AQA A - Level Chemistry - Organic Analysis

what is the test for an alkene?

bromine water

what are the observations for bromine water test on alkene?

• water decolourised

• orange → colourless

what is the test for primary/secondary alcohols?

reacts with acidified K2Cr2O7

what are observations for the test for primary/secondary alcohols?

• positive: orange → green colour change

• aldehyde/carboxylic acid/ketone produced

• negative: stays green otherwise

what are the two aldehyde tests?

• Tollen’s reagent

• Fehling’s solution

describe the observations for the Tollen’s reagent test for aldehyde

• positive: silver mirror forms on the walls of the test tube

• negative: remains colourless

describe how the Tollen’s reagent test works

• a solution of AgNO3 and NH3 → [Ag(NH3)2]+ (in the solution)

• when added to aldehyde, this reacts to form Ag (s)

  • Ag+ + e- → Ag (s)

  • RCHO + [O] → RCOOH

describe the observations for the Fehling’s solution test for aldehydes

• positive: brick red precipitate forms

• negative: remains blue colourless solution

describe how the Fehling’s solution test works

• originally solution of Cu2+

  • mild oxidising agent

• Cu2+ ions are oxidised in in presence of aldehyde to make Cu+ ions which are a precipitate

  • Cu2+ + e- → Cu+

  • RCHO + [O] → RCOOH

what can the tests for aldehydes also be used to identify in comparison?

ketones as they will not react in any of the tests

what is the test for a carboxylic acid?

• add sodium carbonate

• bubble gas through limewater

what are the observations for the test of a carboxylic acid?

• positive: bubbles when NaCO3 added, gas turns limewater cloudy

• negative: no bubbles

what is the test for a haloalkane (both parts)?

• add ethanolic silver nitrate

• add dilute and concentrated ammonia to precipitates produced

what are the observations for the test for haloalkanes?

• precipitates form - white, cream, yellow for Cl, Br, I

• AgCl dissolves in dilute ammonia, AgBr in concentrated, AgI not at all

what two spectrometry techniques are used to analyse organic compounds?

• mass spectrometry

• infrared spectrometry

how can mass spectrometry be used in organic analysis?

• can be used to identify organic compounds

• can also be used to differentiate between isomers of the same compound

  • as they have the same M+ peak but different fragmentation patterns