Aldehydes & Ketones: Reaction with 2,4-Dinitrophenylhydrazine, Fehling’s and Tollen’s Reagent Tests and Iodoform Test.

Experiment title:

Aldehydes & Ketones: Reaction with 2,4-dinitrophenylhydrazine, Fehling’s and Tollen’s reagent tests and iodoform test.

Objective:

The aim of this experiment is to understand the similarities and the differences of the chemical properties of aldehydes and ketones.

Introduction:

Aldehydes and ketones are very important organic compounds. They both contain the carbonyl group (C=O), so they share much of their chemistry but they are different enough to be considered different classes of compounds. The oxygen in the carbonyl group of aldehydes and ketones are far more electronegative than carbon and has a strong tendency to pull electrons in carbon-oxygen bond towards itself. One of the two pairs of electrons that make up a carbon-oxygen double bond is even more easily pull towards the oxygen. This makes the carbon-oxygen double bond very highly polar. So, both aldehydes and ketones can react with nucleophiles (negatively charged ion) for example cyanide ion and ammonium derivatives such as 2,4-dinitrophenylhydrazine to form solid products. The theory says that a positive test is when a yellow, orange or red precipitate is formed and if the precipitate was red then it’s a conjugated carbonyl if it’s yellow or orange then it is unconjugated. Aldehydes are produced by the oxidation of primary alcohols, which in turn are further oxidized to carboxylic acids. Meanwhile ketones can be prepared by the oxidation of secondary alcohols, but they are not further oxidized. (Arun Bahi, 2006)

Therefore, both aldehydes and ketones can be differentiated by using oxidizing agent such as Fehling’s solution (Positive test is the formation of brick-red precipitate) and Tollen’s reagent (Positive test is the formation of silver-mirror layer). This is because these agents can only oxidize aldehydes (fehling only oxidize aliphatic aldehydes) but not ketones. Also, carbonyls containing CH3CO- group can be identified using Iododform test which a positive test would...