Tests for alcohols, aldehydes, alkenes and carboxylic acids
Alcohols
Aldehydes
Alkenes
Carboxylic acids
Videos
The sodium dichromate solution is an oxidising agent.
Primary alcohols
In a primary alcohol, the carbon atom that carries the -OH group is only attached to one alkyl group. Primary alcohols oxidise to an aldehyde.
Secondary alcohols
In a secondary alcohol, the carbon atom with the -OH group attached is joined directly to two alkyl groups, which may be
the same or different. Secondary alcohols oxidise to a ketone.
Tertiary alcohols
In a tertiary alcohol, the carbon atom holding the -OH group is attached directly to three alkyl groups, which may be any combination of the same or different groups
These tests work on any compound containing an aldehyde group.
Tollens' reagent (the silver mirror test)
Tollens' reagent is made from silver(I) nitrate solution by adding a drop of sodium hydroxide solution to give a precipitate
of silver(I) oxide, and then add just enough dilute ammonia solution to redissolve the precipitate.
Tollen's reagent contains the diamminesilver(I) ion. This is reduced by the aldehyde to form metallic silver
that is deposited on the inside surface of the test tube, forming the "silver mirror".
Fehling's solution contains copper(II) ions complexed with tartrate ions in sodium hydroxide solution.
Complexing the copper(II) ions with tartrate ions prevents precipitation of copper(II) hydroxide.
Aldehydes reduce the complexed copper(II) ion to copper(I) oxide which precipitates.
The hydrocarbons in the animation differ by the presence of a double bond. The test will work for any alkene.
Alkenes with fewer than six carbon atoms are gases and you would need to bubble the gas through bromine water.
The test works because the bromine reacts with the double bond. This is called an "addition reaction".
Alkanes and alkene are immiscible with water which is why the test tubes need to shaken or stirred. Left to stand, the layers will separate out
again with the more dense aqueous layer at the bottom. Bromine is soluble in alkanes, so the top layer in the alkane tube will be coloured.
Like other acids, carboxylic acids will react carbonates and hydrogencarbonates to form a salt, carbon dioxide and water.
