The reaction is usually carried out by dissolving the alkenes in a solvent such as
ethyl alcohol or ethanol (C2H5OH), adding metal catalyst and then exposing the
mixture to hydrogen gas under pressure in a special apparatus.
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G.
Polymerization of alkenes
Polymers are compounds that consists of very large molecules made up of many
repeating units. The molecular units that are used to synthesise polymers are called
monomers, and the reactions by which monomers are joined together are called
polymerization reactions.
Propenes, for example, can be polymerized to form polypropene. This polymerization
occurs by an addition reaction and as a consquence, polymers such as polypropene are
called addition polymer.
The addition reactions may occur through free-radicals, cationic, or anionic mechanisms
depending on how they are initiated.
More examples of addition polymers:
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Addition polymerization can occur between two kinds of monomers:
A mixture of vinyl chloride and vinylidene chloride polymerizes to form what is known
as a copolymer. The subunits do not necessarily alternate regularly along the polymer
chain.
Classwork:
H.
Predict the structure of polymer formed. Explain.
Cleavage of alkenes
i.
Ozonization (ozonolysis) of alkenes
Ozone reacts vigorously with alkenes to form unstable compounds called initial
ozonides which rearrange spontaneously to form compounds known as ozonides.
Ozonides themselves are very unstable compounds and low molecular weight
ozonides often explode violently. They usually are reduced directly by zinc and
water.
The identities of the aldehydes or ketones disclose the location of the double bond in
the original alkene.
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Classwork:
a.
b.
c.
d.
Write the general structure of the alkenes that would produce the following
products when treated with ozone and then zinc and water.
CH3CH2CHO + CH3CHO
(CH3)2C=O only (two moles are produced from one mole of alkene)
(CH3)2CHCH2CHO + HCHO
OHCCH2CH2CH2CH2CHO only
ii.
Oxidative cleavage of alkenes
Alkenes are oxidatively cleaved to salts of carboxylic acid by hot permanganate
solution.
Acidification of the mixture, after the oxidation is complete, produces two moles of
ethanoic acid from each mole of 2-butene. The terminal group of a 1-alkene is
completely oxidized to carbon dioxide and water.
Classwork
An unknown alkene with formula C8H16 was found, on oxidation with hot
permanganate, to yield a five-carbon carboxylic acid (pentanoic acid) and a
three-carbon carboxylic acid (propanoic acid).
Predict the structure of the alkenes.