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Questions and Answers
What is the general formula for ketones?
What is the general formula for ketones?
Which reagent is used to differentiate between aldehydes and ketones based on oxidation potential?
Which reagent is used to differentiate between aldehydes and ketones based on oxidation potential?
What type of precipitate do aldehydes and ketones form when reacted with 2,4-Dinitrophenylhydrazine?
What type of precipitate do aldehydes and ketones form when reacted with 2,4-Dinitrophenylhydrazine?
What result indicates the presence of an aldehyde during Tollen's test?
What result indicates the presence of an aldehyde during Tollen's test?
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How do aldehydes differ from ketones in their chemical reactivity?
How do aldehydes differ from ketones in their chemical reactivity?
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What is the purpose of ammonium hydroxide in Tollen's reagent?
What is the purpose of ammonium hydroxide in Tollen's reagent?
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Which of the following statements about formaldehyde is true?
Which of the following statements about formaldehyde is true?
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Which reaction condition is necessary for the successful oxidation of aldehydes in Tollen's test?
Which reaction condition is necessary for the successful oxidation of aldehydes in Tollen's test?
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What is the main distinguishing feature of aldehydes in the Fehling's test?
What is the main distinguishing feature of aldehydes in the Fehling's test?
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Which of the following aldehydes is NOT mentioned as undergoing the Cannizzaro reaction?
Which of the following aldehydes is NOT mentioned as undergoing the Cannizzaro reaction?
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What color complex indicates a positive result in the sodium nitroprusside test?
What color complex indicates a positive result in the sodium nitroprusside test?
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What occurs during the Cannizzaro reaction?
What occurs during the Cannizzaro reaction?
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Which of the following statements about ketones in Fehling's test is correct?
Which of the following statements about ketones in Fehling's test is correct?
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During the polymerization reaction, what indicates a positive result when mixing formaldehyde and resorcinol?
During the polymerization reaction, what indicates a positive result when mixing formaldehyde and resorcinol?
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Which of the following compounds can give a positive result in the Iodoform test?
Which of the following compounds can give a positive result in the Iodoform test?
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What is the expected result when benzaldehyde is reacted with concentrated hydrochloric acid after the Cannizzaro reaction?
What is the expected result when benzaldehyde is reacted with concentrated hydrochloric acid after the Cannizzaro reaction?
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Study Notes
Aldehyde and Ketone Identification
- Aldehydes: Compounds with the general formula RCHO. R can be aliphatic or aromatic. Formaldehyde has a hydrogen atom for R.
- Ketones: Compounds with the general formula RCOR'. R and R' can be aliphatic or aromatic.
- Functional Group: Both contain a carbonyl group (C=O), which is key to their chemical and physical properties. Examples of aldehydes include formaldehyde, acetaldehyde, propionaldehyde, benzaldehyde, and salicylaldehyde. Ketones include acetone, acetophenone, benzyl methyl ketone, and benzophenone.
Chemical Reactions
- 2,4-Dinitrophenylhydrazine (2,4-DNPH) Test: Both aldehydes and ketones react with 2,4-DNPH to form a yellow or orange precipitate. This precipitate is a 2,4-dinitrophenylhydrazone.
- General Test Procedure: Add 2-3 drops of the compound to 3 drops of the reagent. If the compound is insoluble in water, dissolve it in 1 mL of methanol first.
Differentiation Tests
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Tollen's Test: Aldehydes react with Tollen's reagent (ammoniacal silver nitrate) to form a silver mirror (Ag). This reaction involves the oxidation of the aldehyde to a carboxylic acid.
- Procedure: Add 2-3 drops of the compound to 2-3 mL of Tollen's reagent. Warm the test tube if no reaction occurs.
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Fehling's Test: Only aldehydes reduce Fehling's reagent (a deep blue solution) to give a red cuprous oxide (Cu₂O) precipitate.
- Procedure: Add 5 drops of the compound to 1 mL of Fehling’s solution and heat in a water bath for 5 minutes (shake for water insoluble compounds).
- Important Note: Ketones do not react positively with either Tollen's or Fehling's reagent.
Special Tests for Terminal Methyl Groups
- Iodoform Test: This test identifies aldehydes and ketones with a terminal methyl group.
- Sodium Nitroprusside Test: Also tests for compounds with a terminal methyl group causing a red color
- Polymerization reaction: Formaldehyde or salicylaldehyde will polymerize with resorcinol in the presence of sulfuric acid.
Cannizzaro Reaction
- Conditions: Benzaldehyde, salicylaldehyde, and formaldehyde (no alpha-hydrogen atoms) undergo a self-oxidation-reduction reaction in a strong base (e.g., sodium hydroxide).
- Products: The reaction produces a mixture of an alcohol and a salt of the corresponding carboxylic acid.
- Procedure: Add a few drops of aldehyde to 0.5 mL of 30% sodium hydroxide and heat gently for 5 minutes. Then add water and concentrated hydrochloric acid for reaction confirmation.
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Description
Test your knowledge on aldehyde and ketone identification, including their structural formulas and reactions. This quiz covers essential tests like 2,4-DNPH and differentiation tests such as Tollen's Test. Perfect for chemistry students looking to strengthen their understanding of organic compounds.
