Organic Functional Group Tests
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flagWhat you'll discover
- arrow_forwardPerform and interpret the sodium metal test for the −OH group
- arrow_forwardDistinguish aldehydes with Tollens' silver mirror and Fehling's red precipitate
- arrow_forwardUse the NaHCO₃ test to separate carboxylic acids from phenol
- arrow_forwardRecognise phenol by the violet FeCl₃ colouration
- arrow_forwardCombine positive AND negative results into a logical identification
Hydroxyl-containing compounds: Na, NaHCO₃ and FeCl₃
Sodium metal fizzes with any acidic O−H hydrogen, releasing H₂: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. Alcohols, phenol and carboxylic acids all give the effervescence, so a positive sodium test says "some −OH is here" without naming it. The discrimination comes from acidity strength. NaHCO₃ liberates CO₂ only from acids stronger than carbonic acid: carboxylic acids fizz briskly (CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂), but phenol — about a hundred thousand times weaker — cannot displace CO₂ and gives nothing. That one negative cleanly separates phenol from the acids.
Neutral FeCl₃ then names phenol directly: a striking violet complex, [Fe(OC₆H₅)₆]³⁻. (Acetate ions give a red colouration instead — a different positive worth knowing so you don't confuse them.) Logic chain: Na fizz + NaHCO₃ fizz = carboxylic acid; Na fizz + no NaHCO₃ fizz + violet FeCl₃ = phenol; Na fizz + both others negative = alcohol.
Aldehydes: the two great reduction tests
Aldehydes are distinguished by being easily oxidised — so reagents containing reducible metal ions light up with them. Tollens' reagent is ammoniacal silver nitrate, [Ag(NH₃)₂]⁺. Warm it gently with an aldehyde and the silver ions are reduced to metallic silver that deposits on the clean glass wall as a brilliant mirror: CH₃CHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → CH₃COO⁻ + 2Ag↓ + 4NH₃ + 2H₂O.
Fehling's solution is alkaline copper(II) tartrate, deep blue. Boil it with an aldehyde and Cu²⁺ is reduced to red-brown Cu₂O precipitate. Ketones fail both tests (no easily-removed carbonyl hydrogen), which is precisely how aldehydes and ketones are told apart. Two cautions: Tollens' reagent must be freshly prepared and discarded after use (on standing it can form explosive silver fulminate), and the tube must be scrupulously clean or the silver deposits as grey sludge instead of a mirror.
Strategy: negatives are evidence too
In the exam you receive one unknown and limited time, so test economically and think in a grid. A fizzing NaHCO₃ test alone almost settles carboxylic acid; confirm with the ester test — warm the liquid with ethanol and a few drops of concentrated H₂SO₄, pour into water, and the fruity smell of ethyl ethanoate is unmistakable: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O.
Equally, absences convict: a liquid that fizzes with sodium but ignores NaHCO₃, Tollens' and FeCl₃ can only be an alcohol — three negatives plus one positive is a complete proof. Record every observation, positive or negative, in your table. The examiner marks the reasoning chain: observation → inference → conclusion. A correct answer with no recorded negative evidence earns less than a fully-argued one.