Qualitative organic
analysis
Reactions of alkanes with bromine.
Alkane
layer
Water
Alkanes
decolourise
bromine in the
light.
But not in the
dark!
Reactions of
alkenes with
bromine
Alkenes decolourise
bromine in both the
dark and the light.
Reactions of alkenes with
potassium manganate (vii)
Alkenes decolourise
acidified manganate
(vii)
Manganese is reduced
from purple +7
To colourless +2
If there is insufficient
alkene…
the intermediate
brown Manganese (iv)
is formed.
Reactions of alkanes with manganate (vii)
Alkanes do not
decolourise
acidified
manganate (vii) as
they are saturated.
Reactions of haloalkanes with silver
nitrate
Silver nitrate is
colourless
Chloroalkanes give a
white ppt, which
darkens to purple on
exposure to bright
light.
Reactions with silver nitrate
Bromoalkanes give a
buff ppt, which darkens
on exposure to bright
light.
Silver nitrate is
colourless
Reactions with silver nitrate
Iodoalkanes give a
light yellow ppt,
which is not photo
sensitive.
Silver nitrate is
colourless
Alcohols give effervescence
with sodium
This is because they are extremely weak
acids.
Hydrogen gives a “pop” with a lighted splint.
But as alcohols are extremely weak acids
they are unable to neutralise bases.
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 to
green chromium
(iii) when heated
with acid.
Oxidation of primary alcohols.
Primary alcohols
are oxidised as
orange
chromium (vi) is
reduced to
green chromium
(iii) when heated
with acid.
Two organic products are possible;
Aldehydes or Carboxylic Acids.
Oxidation of aldehydes.
Aldehydes are
oxidised as
orange
chromium (vi) is
reduced to
green chromium
(iii) when heated
with acid.
Only one organic product is possible;
a Carboxylic Acids.
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.
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.
Testing for carbonyl compounds.
Both aldehydes and ketones
give an orange/red
precipitate with 2,4 dinitro
phenyl hydrazine.
Distinguishing between
aldehydes and ketones.
Aldehydes are
oxidised as
orange
chromium (vi) is
reduced to
green chromium
(iii) when heated
with acid.
Only one organic product is possible;
a Carboxylic Acids.
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.
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+
Ketones do not react
with Fehling’s
Solution as they are
not oxidised by mild
oxidising agents.
Reaction with Tollen’s Reagent
Aldehydes 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)
But Ketones do not
react with Tollen’s
reagent as they are
not oxidised by mild
oxidising agents.
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.
Reaction of carboxylic acids with
sodium carbonate.
Acids react
with
carbonates
to give a salt,
water and
carbon
dioxide.
Effervescence
(CO2)
Phenols and
alcohols are
weaker
acids and
will not
react.
Reactions with Universal Indicator
Alcohols, carbonyl
compounds and esters
are neutral so turn
Universal Indicator
green.
Alcohols might react with sodium like acids,
but…
they are unable to
neutralise bases.
Carboxylic Acids are
acidic, so turn Universal
Indicator red.
Carboxylic acids neutralise bases.
Amines are basic so
turn Universal
Indicator blue.
Amines neutralise acids.