Identification of Aldehydes and Ketones

Identification of Aldehydes and Ketones

Abstract

        Aldehydes and Ketones are representative of compounds which possess the carbonyl group. Specific groups of atoms in an organic molecule determine its physical and chemical properties. These groups are called functional groups and are responsible for the characteristic reactions of a particular compound. The objectives of the experiment were to investigate the chemical properties of representative aldehydes and ketones and determine the unknown. Several tests were done and these are the chromic acid test, Tollen’s, Iodoform, 2,4,-dinitrophenylhydrazine and Fehling’s test. The test compounds were acetaldehyde, acetone, butyraldehyde, benzaldehyde, acetophenol, and an unknown. The unknown resulted positive in Tollen’s test, 2,4-dinitrophenylhydarzine test and Fehling’s test. The unknown was concluded to be an aldehyde.

Keywords: Aldehydes, Ketones, Chromic Acid, Tollen’s, Fehling’s, 2,4-DNP, Iodoform

Introduction

        Aldehydes and Ketones are representative of compounds which possess the carbonyl group.

[pic 1]

The difference between an aldehyde and a ketone is that an aldehyde has at least one hydrogen attached to the carbonyl carbon, and ketones have no hydrogens directly attached to the carbonyl carbon only carbon containing R groups.[pic 2]

Classification Tests:

1. Chromic Acid Test

Aldehydes are oxidized into carboxylic acids and ketones remain the same. Positive test results in formation of a blue-green solution.[pic 3]

1. Tollen’s Reagent

Aldehydes reduce Tollen’s reagent to give a precipitate of silver metal. The free silver forms a silver mirror on the sides of the test tube. Aldehydes are oxidized to a carboxylic acid.

2. Iodoform Test[pic 4]

This test is used to distinguish a methyl ketone from other ketones and aldehydes. The presence of iodoform, a yellow precipitate, is the indication of a positive result.[pic 5]

1. 2,4-dinitrophenylhydrazine test

This test is a tool used to verify if a certain compound is an aldehyde or a ketone.The appearance of a yellow to orange precipitate indicates a positive result.

1. Fehling’s Test

This test is done to distinguish aliphatic aldehydes from aromatic aldehydes. A positive result for the Fehling’s Test is the appearance of a red precipitate. Aromatic aldehydes and ketones give a negative result to Fehling’s test since they lack alpha hydrogens that cannot form an enolate.[pic 6]

Methodology

        The test tubes were labeled one for each test sample which are acetaldehyde, butyraldehyde, acetone, benzaldehyde, acetophenone and an unknown.

        First test that was done was the chromic acid test, five drops of each sample were poured onto separate test tubes. Twenty drops of acetone and four drops of chromic acid were added and was placed in a 60°C water bath for five minutes.  Next was the Tollen’s test. The test tubes were filled with the Tollen’s reagent each (1mL) and then was added five drops of each sample into designated test tubes. The mixture was shaken and allowed to stand for ten minutes. The samples that had no reaction were subjected to water bath at about 35-50°C bath for five minutes. Next was the Iodoform test. Five drops of the test samples were poured into designated test tubes and twenty drops of Potassium iodide solution and twenty drops of Sodium bicarbonate (5%) were added. Next test that was performed was the 2,4-dinitrophenylhydrazine test. Five drops of each sample were poured onto designated test tubes, five drops of ethanol (95%) was added. The mixture was shaken and three drops of the test reagent was added and if no precipitate was formed, it was allowed to stand for at least fifteen minutes. Lastly, the Fehling’s test, the test reagent solutions was already prepared and was poured into separate test tubes and three drops of each sample were poured into designated test tubes, were subjected to boiling water and was observed.

Results and Observations