Identification of Aldehydes and Ketones

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Questions and Answers

What is the color change observed when an aldehyde is added to Fehling's solution?

Blue to green to red

Blue to green to orange to red (correct)

Blue to purple to white

Blue to yellow to orange to red

Which of the following compounds does not contain an alpha hydrogen and can undergo the Cannizzaro reaction?

Acetaldehyde

Acetone

Salicylaldehyde (correct)

Benzaldehyde (correct)

Which test can be applied to identify compounds that contain a terminal methyl group?

Tollen's test

Lucas test

Iodoform test (correct)

Benedict's test

What is produced when an aldehyde undergoes the Cannizzaro reaction?

A mixture of alcohol and carboxylic acid salt Signup and view all the answers

What is a positive result of the sodium nitroprusside test indicated by?

A red color complex Signup and view all the answers

Which aldehydes do not yield sharp results in Fehling's test?

Aromatic aldehydes Signup and view all the answers

What condition is required for a positive Cannizzaro reaction?

Absence of alpha hydrogen Signup and view all the answers

What product is obtained before purifying benzoic acid in the Cannizzaro reaction?

Sodium benzoate Signup and view all the answers

What general formula do ketones follow?

RCOR' Signup and view all the answers

Which reagent will produce a yellow or orange precipitate when reacted with aldehydes and ketones?

2,4-Dinitrophenylhydrazine Signup and view all the answers

What is the main purpose of using Tollen's reagent in the identification process?

To distinguish between aldehydes and ketones Signup and view all the answers

Which of the following aldehydes will give a positive Tollen's test?

Benzaldehyde Signup and view all the answers

What is formed when aldehydes are oxidized in the presence of Tollen's reagent?

Carboxylic acids Signup and view all the answers

What role does ammonium hydroxide play in Tollen's reagent?

Complexes with silver oxide to maintain solubility Signup and view all the answers

Which one of the following statements is true regarding the oxidation of ketones?

They require very strong oxidizing agents for oxidation. Signup and view all the answers

If no precipitate forms in the reaction of an unknown compound with 2,4-Dinitrophenylhydrazine, what can be concluded?

The compound may not contain a carbonyl group. Signup and view all the answers

Study Notes

Experiment 4: Identification of Aldehydes and Ketones

Aldehydes: Compounds with the general formula RCHO. R can be alkyl or aryl groups, and in formaldehyde, R is a hydrogen atom.
Ketones: Compounds with the general formula RCOR'. R and R' can be alkyl or aryl groups.
Carbonyl Group (C=O): A key functional group found in both aldehydes and ketones. It's responsible for many of their chemical and physical properties.
Examples of Aldehydes: Formaldehyde, acetaldehyde, propionaldehyde, benzaldehyde, salicylaldehyde
Examples of Ketones: Acetone, acetophenone, benzyl methyl ketone, benzophenone
General Test (2,4-Dinitrophenylhydrazine): Both aldehydes and ketones react with 2,4-Dinitrophenylhydrazine to form a yellow or orange precipitate.
Procedure (General Test): Add 2 drops of the compound to 3 drops of the reagent. Observe for a precipitate. If insoluble in water, dissolve compound in 1 ml methanol first, then add the reagent.

Differentiation Tests (Aldehyde vs. Ketone)

Tollen's Test (Reduction of Ammonical Silver Nitrate): This reagent distinguishes aldehydes, which form a silver mirror, from ketones, which do not.
- Procedure (Tollen's Test): Add 2-3 drops of the compound to 2-3 ml of Tollen's reagent. Formation of a silver mirror indicates an aldehyde. Warm the test tube in a water bath if no reaction occurs.
Fehling's Test: Aldehydes, but not ketones, reduce Fehling's reagent (a deep blue solution), forming a red precipitate of cuprous oxide (Cu2O).
- Procedure (Fehling's test): Add 5 drops of the compound to 1 ml of Fehling's solution. Heat in a water bath for 5 minutes (with shaking for water insoluble compounds). A change in color to green, orange, and then red precipitate or copper mirror suggests an aldehyde.
lodoform Test: This test is specific for compounds containing a terminal methyl group (CH3) on a carbonyl group.
Sodium Nitroprusside Test: Used for compounds with terminal methyl groups. A red color complex indicates a positive test.
Polymerization Reaction: Formaldehyde and salicylaldehyde react with resorcinol and sulfuric acid to form a colored polymer.

Cannizzaro Reaction

Cannizzaro Reaction: Aldehydes without alpha hydrogens can undergo self-oxidation-reduction reactions in a strong basic medium. The reaction produces an alcohol and a carboxylic acid salt
- Procedure (Cannizzaro): Add a few drops of the aldehyde (e.g., benzaldehyde) to 0.5 ml of 30% sodium hydroxide, heat slowly, and add distilled water. Concentrated hydrochloric acid can be added to liberate benzoic acid as a precipitate.

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Description

This quiz focuses on the identification of aldehydes and ketones, two important classes of organic compounds. It covers their general formulas, chemical properties, examples, and a specific testing method using 2,4-Dinitrophenylhydrazine. Test your knowledge on the characteristics and reactions of these carbonyl compounds.