PAG 7.3 Test For Alcohol, Aldehydes, Alkenes, Carboxylic Acids

Question 1

Test for alcohols

Answer 1

Add acidified (sulphuric acid) potassium dichromate (K2CrO7) solution to each of the test tubes
Stand test tubes in beakers of warm water
Primary and secondary alcohols are oxidised by the dichromate solution to form a dark green precipitate
Tertiary alcohols are not oxidised
Orange > dark green

Question 2

Test for aldehyde

Answer 2

Add few drops of aldehyde to Tollens’ reagent and Fehling’s solution
Head gently in water bath for a few minutes
Aldehyde + Tollens’ = colourless > silver mirror
Aldehyde + Fehling’s = blue solution > red precipitate
If not provided with Tollens’ reagent:
Add a drop of NaOH to silver nitrate solution then add dilute ammonia solution dropwise till the brown precipitate dissolves to form a colourless solution

Question 3

Test for alkenes

Answer 3

Add bromine water to solution
Stir the mixture
Alkene + bromine water = orange >colourless

Question 4

Text for carboxylic acids

Answer 4

Add carboxylic acid to solution of sodium carbonate or sodium carbonate powder
Test tube which contains effervescence has the acid
Carbon dioxide gas is released which turns lime water milky

Question 5

Test for haloalkanes

Answer 5

Heat sample with NaOH under reflux to release any halide ions
Add small amount silver nitrate and ethanol to the solution
Place test tubes in warm water bath
Test tubes which a precipitate (cream + yellow) forms, contain the haloalkanes
If need to distinguish between Br- and I-:
Add conc ammonia
If precipitate dissolves > contains Br-

Question 6

Test for phenol

Answer 6

Add bromine water
If phenol present, solution changes from orange to colourless + white precipitate forms