Flashcards · 7405 · AS Topic 24

Organic analysis (AS Level) flashcards

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Required practical 6: test for a carboxylic acid: Add Na₂CO₃ or NaHCO₃: effervescence; the CO₂ turns limewater cloudy.
How does acidified dichromate distinguish alcohols?: Primary and secondary alcohols turn it from orange to green. A tertiary alcohol leaves it orange.
In a test sequence, why test for an aldehyde before using dichromate?: Aldehydes are also oxidised by dichromate (orange → green), so an aldehyde would be mistaken for an alcohol.
How does high-resolution mass spectrometry find a molecular formula?: Molecules with the same whole-number Mr have different precise masses. The m/z of the molecular ion to four decimal places matches only one formula.
IR absorptions: alcohol O–H vs carboxylic acid O–H: Alcohol O–H: 3230–3550 cm⁻¹, broad. Acid O–H: 2500–3000 cm⁻¹, very broad.
IR absorption range for C=O: 1680–1750 cm⁻¹, strong and sharp.
Fingerprint region: The region below about 1500 cm⁻¹, with a pattern unique to each molecule. Comparing it with a known spectrum identifies the compound.
Why do CO₂, methane and water vapour contribute to global warming?: Their bonds (C=O, C–H, O–H) absorb infrared radiation emitted by the Earth’s surface, and they re-emit energy, some back towards the surface.