Topic 5.5: Organic analysis
Tests for functional groups
• Alkenes (C–
–C)
i) Reaction with bromine water (electrophilic addition)
Shake 1 cm
3
of the compound with 1 cm
3
of bromine water (or bubble it through if a gas) in the dark. If the orange
bromine is decolourised, this confirms an alkene
e.g. H
2
C–
–CH
2
+ Br
2
Br–CH
2
–CH
2
–Br
ii) Reaction with acidified potassium manganate(VII) (oxidative addition)
Shake 1 cm
3
of the compound with 1 cm
3
of potassium manganate(VII) solution acidified with a little dilute sulphuric acid
(or bubble it through if a gas). If the purple manganate(VII) is decolourised, this confirms an alkene
e.g. H
2
C–
–CH
2
+ [O] + H
2
O
KMnO
4
, H
+
/ OH
-
HO–H
2
C–CH
2
–OH
• Halogenoalkanes (R-X)
Reaction with aqueous sodium hydroxide (nucleophilic substitution)
Add 1 cm
3
of the compound to 1 cm
3
of aqueous sodium hydroxide in a test tube and heat to 50ºC in a water bath; neutralise
excess hydroxide with dilute nitric acid; add 1 cm
3
of aqueous silver nitrate
White ppt of AgCl ⇒ R-Cl; cream ppt of AgBr ⇒ R-Br; yellow ppt of AgI ⇒ R-I
e.g. R-X + OH
-
aqueous KOH
warm
R–OH + X
-
• Alcohols (R-OH)
Reaction with phosphorus pentachloride
Add a spatula measure of PCl
5
solid to 1 cm
3
of the liquid to be tested. An exothermic reaction which evolves steamy fumes
(HCl) which turn damp litmus red and form a white ‘smoke’ with ammonia gas confirms the presence of an –OH group in the
molecule. As this also gives a positive result with water (H-OH) and carboxylic acids (RCOOH), you must also be sure that
the ‘unknown’ is dry and neutral before you can confirm it is an alcohol
e.g. R–OH + PCl
5
R–Cl + POCl
3
+ HCl
Once you have confirmed an alcohol is present, you may need to identify it as primary, secondary or tertiary
a) Primary alcohols (R–CH
2
–OH)
Oxidise with potassium dichromate acidified with a little dilute sulphuric acid, distilling off the product. If the product
shows a positive test for an aldehyde (see below) the ‘unknown’ must be a primary alcohol
e.g. R–CH
2
–OH + [O]
K
2
Cr
2
O
7
distil off product
,H
+
CR
H
O
+ H
2
O
b) Secondary alcohols (R
2
CH–OH)
Oxidise with potassium dichromate acidified with a little dilute sulphuric acid, distilling off the product. If the product
shows a positive test for a carbonyl group but a negative test for an aldehyde (see below) the product must be a ketone
and the ‘unknown’ must be a secondary alcohol
e.g. R
2
CH–OH + [O]
K
2
Cr
2
O
7
distil off product
,H
+
CR
R
O
+ H
2
O
c) Tertiary alcohols (R
3
C–OH)
Attempt to oxidise by warming with potassium dichromate acidified with a little dilute sulphuric acid. If the potassium
dichromate remains orange, no oxidation has taken place and the ‘unknown’ must be a tertiary alcohol
e.g. R
3
C–OH + [O]
K
2
Cr
2
O
7
,H
+
no reaction
d) Compounds containing –CH(CH
3
)OH – the iodoform (triiodomethane) reaction
To 2 cm
3
of aqueous sodium hydroxide add 1 cm
3
of the ‘unknown’ followed by a few drops of iodine solution, stopper
and shake well. The production of a pale yellow crystalline precipitate (of triiodomethane) confirms the presence of either
the -CH(CH
3
)OH group (present in secondary methyl alcohols or ethanol) or a methyl ketone (see below) which must be
distinguished by additional tests.
e.g. RCH(CH
3
)OH + 6 OH
-
+ 4 I
2
CHI
3
+ RCOO
-
+ 5 I
-
+ 5 H
2
O
• Carbonyl compounds (R
2
C–
–O)
Reaction with 2,4-dinitrophenylhydrazine (condensation)
Add a few drops of the ‘unknown’ to 1cm
3
of a solution of 2,4-DNP in methanol. A dense yellow or orange precipitate
confirms the presence of a carbonyl group
e.g.
C
R
R
O
+
O
2
N
NO
2
NN
H
H
H
K
2
Cr
2
O
7
,H
+
O
2
N
NO
2
NN
H
R
1
R
2
+ H
2
O
Once a carbonyl group has been identified, it may be confirmed as either an aldehyde or ketone by one of the following
procedures:
