3.3.6.1 Identification of functional groups by test-tube reactions

Question 1

How do you test for alkenes?

Answer 1

Shake with bromine water, turns from orange to colourless

Question 2

What is the test for alcohols?

Answer 2

Add acidified K2Cr2O7: orange colour turns green with primary or secondary alcohols (also with aldehydes)
Add sodium metal: sodium replaces the hydrogen in the functional group to form hydrogen gas, effervescence

Question 3

What is the test for aldehydes?

Answer 3

Warm with Fehling’s solution: blue colour turns to red precipitate
Warm with Tollens’ solution: silver mirror forms

Question 4

What is the test for carboxylic acids?

Answer 4

Compound is acidic
Add NaHCO3 (aq): CO2 is given off, bubbles formed

Question 5

How does the Fehling’s test work?

Answer 5

Fehling’s reagent is a gentle oxidising agent
It contains blue copper (II) complex ions which oxidises aldehydes but not ketones
During the oxidation the blue copper (II) ions change to a precipitate of red copper (I) ions

Question 6

How does the test for carboxylic acids work?

Answer 6

R - COOH + NaHCO3 = R - COONa + H2O + CO2