Tests for alcohols, aldehydes, alkenes and carboxylic acids

Question 1

Test for primary, secondary and tertiary alcohols

Answer 1

Add H2SO4 and sodium dichromate solution to each of the test tubes. Stand the test tubes in beakers of warm water and wait a few minutes. (initial colour of alcohol and Na2Cr2O7 is orange)
- Primary and secondary alcohols are oxidised by the dichromate solution to form a dark green precipitate.
- Tertiary alcohols are not oxidised, stays orange

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.

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.

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

Question 2

Tollens’ reagent (the silver mirror test)

Answer 2

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 adding just enough dilute ammonia solution to dissolve the precipitate.
Tollens’ reagent contains the diamine silver(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”.
2Ag(NH3)2+ + R-CHO + 3OH− → 2Ag + R-CHOO− + 4NH3 + 2H2O

Question 3

Fehling’s solution

Answer 3

Add a few drops of the aldehyde to each test tube. Place the test tubes in a water bath and heat gently for a few minutes, blue to red precipitate will form.

Fehling’s solution contains copper(II) ions complexed with tartrate ions in sodium hydroxide solution. Complexing the copper(II) ions with tartrate ions prevents the precipitation of copper(II) hydroxide.
Aldehydes reduce the complexed copper(II) ion to copper(I) oxide which precipitates.
R-CHO+ Cu2+ + 5OH − → R-COO− + Cu2O + 3H2O

Question 4

Test for alkene

Answer 4

Add bromine water to the test tube, bromine water is immiscible with alkene so two layers are formed. Allow the test tubes to stand and the layers separate out again. The bromine reacts with the alkene, so it turns colourless.

Question 5

Test for carboxylic acids

Answer 5

Add the solution to a solution of sodium carbonate or to sodium carbonate powder, then bubble the gas produced through lime water, if the lime water turns milky carbon dioxide gas is present meaning the initial solution contained a carboxylic acids