Organic Compound Tests Flashcards
How do you test for a carbonyl group?
Positive test for: Aldehydes and Ketones
Type: Condensation/Nucleophilic Addition
Reagent: Brady’s Reagent -> 2,4-DNPH in dilute acid
Observation if +ve: Orange/Red precipitate forms
After a positive 2,4-DNPH test, how do you identify the starting Aldehyde or Ketone?
The precipitate formed is filtered off and purified (recrystallised). The melting temperature of the solid is then taken and compared with text book values of melting temperatures.
Why do Tollen’s reagent and Fehling’s test give a positive result for an Aldehyde but not a Ketone?
The hydrogen atom on an Aldehyde is quite reactive so aldehydes are easily oxidised. Both Tollen’s reagent and Fehling’s reagent are reduced by the aldehyde.
Ketones are unable to react like this because the carbonyl group hasn’t got a hydrogen bonded to it.
Describe Tollen’s Test
Also known as the Silver Mirror Test
Positive test for: Aldehydes
Reagent: Tollen’s Reagent -> Aqueous Silver Nitrate + Aqueous Ammonia
Observation if +ve: Silver mirror forms
Silver ions are reduced to Silver
Ag+(aq) + e- -> Ag(s)
Describe Fehling’s Test
Benedict’s solution can also be used
Both contain Cu2+ ions dissolved in aqueous alkali
Positive test for: Aldehydes
Observation: Brick red precipitate of Cu2O
Cu2+ ions are reduced to Cu+ ions
2Cu2+(aq) + 2e- + 2OH-(aq) -> Cu2O(s) + H2O(l)
How do you test for a methyl carbonyl group?
Positive test for: Ethanal, all Ketones, Ethanol and Propan-2-ol
Type: Condensation/Nucleophilic Addition
Reagent: Compound with I2 in Aqueous Sodium Hydroxide
Observation if +ve: Yellow precipitate of triiodomethane (iodoform), CHI3, is formed
How can acidified dichromate solution be used to test for an Aldehyde?
Acidified dichromate solution will be reduced by an aldehyde but not a ketone. The orange CrO7^2- are reduced to green chromium(III) ions, Cr^3+.
