Organic Chemistry Laboratory II
Identification of Alcohols by Chemical
Methods
Experiment Description and Background
When an alcohol
behaves as a base (in low pH or acidic
environments), the
atom of the hydroxyl group accepts a proton to generate an oxonium ion |
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When an alcohol behaves
as an acid (in high pH or basic environments), the
hydrogen atom of the hydroxyl group donates a proton
to generate an alkoxide ion. |
Mechanism of the
iodoform reaction of a methyl ketone |
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Some
secondary alcohols are oxidized to methyl
ketones under the conditions of the iodoform
reaction. Secondary alcohols where the
hydroxyl group is bonded to the second carbon
atom from a terminal chain will be oxidized to
methyl ketones. 2-pentanol is an example
of this knid of alcohol (see below). The
methyl ketone that results form this oxidation
can then proceed into the iodoform reaction and
will result in giving a positive result for this
test.
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Permaganate
(KMnO4) Test
The permaganante test is generally used to test for alkene or alkyne functinal groups. Reaction of potassium permanganate with alkenes or alkynes occurs through an oxidation process. The permanganate oxygen atoms add to the pi system of the alkene or alkyne to generate a cyclic intermedicate that decomposes to a diol product. The diol product generated from alkynes reacts further to provide a a-hydroxcarbonyl compound as the final product. The purple permanganate reagent is reduced during the course of the oxidation reaction to give a colorless, yellow or brown solution. The transformation of the reaction mixture from a deep purple color to a yellow or brown solution indicates a positive result for this test. The pi bonds of an aromatic ring are not susceptible to oxidation by the permanganate reagent. However, permanganate may react with some benzylic atoms (atoms bonded to an sp2 hybridized C of a benzene ring) through a somewhat different reaction mechanism. The benzylic atom of toluene (for example) will be oxidized to a carboxylic acid. A postive test result will also frequently be observed when phenols are subjected to the permanganate reaction. |
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Bromine/Water (Br2/H2O)Test The bromine water test
is generally used to identify alkenes, alkynes and
phenols. The bromine in water reagent reacts
with sites of unsaturation, even aromatic rings,
through a complex addition reaction.
For alkenes and alkynes, the reaction occurs
through a simple electrophilic addition. The pi
bond of enols will also react with bromine. But
for pi bonds of an aromatic ring, a hydroxyl group
must be bonded to the ring to stimulate the reaction
(i.e., a phenol). The hydroxyl group of the
phenol activates the ring toward reaction with the
electrophilic bromine. Bromine is a dark brown color but when
it reacts and becomes incorported into the product
via the addition react, the color dissipates and the
reaction mixture becomes yellow or sometimes
colorless. The dissipation of the brown color
of the bromine reagent indicates that the tested
compound contains an unsaturated hydrocarbon (alkene
or alkyne), phenol, or enol. The
advantage to using water as the solvent in this
reaction is that the polarity significantly enhances
the stability of the reaction intermediate and
increases the overall rate of the reaction. Additional catalysts are not
necessary. Aromatic
compounds without the phenol functional group rarely
react with bromine through this mechanism.
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Lucas Test The Lucas test is used
to identify compounds that contain alcohol functional
groups. The Lucas reagent is an aqueous
solution of strong acid (HCl) and zinc chloride. The alcohol starting
material must be sufficiently soluble in aqueous
environments (i.e., the Lucas reagent) for the
reaction to take place. Therefore,
only water-soluble, allylic (primary, secondary and
tertiary), tertiary
alkyl and very
few secondary
alcohols of low molecular weight will provide
positive results in this test. The reaction
that occurs in the Lucas test is an SN1
nucleophilic substitution. Only
alcohols that can generate stable carbocation
intermediates will undergo the reaction. The acid catalyst
activates the OH group of the alcohol by protonating
the oxygen atom. The
C-OH2+ bond breaks to generate
the carbocation, which in turn reacts with the
chloride ion (nucleophile) to generate an alkyl
halide product. An insoluble layer,
cloudiness, a change in color (red or orange) or
an emulsion will form with 1°, 2° or 3° allylic, 3° alkyl and some 2 ° alkyl alcohols. The
insoluble layer is the organic halide that forms
as the product in this SN1 reaction and this
organic halide (no hydrogen bonding) is not water
soluble, even if the starting alcohol (has
hydrogen bonding) is water soluble.
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