- 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.
- 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.
