3.3.6.1 Identification of functional groups by test-tube reactions
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How do you test for alkenes?
Shake with bromine water, turns from orange to colourless
What is the test for alcohols?
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
What is the test for aldehydes?
Warm with Fehling’s solution: blue colour turns to red precipitate Warm with Tollens’ solution: silver mirror forms
What is the test for carboxylic acids?
Compound is acidic Add NaHCO3 (aq): CO2 is given off, bubbles formed
How does the Fehling’s test work?
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
How does the test for carboxylic acids work?
R - COOH + NaHCO3 = R - COONa + H2O + CO2