26.2 - Identifying Aldehydes & Ketones Flashcards Preview

OCR A Chemistry A Level - Chapter 26 > 26.2 - Identifying Aldehydes & Ketones > Flashcards

Flashcards in 26.2 - Identifying Aldehydes & Ketones Deck (5)
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How do you identify carbonyl compounds?

USE 2,4-DNP (Brady's Reagent

  • In the presence of carbonyl, a yellow/orange precip. forms
  • Solid 2,4-DNP is dangerous, friction causes explosion
    • typically dissolved in methanol and sulfuric acid


  1. Add 5cm3 2,4,-DNP in excess to clean test tube
  2. Add 3 drops of the unknown compound
  3. If no crystals form, add sulfuric acid
  4. In the presence of C=O, a yellow/orangle precip. forms


How do you identify carbonyls by melting point?

  • The 2,4 - dinitrophenylhydrazone precipitate from the 2,4- DNP test is filtered from solution and recrystallised giving a pure sample of crystals
  • The melting point of the pure 2,4- dinitrophenylhydrazone is measured, recorded and compared to a set of knwon melting point values to identify the original carbonyl compound


How do you distinguish between aldehydes and ketones?

  • After using 2,4-DNP to identify the carbonyl
    • TOLLEN'S REAGENT (AgNO3 in NH3) is used to distinguish between aldehydes and ketones
    • Forms a silver mirror in the presence of aldehydes
    • Tollen's has a short shelf-life, it must be used as soon as prepared



How do you make Tollen's Reagent?

  1. Add 3cm depth AgNO3, then NaOH(aq) until the brown precip. of Ag2O forms
  2. Add dilute NH3 unti lthe brown precip forms and disappears
    • That is Tollen's



Describe the test using Tollen's Reagent.

  1. 2cm depth of unknown solution in a test tube
  2. Add equal volume of Tollen's
  3. Leave the testu tube to stand in 50oC water bath (10-15mins), look for silver mirror (aldehyde) 


Tollen's has Ag+ ions (oxidising agent)

Ag+ + e→ Ag(s)