Identification of Aldehydes and Ketones

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Questions and Answers

What is the general formula of aldehydes?

R2C=O

RCHO (correct)

RCOR'

RCOOH

Which reagent can be used to differentiate between aldehydes and ketones?

Sodium hydroxide

Hydrochloric acid

Tollen's reagent (correct)

Benedict's reagent

What phenomenon occurs when an aldehyde reacts with Tollen's reagent?

Color change to blue

Production of a gas

Generation of a silver mirror (correct)

Formation of a precipitate

Which of the following compounds is classified as a ketone?

Acetone Signup and view all the answers

Which property differentiates aldehydes from ketones?

Ability to be oxidized easily Signup and view all the answers

What is the main role of ammonium hydroxide in Tollen's reagent?

It complexifies silver ions Signup and view all the answers

Which of the following statements about the carbonyl group is correct?

It is present in both aldehydes and ketones Signup and view all the answers

What is typically observed when aldehydes react with the 2,4-Dinitrophenylhydrazine reagent?

Formation of a yellow or orange precipitate Signup and view all the answers

What color change indicates a positive result when testing for aldehydes using Fehling's reagent?

From blue to green, then to orange and red precipitate Signup and view all the answers

Which aldehydes are known to undergo the Cannizzaro reaction?

Benzaldehyde, salicylaldehyde, and formaldehyde Signup and view all the answers

What is observed during a positive Iodoform test for aldehydes with terminal methyl groups?

A yellow precipitate appears Signup and view all the answers

Which statement is true about ketones in relation to Fehling's test?

They do not change the color of Fehling's reagent. Signup and view all the answers

What happens when an aldehyde is subjected to the polymerization reaction with resorcinol in sulfuric acid?

It forms a white ring that turns red. Signup and view all the answers

What is the role of sodium hydroxide in the Cannizzaro reaction?

It provides a strong basic medium for the reaction. Signup and view all the answers

What distinguishes aromatic aldehydes in relation to Fehling's test?

They do not produce sharp results. Signup and view all the answers

In the sodium nitroprusside test, what indicates a positive result?

Development of a red color complex Signup and view all the answers

Study Notes

Aldehyde and Ketone Identification

Aldehydes: Compounds with the general formula RCHO. R can be aliphatic or aromatic; in formaldehyde, R is a hydrogen atom.
Ketones: Compounds with the general formula RCOR'. R and R' can be aliphatic or aromatic.
Functional Group: Both contain a carbonyl group (C=O). This group is crucial to their chemical and physical properties.
Examples: Formaldehyde, acetaldehyde, propionaldehyde, benzaldehyde, salicylaldehyde, acetone, acetophenone, benzyl methyl ketone, and benzophenone.

Chemical Reactions

2,4-Dinitrophenylhydrazine (2,4-DNPH) Test: A general test for both aldehydes and ketones. A yellow or orange precipitate forms.
- This reagent is added to the unknown compound (2 drops).
- A precipitate indicates the presence of either an aldehyde or ketone.
- If insoluble in water, dissolve in methanol before adding the reagent.

Differentiation Tests

Tollen's Test: Used to distinguish aldehydes from ketones.
- Positive result: Formation of a silver mirror.
  - This is due to the oxidation of the aldehyde, and reduction of silver ions.
- Negative result: No silver mirror forms if the compound is a ketone.
- Alkaline medium is necessary. Ammonium hydroxide is used to prevent precipitation of silver oxide.
Fehling's Test: Also used to distinguish aldehydes from ketones.
- Positive result: Formation of a red cuprous oxide precipitate (Cu2O).
  - Deep blue Fehling's solution changes color to green, orange, or red, with final precipitate.
- Negative result: No change in Fehling's solution if the compound is a ketone.

Special Tests for Terminal Methyl Groups

Iodoform Test: For aldehydes/ketones containing a terminal methyl group in their structure. More details needed on the procedure to determine a positive test.
Sodium Nitroprusside Test: Positive result is a red complex for compounds containing a terminal methyl group.
Cannizzaro Reaction: This reaction occurs for aldehydes (like benzaldehyde, salicylaldehyde, formaldehyde) that lack alpha hydrogen atoms. The aldehyde undergoes self-oxidation-reduction. The result is the formation of an alcohol and salt of a carboxylic acid and/or the acid itself. Details of the procedure will determine positive observations.

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Description

Explore the chemistry of aldehydes and ketones in this quiz. Learn about their definitions, structures, and characteristic tests such as the 2,4-DNPH and Tollen's test. This quiz will enhance your understanding of carbonyl compounds and their identification.