Test for Aldehydes and Ketones

Theoretical Background / Context

• Aldehydes and ketones are two classes of organic compounds that possess at least one carbonyl group (C=O). 
• Aldehydes (R – CHO) are characterized by having a terminal CHO in their structure, while ketones (R – CO – R’) are characterized by having C=O within their structures. 
• R and R’ represent the alkyl or aryl substituents on the carbon atom of the carbonyl group. 
• In other words, the carbonyl group of an aldehyde is linked to one alkyl/aryl group and an H atom, while that of ketone is linked to two alkyl/aryl groups.
• The role that aldehydes and ketones play in our real life is controlled by their properties. 
• Aldehydes and ketones are used in various applications in different fields such as the food industry, cosmetics, pharmaceutics, etc. 

   First: Common Uses of Aldehydes:

• Formaldehyde, a gaseous aldehyde compound is used to prepare 40% w/v solution. 
• This solution is known commercially as formalin and used to preserve biological samples.
• Formaldehyde also reacts with phenol to form Bakelite that is used in different industries e.g. adhesives, coatings. Polymers and plastics. It also acts as a fungicide or insecticide. 
• Finally, it is used in photography.
• Acetaldehyde, another common example of acetaldehyde, is used as a raw material for the preparation of some chemicals such as acetic acid and pyridine derivatives.
• Benzaldehyde, an aromatic aldehyde, is a fundamental ingredient in cosmetics, dyes, and perfumes. 
• It is also used as a flavoring additive (almond-like) in many food products. It also acts as a bee repellent.

• Many other aldehydes are responsible for specific fragrances and flavors in baking and foods such as cinnamon, vanilla, and coriander. 
• Aldehydes are also important ingredients in the caramelization of sugars as they are responsible for the color and taste of caramel.
•  Finally, some aldehydes are also responsible for sweet odors in fragrance and perfume production.

 Second: Common Uses of Ketones:

• Generally, a lot of ketones are used as solvents in the plastics and synthetic fibers industry.
• Propanone (acetone), commonly and commercially known as acetone, is used as a paint thinner and nail paint remover. 
• It is also used in medical and pharmaceutical applications such as chemical peeling and acne treatments.
• Ethyl methyl ketone, also known as butanone, is one of the commonly used solvents in many industries such as in textiles, paints & varnishes, paraffin wax, polymers & plastic production, etc.
• Cyclohexanone, a cyclic ketone, is an important raw material in the nylon industry.
• Finally, many ketones such as acetophenone have a characteristic sweet odor (such as cherry, jasmine, honeysuckle, almond, strawberry, etc.), so they are included in perfume industries.
• The test for aldehydes and ketones is designed in order to confirm the presence of aldehydes and ketones in certain samples. In addition, it can differentiate between aldehydes and ketones. 
• In other words, the experiment is designed for the qualitative analysis of aldehydes and ketones.

Principle of Work

• In the test for aldehydes and ketones, aldehydes and ketones are tested by detecting their carbonyl groups through different chemical tests. 
• In addition, these tests can be used to differentiate between the two classes. 
• This is done through conducting different tests using different reagents such as phenylhydrazine, Tollen’s Test, Schiff’s Test, and Fehling's test for aldehydes and ketones.

First: Phenylhydrazine Test 

• In this test, 2,4-dinitrophenylhydrazine is used which reacts with carbonyl groups, forming phenylhydrazones. 
• Phenylhydrazones are colored compounds and their color differs according to whether the sample contains an aldehyde or ketone. 
• Specifically, it gives a red color in the case of aldehyde, while yellow or orange in the case of ketones.

Second: Tollen’s (Silver Mirror) Test

• It depends on the fact that aldehydes can be oxidized into their corresponding carboxylic acids, while ketones cannot. 
• This test is named after the name of the used reagent, Tollens’ reagent. Tollens’ reagent is a colorless basic aqueous solution composed of silver ions linked to ammonia [Ag(NH3)2+]. It is prepared through two steps:

Step 1: Preparation of Silver Oxide (Ag2O)

AgNO3 + NaOH → AgOH + HNO3

2AgOH → Ag2O↓ + H2O

Step 2: Preparation of Tollen’s reagent

Ag2O + 4 NH3 + H2O → 2Ag(NH3)+2 + 2OH-

• In this test, Tollen’s reagent differentiates between aldehydes and ketones since it is able to oxidize aldehydes only to their corresponding carboxylic acid. 
• During the reaction, the silver ins are reduced to precipitate as metallic silver (Ag0) forming a mirror at the bottom of the test tube as shown in figure 1 in the left-hand side tube. 
• On the other hand, no reaction proceeds in the case of ketones ending up with a negative test result as shown in Figure 1 in the right-hand side tube.

Third: Schiff’s Test 

• Schiff’s reagent is prepared by mixing concentrated hydrochloric acid, sodium bisulfite, and triphenylmethane dye together. Schiff’s reagent itself has no color. 
• However, it reacts with an aldehyde in the presence of sulfurous acid giving a complex with red brick color.
• Schiff’s reagent (Colorless) Complex with an aldehyde (Brick red Color).
• On the other hand, ketone gives a negative result in this test as it cannot form a complex with the reagent.

 Forth: Fehling Test: 

• Fehling test is used to detect reducing sugars so it is used to differentiate between aldehydes and ketones. Using the Fehling test, aldehydes give positive results (reddish-brown precipitate), while ketone does not.
• Fehling reagent is composed of two solutions; Fehling A and Fehling B. Fehling A is an aqueous copper sulfate solution (CuSO4), while Fehling B is an alkaline aqueous solution of sodium potassium tartrate (Rochelle salt). Rochelle salt acts as a chelating agent. 
• Fehling A and Fehling B are mixed in equal amounts prior to being used to differentiate between aldehydes and ketones.
•  Fehling test gives a positive result with an aldehyde, while a negative result with a ketone. 
• Upon heating the aldehyde is oxidized, its corresponding carboxylic acid, while cupric oxide (CuO) is reduced to cuprous oxide (Cu2O) which is a reddish-brown precipitate.