6.1.8

• 2,4-dinitrophenylhydrazine

  Testing for a carbonyl functional group

  • Brady’s reagent is an orange transparent mixture of methanol , sulphuric acid + solution of 2,4-DNP

  → when added to :

  aldehyde or ketone ~ yellow/orange precipitate of 2,4-DNP derivative

  carboxylic acid or ester ~ no precipitate observed
• Identifying specific C=O

  after positive Brady’s reagent, further processing of a precipitate allows specific aldehyde or ketone to be identified

  • 2,4-DNP derivative can be collected by filtration + purified using recrystallisation

  → after drying, accurate melting point of pure product can then be measured through experiment
• Identifying specific C=O 2

  • using a database, you can find accurately measured melting points for aldehydes or ketones
  • ketones with a similar length chain have similar melting points, making it challenging to experimentally distinguish between them

  → however 2,4-DNP (hydrozone) derivative have different melting points
• Tollen’s Reagent

  aka ammoniacal silver nitrate is a colourless chemical made in a 2-stage process

  1. sodium hydroxide solution is added to silver nitrate solution until a brown precipitate is formed
  2. Dilute ammonia is added drop-wise until brown precipitate redissolves
• Tollen's Reagent 2

  • can be used to distinguish between aldehyde + ketone
  • weak oxidising agent + can be react with C=O with aldehyde but not with ketone

  → ketones cannot be oxidised further

  oxidation of silver ions causes silver metal to be precipitated out of + this is observed as a silver mirror effect on inside of reaction vessel

  • the aldehyde would oxidise into carboxylic acid