Download Testing Organic Functional Groups 1

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Tests for organic groups – the OH group.
Tests for organic groups – the alkene (>C=C<) group (test one of two).
Reagents and conditions
Add a small amount of PCl5 to the test substance in a clean, dry test tube in a fume
cupboard at room temperature. Test for any gas evolved with an ammonia stopper and
damp blue litmus paper.
Reagents and conditions
Mix the alkene (e.g. bubble it through or shake together with) a solution of bromine (e.g.
bromine water, bromine in CCl4 or hexane) at room temperature, in a fume cupboard.
Observations
Steamy fumes (of HCl) produced, which make a dense white smoke (NH4Cl) with the
ammonia stopper, and turn the damp blue litmus paper red.
Equations
PCl5 + CH3 CH2OH
PCl5 + CH3 CO2H
→
→
POCl3 + HCl + CH3CH2Cl
POCl3 + HCl + CH3COCl
Notes
This tests for the OH group in either alcohols or carboxylic acids, or even phenols if it is
warmed. Dry conditions are needed as PCl5 is hydrolysed, as water also contains an OH
group.
An alternative test is to add sodium at room temperature under anhydrous conditions,
bubbling of a colourless gas that ignites with a squeaky pop (H2) is the positive result.
Tests for organic groups – alcohols
Observations
Decolourisation of the brown/orange/red bromine solution to colourless.
Equations
For bromine water
CH3 CH=CH2 + Br2 + H2O
→
CH3CH(OH)CH2Br + HBr
For bromine in CCl4 or hexane
CH3 CH=CH2 + Br2
→
CH3CHBrCH2 Br
Notes
When bromine water is used, the water can act as a nucleophile, attacking the more stable
carbocation formed in the second step of the mechanism. This is avoided when an inert
solvent such as CCl4 or hexane is used.
Note that CCl4 is carcinogenic, whilst hexane is flammable.
Tests for organic groups – the alkene (>C=C<) group, (test two of two).
Reagents and conditions
Boil the test substance for a minute with an equal volume of anhydrous ethanoic acid and
a few drops of conc. H2SO4 in a boiling tube, with a few anti-bumping granules. Remove
from the heat and add carefully to a solution of sodium carbonate. Smell carefully.
Reagents and conditions
Mix the alkene (e.g. bubble it through or shake together with) a solution of cold, dilute
potassium manganate (VII) and sodium carbonate.
Observations
A sweet or fruity smell (indicates an ester has been formed).
Observations
2Colour change from purple (MnO4 ) to green (MnO4 ), followed by a brown precipitate
(of MnO2).
Equations
CH3CH2CH2OH + CH3 CO2H
Equations
CH3CH=CH2 + [O] + H2 O
↔
CH3CO2CH2CH2CH3 + H2O
Notes
The solution of sodium carbonate is to neutralise the ethanoic acid, which would mask
any ester smell with its pungent odour.
This test can be performed as a test for a carboxylic acid by adding anhydrous ethanol
instead of anhydrous ethanoic acid.
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→
CH3CH(OH)CH2OH
Notes
Under typical conditions, such as hot and acidified with dil. H2SO4, potassium manganate
(VII) would react with many compounds such as primary and secondary alcohols, and
aldehydes. Here it can be a test for an alkene because it is cold and alkaline, both of which
lower its oxidising power.
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