16.1 Test-tube reactions

• Some tests are very straightforward.

  • Is the compound acidic (suggests carboxilic acid)?
  • Is the compound solid (suggests long carbon chain or ionic bonding), liquid (suggests medium length carbon chain or polar or hydrogen bonding), or gas (suggests short carbon chain, little or no polarity)?
  • Does the compound dissolve in water (suggests polar groups) or not (suggests no polar groups)?
• Adding bromine water...

  Identifies alkenes if the orange colour decolourises.
• Adding Fehling's solution...

  identifies aldehydes if the blue solution turns to red precipitate
• Adding Tollen's reagent...

  identifies aldehydes if silver mirror is formed
• How do you identify carbolixic acids?

  Add sodium carbonate. Effervescence of CO2 shows the presence of carboxilic acids (bubble through limewater to identify CO2).
• How do you identify primary and secondary alcohols and aldehydes?

  Add sodium dichromate and sulfuric acid. If they're present, there'll be an orange to green colour change. (acidified sodium dichromate is an oxidising agent)
• How do you identify halogenoalkanes?

  1. add NaOH (aq) and warm
  2. acidify with HNO3
  3. add AgNO3 (aq)

  Cl= white Br= cream I= yellow
• In the test for halogenoalkanes, why is it necessary to acidify the dilute acid before adding silver nitrate? And why would acidifying with hydrochloric acid not be suitable?

  1. To remove any CO3 2- or OH- ions present which would also form a precipitate.
  2. This would form a precipitate of AgCl.
• Tollen's reagent is formed by mixing aqueous ammonia and silver nitrate.
• What's an issue for the test for alcohols?

  The result is the same for primary and secondary alcohols, so distil the product and check for aldehyde or ketone. Primary alcohols are oxidised to aldehydes. Secondary alcohols are oxidised to ketones.
• Water vapour is a greenhouse gas because the bonds absorb and re-emit IR radiation that warms the lower atmosphere.