Organic Analysis: Identification of Functional Groups by Test-tube Reactions

Question 1

How do you test for primary, secondary and tertiary alcohols?

Answer 1

• Chemical used

• Acidified potassium dichromate(VI)
• Acidified with sulfuric acid
• K2Cr2O7
• Can use [O] to represent potassium dichromate, the oxidising agent

• Observation

• Orange solution turns green as aldehyde or ketone forms with primary and secondary alcohols
• No colour change seen with tertiary alcohols

Question 2

How do you distinguish between primary and secondary alcohols?

Answer 2

• Carboxylic acid

• If you oxidise an alcohol under reflux
• And it tests positive for being a carboxylic acid
• It’s a primary alcohol

• Aldehyde

• If you oxidise an alcohol under distillation conditions
• And it tests positive for being an aldehyde
• It’s a primary alcohol

• Ketone

• If you oxidise an alcohol under reflux (or distillation)
• And it tests positive for being a ketone
• It’s a secondary alcohol

Question 3

What two chemicals can be used to test for aldehydes and ketones?

Answer 3

• Fehling’s solution

- Tollen’s reagent

Question 4

Describe the Fehling’s solution way of testing for aldehydes and ketones

Answer 4

• Chemical used

• Fehling’s solution
• Contains copper(II) ions

• Observation

• Turns from blue to a brick red precipitate with aldehydes
• No reaction with ketone, solution stays blue

Question 5

Describe the Tollen’s reagent way of testing for aldehydes and ketones

Answer 5

• Chemical used

• Tollen’s reagent
• Uses [Ag(NH3)2]+ complex

• Observation

• Forms silver mirror with aldehydes
• No reaction with ketone, solution stays colourless

Question 6

How do you test for carboxylic acids?

Answer 6

• Chemical test
- Sodium carbonate

• Observation

• Effervescence
• Limewater turns cloudy

Question 7

How do you test for alkenes?

Answer 7

• Chemical test
- Bromine water

• Observation
- Orange bromine water turns colourless

Question 8

How do you test for halogenoalkanes?

Answer 8

• Chemical test

• Sodium hydroxide
• Silver nitrate

• Observation

• Precipitate will form
• Chloride ions - white precipitate
• Bromide ions - cream precipitate
• Iodide ions - yellow precipitate

Question 9

What colour precipitate is produced from the test for halogenoalkanes?

Answer 9

• Chloride ions
- White

• Bromide ions
- Cream

• Iodide ions
- Yellow