Qualitative tests: organic functional groups

Question 1

what is the functional group for alkenes

Answer 1

C=C

Question 2

What is the test and result for alkenes

Answer 2

Add Br₂ dropwise → C=C declolourises bromine water
orange→colourless

Question 3

What is the test for haloalkanes and results

Answer 3

Add aqueous AgNO₃ in ethanol
a precipitate will form (AgX)
if X = Cl⁻ white precipitate
if X = Br⁻ cream precipitate
if X = I⁻ yellow precipiate

Question 4

what is the test for alcohols and results

Answer 4

Add acidified dichromate (Cr₂O₇²⁻/ H⁺) - this may need to be warmed.
The colour will change from orange to green for primary and secondary alcohols.
Tertiary alcohols won’t react, however aldehydes will

Question 5

what is the test for carboxylic acids and results

Answer 5

• Add carbonate e.g. Na₂CO₃ and will show bubble/ effervessing/ fizzing
• universal indicator will show the carboxylic acid to be weakly acidic

Question 6

what is the test for phenols and results

Answer 6

• Universal indicator will show it to be weakly acidic but it will not react with a carbonate
• Add Fe³⁺ and the solution turns violet

Question 7

what is the test for carbonyls and results

Answer 7

2,4-DNPH (brady’s reagent)
reacts with aldehydes and ketones to give an orange precipitate which has a characteristic melting point reflective of the carbonyl

Question 8

what is the test for aldehydes and results

Answer 8

• Tollen’s reagent - gives a silver mirror
• Acidified dichromate (Cr₂O₇²⁻/ H⁺) - colour changes from orange to green