Flashcards · 9620 · AS Topic 24

Organic analysis (AS Level) flashcards

13 cards on AS Topic 24, subtopics 3.3.6.1 to 3.3.6.3. Pick a subtopic or study them all.

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Test for a primary or secondary alcohol: Warm with acidified potassium dichromate(VI): orange → green. A tertiary alcohol (or a ketone) leaves it orange.
Test for a carboxylic acid: Add Na₂CO₃(aq): fizzing; the CO₂ turns limewater milky.
Why must aldehydes be ruled out before using dichromate(VI) to test for an alcohol?: Aldehydes are also oxidised by dichromate(VI), so they also turn it from orange to green.
Which two compound types does no AS test-tube reaction separate?: A ketone and a tertiary alcohol: neither reacts with dichromate(VI), Fehling’s or Tollens’. Use IR (C=O vs O–H).
Required practical 5: why heat organic test mixtures in a water bath?: Most of the organic liquids are flammable, so no naked flame.

How does high-resolution mass spectrometry identify a molecular formula?: Formulae with the same integer Mr have different precise masses. The m/z of the molecular ion to 4 decimal places is matched against the precise mass of each candidate formula.
Molecular ion at m/z 90.0678: C₄H₁₀O₂ or C₃H₆O₃? (C 12.0000, H 1.0078, O 15.9949): C₄H₁₀O₂: 48.0000 + 10.0780 + 31.9898 = 90.0678 (match). C₃H₆O₃: 36.0000 + 6.0468 + 47.9847 = 90.0315.
What can mass spectrometry of the molecular ion not tell you?: The structure: many isomers share a molecular formula. IR, test-tube reactions or fingerprint comparison are needed to go further.

IR absorptions: O–H in alcohols vs O–H in carboxylic acids: Alcohol O–H: broad, 3230–3550 cm⁻¹. Acid O–H: very broad, 2500–3000 cm⁻¹ (merges with C–H).
IR absorptions for C=O, C=C and C≡N: C=O: 1680–1750 cm⁻¹ (strong, sharp). C=C: 1620–1680 cm⁻¹. C≡N: 2220–2260 cm⁻¹.
Fingerprint region: Below about 1500 cm⁻¹. Its pattern is characteristic of the whole molecule, so comparing it with spectra of known compounds identifies the substance.
A product expected to be an aldehyde shows a very broad band at 2500–3000 cm⁻¹. What does it show?: Impurity of a carboxylic acid, from over-oxidation.
How do CO₂, methane and water vapour contribute to global warming?: Their C=O, C–H and O–H bonds absorb infrared emitted by the Earth’s surface; the energy is re-emitted in all directions, some back towards the surface.