Identification of Amines

Amines are basic organic compounds that are considered as derivatives of

ammonia. They are classified as primary, secondary, or tertiary according to the
number of groups attached to the nitrogen atom: RNH 2 , R 2 NH, or R 3 N
respectively where R is any alkyl or aryl group.

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Physical properties :
 Like ammonia, amines are polar compounds and all of them can form
intermolecular hydrogen bonds except tertiary amines.
 They have lower boiling points than alcohols or carboxylic acids of the same
molecular weight but higher boiling points than nonpolar compounds.
 Methylamine is gas while o-phenylenediamine and p,p
diaminodiphenylmethane are solids The others are liquids.
 All amines are capable of forming hydrogen bonds with water, thus those
with six carbon atoms or less are quite soluble in water They are soluble in
organic solvents as ether, alcohol and benzene.
 All of them have fish like odour except the methylamines and ethylamines
which smell just like ammonia.
 Aromatic amines are colorless when pure, but they are easily oxidized by air
becoming colored.
 They are generally very toxic and can be absorbed through the skin.

Chemical reactions
All classes of amines (primary, secondary, and tertiary) have an unshared pair of
electrons on the nitrogen atom, just like ammonia That is why they are similar to
ammonia in their chemical behavior (mainly basicity and neocleophilicity) .

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1- Ramini and Simon tests (Sodium Nitroprusside tests).

conventional Ramini and Simon tests

In Ramini test the amine reacts with acetone and the product interacts with sodium
Nitroprusside (Na2[Fe(NO)(CN)5]2H2O) that is dissolved in 50% aqueous
methanolic solution to produce a colored complex In Simon test acetone is
replaced by 2.5 M acetaldehyde solution These two tests distinguish between
primary and secondary aliphatic amines.

To distinguish between aromatic amines ( secondary and tertiary) the modified

Ramini and Simon tests are applied. These tests use the same reagents and
procedure of the conventional tests but the modifications are the replacement of the
methanolic solution of sodium Nitroprusside by a solution of sodium Nitroprusside
in dimethylsulfoxide (modified sodium Nitroprusside reagent) and the use of a
saturated aqueous solution of zinc chloride instead of water.

Procedure:

 Ramini test

1 mL of methanolic sodium Nitroprusside solution add 1 mL of distilled water, 5

drops of acetone, and about 30 mg of the amine In most cases the characteristic
color appears in a few seconds, although in some cases 2 minutes may be
necessary.

 Simon test

Follow the above procedure exactly but use 5 drops of 2.5 M acetaldehyde
solution instead of acetone Up to 2 minutes may be needed for the color to
develop.

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  Modified Ramini test

To 1 mL of the modified sodium Nitroprusside reagent add 1 mL of saturated

aqueous zinc chloride solution, 5 drops of acetone, and about 30 mg of the amine
Primary and secondary aromatic amines produce orange red to red brown colors
within a period of few seconds to 5 minutes Tertiary aromatic amines give a color
that changes from orange red to green over a period of about 5 minutes.

 Modified Simon test

Follow the above procedure exactly but use 5 drops of 2.5 M acetaldehyde solution
instead of acetone Primary aromatic amines give an orange red to red brown color
within 5 minutes secondary aromatic amines give a color changing from red to
purple within 5 minutes tertiary aromatic amines give a color that changes from
orange red to green over a period of about 5 minutes.

Modified Modified
Amine Ramini test Simon test
Ramini test Simon test
tert- Butylamine Deep red Red-brown
Violet with
Dicyclohexylamine Deep red
precipitate
Diethylamine Red-brown Deep blue
Turbid orange change
Aniline Brown
to henna color
Turbid Turbid henna
o-Phenylenediamine
red-brown color
Brown changes
Brown changes to
N,N-Dimethylaniline to green (rapid
green
change)
Orange-red to red-
Diphenylamine Pale orange
brown

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 2- Nitrous acid test.

The reaction of amines with nitrous acid (HNO2) is another test that classifies the
amine not only as primary, secondary, or tertiary, but also as aliphatic or aromatic.

 Primary aromatic and aliphatic amines react with nitrous acid to form an
intermediate diazonium salt The aliphatic diazonium salts decompose
spontaneously by rapid loss of nitrogen, particularly when the original amino
group is attached to a secondary or tertiary carbon, while most aromatic
diazonium salts are stable at 0°C but lose nitrogen slowly on warming to
room temperature.

 Secondary amines undergo a reaction with nitrous acid to form

N-nitrosoamines, which are usually yellow oils or solids These are
carcinogenic compounds therefore, students are advised not to perform
nitrous acid test for secondary amines.

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  Tertiary aliphatic amines do not react with nitrous acid, but they form a
soluble salt.

 Tertiary aromatic amines react with nitrous acid to form the orange colored
hydrochloride salt of the C-nitrosoamine Treating the solution with base
liberates the blue or green C-nitrosoamine

Procedure
Nitrous acid is prepared instantaneously by the reaction of sodium nitrite and
hydrochloric acid:

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In a test tube dissolve 0.5 mL or 0.5 g of the amine in a mixture of (5)mL of
concentrated hydrochloric acid and 2.5 mL of water, and cool the solution to 0°C
in a beaker of ice In another test tube dissolve 0.5 g of sodium nitrite in 2.5 mL of
water, and add this solution drop wise, with shaking to the cold solution of the
amine hydrochloride Move 2 mL of the final solution to another test tube, warm
gently, and examine for evolution of gas.

Results

 The observation of rapid bubbling or foaming as the aqueous sodium nitrite

solution is added at 0°C indicates the presence of a primary aliphatic amine.

 The evolution of gas (bubbling) upon warming to room temperature

indicates that the amine is a primary aromatic amine and the solution should
be subjected to the coupling reaction (test 3).
 If a pale yellow oil (heavier than water) or low melting solid, which is the
N-nitrosoamine, is formed with no evolution of gas, the original amine is
a secondary amine.
 If a dark orange solution or an orange crystalline solid is formed, which is
the hydrochloride salt of the C-nitrosoamine, the amine is tertiary aromatic
Treating 2 mL of this solution with few drops of 10% sodium hydroxide or
sodium carbonate solution produces the bright-green or blue nitrosoamine
base.
 If only solubilization of the amine is obtained with no other results, the
amine is tertiary aliphatic.

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3- Coupling reaction(a test for primary aromatic amines).

Procedure
Dissolve 0.1 g of 2 naphthol in a mixture of 2 mL of 10 % sodium hydroxide
solution and 5 mL distilled water Add 2 mL of the cold diazonium solution and
observe the result The formation of a red orange dye (red precipitate in case of
phenol) with evolution of gas only upon warming indicates that the compound is a
primary aromatic amine

4- . Carbon disulfide reagent test (for secondary aliphatic amines).

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Procedure
In a test tube dissolve ) 50 mg 1-2 drops) of the amine in 5 mL distilled water (or
1-2 drops of concentrated hydrochloric acid if necessary) In another test tube mix
0.5 1 mL of concentrated ammonia solution with 1 mL of nickel chloride in carbon
disulfide reagent (NiCl2 /CS2 Add 0.5-1 mL from the first test tube to the second
one A definite precipitation indicates that the unknown is a secondary amine A
slight turbidity is an indication of a trace of a secondary amine as an impurity.

5- Lignin test ( for primary and secondary aromatic amines)

This test depends on the action of lignin in the newsprint paper.

Procedure

Dissolve 10-20 mg of the amine in a few drops of ethanol and moisten a small
area of newsprint paper with this solution. Place 2 drops of 6N hydrochloric acid
on the moistened spot. The immediate develop of yellow or orange color is a
positive test for a primary or secondary aromatic amine.

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