Test for aldehydes and Ketones

Summary

This video explores various chemical tests for identifying aldehydes and ketones, key organic compounds with a carbonyl group. It covers general tests like the 2,4-dinitrophenylhydrazine test, and specific tests including the iodoform test, Tollens' test, Schiff test, and chromic acid test for aldehydes, and sodium bisulfite, meta-dinitrobenzene, and sodium nitroprusside tests for ketones. The script also offers practical tips for conducting these tests and highlights the importance of preparation and observation in chemical analysis.

Takeaways

• 🧪 Aldehydes and ketones are organic compounds with a carbonyl group (C=O), distinguished by the presence of a hydrogen or R group bonded to the carbonyl carbon.
• 🌿 Aldehydes and ketones occur naturally, such as cinnamaldehyde in cinnamon bark and vanillin in vanilla.
• 💡 The 2,4-dinitrophenylhydrazine (Brady's) test is a general test for both aldehydes and ketones, producing a yellow or orange precipitate.
• 🔍 The iodoform test is specific for aldehydes with an alpha-hydrogen, like acetaldehyde, and some ketones, resulting in a yellow precipitate.
• 📚 Specific tests for aldehydes include the Tollens' test, Schiff test, chromic acid test, and Fehling's test, each producing a distinct reaction.
• 🔬 The Tollens' test uses silver nitrate and sodium hydroxide to produce a silver mirror when an aldehyde is present.
• 🌸 Schiff's test involves a color change to pink upon reaction with aldehydes, indicating the formation of an aldemine group.
• 🍏 The chromic acid test oxidizes aldehydes to carboxylic acids, changing the solution color to green.
• 🍇 Fehling's test produces an orange to red precipitate when an aldehyde reacts with copper(II) ions.
• 🧪 Sodium bisulfite test is used for ketones, forming a crystalline bisulfite adduct.
• 🌈 The meta-dinitrobenzene test is specific for ketones with an alpha-methyl group, resulting in a purple color due to the Janowski reaction.
• 🔴 Sodium nitroprusside test is another specific test for ketones, forming a deep red complex in alkaline conditions.

Q & A

What are the main differences between aldehydes and ketones?

Aldehydes have a hydrogen atom bonded to the carbonyl carbon, while ketones have two R groups (alkyl or aryl groups) bonded to the carbonyl carbon.

What is the 2,4-dinitrophenylhydrazine test and what does it indicate?

The 2,4-dinitrophenylhydrazine test is a general test for aldehydes and ketones. It indicates the presence of a carbonyl group, resulting in a yellow precipitate for aldehydes and an orange precipitate for ketones with a conjugated C=C-O group.

What is the significance of the orange precipitate in the 2,4-dinitrophenylhydrazine test?

An orange precipitate indicates that the carbonyl compound has a C=C-O group that is conjugated with another C=C bond.

What are some natural occurrences of aldehydes and ketones mentioned in the script?

Cinnamaldehyde in the bark of the cinnamon tree, vanillin in vanilla, and acetone used as a nail polish remover are mentioned as natural occurrences of aldehydes and ketones.

What is the iodine test and which compounds give a positive result?

The iodine test, also known as the iodoform test, is a special test for carbonyl compounds containing an alpha hydrogen. Compounds like acetaldehyde, methyl ethyl ketone, acetone, and some alcohols like isopropyl alcohol and ethyl alcohol give a positive result.

What is the Tollens' test and how does it work?

The Tollens' test is a specific test for aldehydes. It involves the oxidation of aldehydes to carboxylic acids by silver oxide, which is reduced to elemental silver, forming a silver mirror on the inner surface of the test tube.

What is the Schiff test and how does it indicate the presence of an aldehyde?

The Schiff test uses Schiff reagent, which is an aqueous solution of peroxaniline hydrochloride. Aldehydes react with this reagent to form a pink color, indicating their presence.

What happens in the Chromic Acid test for aldehydes?

In the Chromic Acid test, the powerful oxidizing agent chromic acid oxidizes aldehydes to carboxylic acids and is reduced to chromium(III), which forms a green chromium(III) sulfate.

What is the Felling's test and what does the formation of an orange to red precipitate indicate?

The Felling's test is specific for aldehydes. The formation of an orange to red precipitate indicates the oxidation of the aldehyde to a carboxylic acid and the reduction of the copper complex to copper(I) oxide.

How does the Sodium Bisulfite test work for ketones?

In the Sodium Bisulfite test, ketones react with sodium bisulfite to form a crystalline product known as a bisulfite adduct.

What is the Meta-dinitrobenzene test and what color change does it produce?

The Meta-dinitrobenzene test is specific for ketones with an alpha-methyl group. It produces a purple color due to the Janowski reaction, forming a Meisenheimer complex.

What is the Sodium Nitroprusside test and what color change does it indicate for ketones?

The Sodium Nitroprusside test is a specific test for ketones. The formation of a deep red color indicates the reaction of ketones with nitroprusside in the presence of an alkali.