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Part 2.7 The chemistry of
ALKENES - unsaturated hydrocarbons
-
reaction of alkenes with
aqueous potassium manganate(VII) - oxidation to form diol alcohols
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Part 2.7 The reaction of
alkenes with aqueous potassium manganate(VII) - oxidation to form
diol alcohols
A redox reaction involving oxidation
of an organic compound and the reduction of an inorganic compound. The old name
for potassium manganate(VII) was potassium permanganate.
2.7
The reaction of alkenes with
aqueous potassium manganate(VII)
If alkenes are
bubbled into, or the liquid shaken with, a neutral aqueous potassium
manganate(VII), KMnO4(aq), an oxidation reaction occurs and a
diol alcohol is formed at room temperature.
The potassium
manganate(VII) solution decolourises and a brown/black precipitate of
manganese dioxide, MnO2
forms.
This is an
oxidation reaction of an organic compound.
The Mn in
KMnO4 is reduced from a +7 oxidation state to a +4 oxidation
state in MnO2.
(i) The
simplified equation for ethane is shown below, which assumes the oxygen atom
originates from the oxidising agent.
(1)
H2C=CH2
+ [O] + H2O ===> HOCH2-CH2OH
The product
is called
ethane-1,2-diol,
(ii) The
simplified equation for propene is:
(2)
CH3CH=CH2
+ [O] + H2O ===> CH3CH(OH)CH2OH
The
product is called
propane-1,2-diol,
,
(iii) The
simplified equations for but-1-ene and
but-2-ene are:
(3) CH3CH2CH=CH2
+ [O] + H2O ===> CH3CH2CH(OH)CH2OH
(3) CH3CH=CHCH3
+ [O] + H2O ===> CH3CH(OH)CH(OH)CH3
The
products are
butane-1,2-diol and butane-2,3-diol
respectively.
See
also
the
structure and naming of alcohols
(iv) The
simplified equation for
2-methylbut-2-ene is:
(CH3)2C=CHCH3
+ [O] + H2O ===> (CH3)2C(OH)CH(OH)CH3
The
product is 2-methylbutane-2,3-diol
The half-reaction
equation for the potassium manganate(VII) is ...
MnO4-(aq)
+ 4H+(aq) + 3e- ===> MnO2(s)
+ H2O(l)
The half-cell
potential for this half-reaction is
Eθ = +1.67 V,
so the manganate(VII) ion is a strong oxidising agent.
See also
the chemistry of the 3d block
transition metal manganese
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