Download - Qualitative organic analysis
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Qualitative organic analysis
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Reactions of alkanes with bromine.
Alkanes decolourise
bromine in the light.
Alkane layer
Water
But not in the dark!
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Reactions of alkenes with
bromine
Alkenes decolourise bromine in both the dark and the light.
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Reactions of alkenes with potassium manganate (vii)
Alkenes decolourise acidified manganate (vii)
Manganese is reduced from purple +7
To colourless +2
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If there is insufficient alkene…
the intermediate brown Manganese (iv) is formed.
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Reactions of alkanes with manganate (vii)
Alkanes do not decolourise
acidified manganate (vii) as they are saturated.
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Reactions of haloalkanes with silver nitrate
Silver nitrate is colourless
Chloroalkanes give a white ppt, which darkens to purple on exposure to bright light.
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Reactions with silver nitrate
Silver nitrate is colourless
Bromoalkanes give a buff ppt, which darkens on exposure to bright
light.
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Reactions with silver nitrate
Silver nitrate is colourless
Iodoalkanes give a light yellow ppt,
which is not photo sensitive.
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Alcohols give effervescence with sodium
This is because they are extremely weak acids.
Hydrogen gives a “pop” with a lighted splint.
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But as alcohols are extremely weak acids they are unable to neutralise bases.
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Reactions with sodium dichromate.Sodium dichromate is an oxidising agent, ie it oxidises other chemicals, being reduced in the process.
Orange chromium (vi) is
reduced togreen chromium (iii) when heated
with acid.
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Oxidation of primary alcohols.
Primary alcohols are oxidised as
orange chromium (vi) is
reduced togreen chromium (iii) when heated
with acid.
Two organic products are possible; Aldehydes or Carboxylic Acids.
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Oxidation of aldehydes.
Aldehydes are oxidised as
orange chromium (vi) is
reduced togreen chromium (iii) when heated
with acid.
Only one organic product is possible; a Carboxylic Acids.
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Oxidation of secondary alcohols.
Secondary alcohols are also
oxidised as orange
chromium (vi) is reduced to
green chromium (iii) when heated
with acid.
Only one organic product is possible; a Ketone.
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Oxidation of tertiary alcohols
Tertiary alcohols cannot be oxidised
by acidified potassium
dichromate.
To oxidise them a much stronger oxidising agent is needed that can break C/C bonds.
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Testing for carbonyl compounds.
Both aldehydes and ketones give an orange/red
precipitate with 2,4 dinitro phenyl hydrazine.
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Distinguishing between aldehydes and ketones.
Aldehydes are oxidised as
orange chromium (vi) is
reduced togreen chromium (iii) when heated
with acid.
Only one organic product is possible; a Carboxylic Acids.
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Distinguishing between aldehydes and Ketones
Ketones cannot be oxidised by acidified
potassium dichromate.
To oxidise them a much stronger oxidising agent is needed that can break C/C bonds.
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Reaction with Fehling’s Solution
Aldehydes change the colour of
Fehling’s Reagent upon heating from
blue to red.
Blue copper (ii) ions are reduced to red
copper (i).
Cu 2+ + e- → Cu+
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Ketones do not react with Fehling’s
Solution as they are not oxidised by mild
oxidising agents.
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Reaction with Tollen’s ReagentAldehydes react with Tollen’s Reagent (ammonical silver solution), depositing a silver mirror on the side of the tube.
Silver ions are reduced to metallic silver.
Ag+(aq) + e- → Ag(s)
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But Ketones do not react with Tollen’s reagent as they are not oxidised by mild oxidising agents.
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Reactions of carboxylic acids
Acids react with metals giving hydrogen.
Carboxylic acids react with sodium, producing effervescence.
Hydrogen gives a “pop” with a lighted splint.
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Reaction of carboxylic acids with sodium carbonate.
Effervescence (CO2)
Phenols and alcohols are weaker acids and will not react.
Acids react with
carbonates to give a salt,
water and carbon dioxide.
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Reactions with Universal Indicator
Alcohols, carbonyl compounds and esters
are neutral so turn Universal Indicator
green.
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Alcohols might react with sodium like acids, but…
they are unable to neutralise bases.
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Carboxylic Acids are acidic, so turn Universal
Indicator red.
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Carboxylic acids neutralise bases.
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Amines are basic so turn Universal Indicator blue.
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Amines neutralise acids.