DISTINGUISHING TESTS

FOR ALKENE AND ALKANE:

Bayer's Test:

It is a quantitative test for the presence of unsaturation (double or triple bonds).
a sample is treated with small amount of elemental bromine either

• as aqueous solution
• as a solution in CCl4
• as solution in CH2Cl2

REACTION:
Deep brown colouration of bromine rapidly disappears as it is consumed by reaction with sample containing alkenes.

Potassium permanganate test:

It is the quantitative test for the presence of unsaturation.
Dilute KMnO4 is purple and the test depends on its ability to act as an oxidizing agent.

REACTION:
When alkenes are oxidized with cold, alkaline KMnO4, dihydroxy compounds (diols or glycols) are formed. The KMnO4 gets decolorized and in its place brown ppt of MnO2 is seen.

A more compressed form in which to express the reaction in is:

ALKANE AND ALKYNE:

Potassium permanganate test:

At room temperature:
Alkynes react with KMnO4 to give "dioics" and decolorize the purple hue of  the oxidizing agent.

At low temperature:
Alkynes may react to give an 'oic' at low temperatures.

ALKENE AND ALKYNE:

The formation of acetylides is used as mark of identification of alkynes.
ACETYLIDES are the dianions with the formula C2−2.They are formed by deprotonation of alkynes and subsequent addition of metals to the carbon atom.

Alkenes do not react as such with metals.

                                                                                         DI SODIUM ACETYLIDE

The above forms DISILVER ACETYLIDE which cools to form a white solid and DICOPPER ACETYLIDE which cools to form a red solid.

ALKANE AND ALKYL HALIDES:

The most efficient way of distinguishing alkyl halides is by a substitution reaction which would convert the halogen into halide ions.The ions can then be tested by Silver Nitrate solution.

REACTION:

Haloalkanes are warmed with sodium hydroxide solution in a mixture of ethanol and water.Everything will    dissolve.

The nitric acid is added to remove other ions that give a confusing precipitate.
Silver Nitrate solution is added.
It may form white ,pale yellow or yellow ppt depending on the halogen.

ALDEHYDE AND KETONE:

Aldehydes are easily oxidized by all sorts of different oxidizing agents: Ketones aren't.
For Aldehydes ,Oxidation under :
Acidic conditions---gain of oxygen
Alkaline conditions---lose of hydrogen and gain of oxygen

Fehling's Test:

Fehling's solution contains copper (II) ions complexed with tartrate ions in sodium hydroxide solution.

Aldehydes reduce the complexed copper (II) ion to copper (I) oxide. 

Because the solution is alkaline, the aldehyde itself is oxidized to a salt of the corresponding carboxylic acid.

ketone         :No change in the colourless solution.  

Aldehyde     :Red ppt of Cu2O.

Tollen's Test:

Tollens' reagent contains the diamminesilver (I) ion, [Ag (NH3)2] +.

Aldehydes reduce the diamminesilver(I) ion to metallic silver. 

Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid.

• ketone     : No change in the colourless solution.  
• aldehyde  : The colourless solution produces a Grey precipitate of silver, or a silver mirror on the test tube.