Section 3: Organic Chemistry
Organic Analysis
Identi cation o f Functional Groups using Test-tube Reactions
Testing for Primary, Secondary and Tertiary Alcohols
Potassium dichromate(VI) is an oxidising agent. It can oxidise primary + secondary alcohols to form
aldehydes + ketones respectively (tertiary alcohols can’t be oxidised).
As primary + secondary alcohols are oxidised (as shown above), potassium dichromate(VI) is reduced.
This is accompanied by a colour change orange ➜ green as the orange dichromate(VI) ion is reduced
to the green chromium(III) ion…
This colour change can be used to test for the presence of primary, secondary or tertiary alcohols.
Here’s the test you need to carry out:
1) Add 10 drops of the alcohol to 2cm3 of acidi ed potassium dichromate solution in a test tube.
2) Warm the mixture gently in a warm water bath.
- If primary/secondary alcohol: orange ➜ green
- If tertiary alcohol: no colour change - remains orange
HOWEVER, this test does not distinguish between primary and secondary alcohols. But you could test
whether an unknown alcohol is primary or secondary by using the oxidised versions of the primary or
secondary alcohol:
• If you oxidise an alcohol under re ux + it tests +ve for being a carboxylic acid, then it’s a primary
alcohol.
• If you oxidise an alcohol under distillation conditions + it tests +ve for being an aldehyde, then it’s a
primary alcohol.
• If you oxidise an alcohol under re ux (or distillation) + it tests +ve for being a ketone, then it’s a
secondary alcohol.
Testing for Aldehydes and Ketones
Aldehydes + ketones are ammable ∴ warmed using a water bath in these tests rather than a bunsen
burner to prevent them from catching alight.
fi
fl fl fi
Organic Analysis
Identi cation o f Functional Groups using Test-tube Reactions
Testing for Primary, Secondary and Tertiary Alcohols
Potassium dichromate(VI) is an oxidising agent. It can oxidise primary + secondary alcohols to form
aldehydes + ketones respectively (tertiary alcohols can’t be oxidised).
As primary + secondary alcohols are oxidised (as shown above), potassium dichromate(VI) is reduced.
This is accompanied by a colour change orange ➜ green as the orange dichromate(VI) ion is reduced
to the green chromium(III) ion…
This colour change can be used to test for the presence of primary, secondary or tertiary alcohols.
Here’s the test you need to carry out:
1) Add 10 drops of the alcohol to 2cm3 of acidi ed potassium dichromate solution in a test tube.
2) Warm the mixture gently in a warm water bath.
- If primary/secondary alcohol: orange ➜ green
- If tertiary alcohol: no colour change - remains orange
HOWEVER, this test does not distinguish between primary and secondary alcohols. But you could test
whether an unknown alcohol is primary or secondary by using the oxidised versions of the primary or
secondary alcohol:
• If you oxidise an alcohol under re ux + it tests +ve for being a carboxylic acid, then it’s a primary
alcohol.
• If you oxidise an alcohol under distillation conditions + it tests +ve for being an aldehyde, then it’s a
primary alcohol.
• If you oxidise an alcohol under re ux (or distillation) + it tests +ve for being a ketone, then it’s a
secondary alcohol.
Testing for Aldehydes and Ketones
Aldehydes + ketones are ammable ∴ warmed using a water bath in these tests rather than a bunsen
burner to prevent them from catching alight.
fi
fl fl fi
