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Syllabus
Syllabus
(c) qualitative analysis of organic functional groups on a test-tube scale; processes and techniques needed to identify the following functional groups in an unknown compound:
(i) Alkenes by reaction with Bromine (see also 4.1.3 f)
(ii) Haloalkanes by reaction with aqueous Silver Nitrate in Ethanol (see also 4.2.2 a)
(iii) Phenols by weak acidity but no reaction with CO32– (see also 6.1.1 h)
(iv) Carbonyl compounds by reaction with 2,4-DNP (see also 6.1.2 d)
(v) Aldehydes by reaction with Tollens’ Reagent (see also 6.1.2 e)
(vi) Primary and secondary Alcohols and Aldehydes by reaction with acidified dichromate (see also 4.2.1 c, 6.1.2a)
(vii) Carboxylic Acids by reaction with CO32– (see also 6.1.3 b).
What does this mean?
What does this mean?
i) Alkenes
i) Alkenes
This is the same test as it was at GCSE.
Alkenes decolourise Bromine water.
Remember that to have the Alkene functional group doesn't mean a substance is a member of the simple Alkene homolgous series with the general formula CnH2n - because substances can contain more than one functional group.
The substance above is Propenoic Acid.
It is both an Alkene and a Carboxylic acid.
So it will decolourise Bromine water and also fizz when added to Carbonate solutions.
ii) Haloalkanes
ii) Haloalkanes
You've already studied this at GCSE too, though not is such detail.
Add Silver Nitrate solution and Ethanol.
Warm Mixture.
Observe colour of precipitate.
Ethanol contains an -OH group just as water does.
So it's no surprise that hydrolysis of haloalkanes happens and that Halide ions (Cl-, Br- and I-) are produced.
The Halides can then precipitate with Silver ions.
Ag+ (aq) + X-(aq) --> AgX(s)
Uncertainty of the colour of the ppt can be cleared up using ammonia solution (see earlier)
iii) Phenols
iii) Phenols
a) The easiest test for a Phenol is to show that it is acidic (using blue litmus paper) but doesn't make carbonate solution fizz as a carboxylic acid would.
b) As we've seen it also decolourises Bromine but produces a white precipitate (which alkenes do not).
iv) Carbonyls
iv) Carbonyls
In case you forgot, "Carbonyls" means Aldehydes and Ketones only
As we've seen 2,4- DNP produces yellow/orange ppts with all carbonyls but not with other substances containing the C=O bond (Acids, Amides, Esters etc)
The presence of a ppt with DNP identifies the substance as a carbonyl but you woudl have to recrystalise, dry and carry out a melting point to know which one.
v) Aldehydes
v) Aldehydes
If you already suspect that the substance is a Carbonyl you can distinguish between an Aldehyde and a Ketone by exploiting the fact that Aldehydes can be oxidised (to acids) but Ketones do not.
As far as A level Chemistry is concerned only aldehydes give a positive test (Silver Mirror) with warm Tollens reagent.
This is because the Silver ions are reduced to Silver (although few students ever manage a silver mirror as good as the one above).
v) Dichromate test
v) Dichromate test
This is a somewhat non-specific test.
Aldehydes can reduce Orange Dichromate to Green Cr3+ ions.
But so can Primary and Secondary Alcohols.
So, it's another way to distinguish Aldehydes from Ketones.
And a way to distinguish primary and secondary alcohols from tertiary ones.
But you couldn't use it to say for sure what a substance was, even if it does give the Orange to Green colour change.
vi ) Carboxylic Acids
vi ) Carboxylic Acids
As already stated Phenols are also acidic and are also in the A level syllabus.
So merely changing the colour of red litmus paper doesn't identify what a substance is but at least it narrows it down.
To show what your Acidic Organic substance is you have to show that it is a stronger acid than Phenols.
Carboxylic Acids react with weak bases like Carbonates but the more weakly acidic Phenols don't.
So, making Sodium Carbonate solution fizz is the test for a Carboxylic acid only.
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