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Halogenoalkanes, Alcohols and Organic Analysis

Distinguishing Aldehydes and Ketones

  • 3 things to know
  • Papers 2 and 3

Overview

Fehling's and Tollens' reagents, and the observations that separate the two.

What you need to know

Answer each one before you reveal it.

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Key term

What reagent is used to distinguish between aldehydes and ketones?

Key term

What observation indicates a positive result when Tollens' reagent reacts with an aldehyde?

Key term

What observation indicates a positive result when Fehling's solution reacts with an aldehyde?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What reagent is used to distinguish between aldehydes and ketones?

    Model answer

    Tollens' reagent or Fehling's solution

  2. 2

    What observation indicates a positive result when Tollens' reagent reacts with an aldehyde?

    Model answer

    A silver mirror forms on the inside of the test tube

  3. 3

    What observation indicates a positive result when Fehling's solution reacts with an aldehyde?

    Model answer

    A brick-red precipitate forms

  4. 4

    Why do ketones not react with Tollens' reagent?

    Model answer

    Ketones cannot be oxidised by mild oxidising agents as they have no hydrogen atom attached to the carbonyl carbon

  5. 5

    Why do ketones not react with Fehling's solution?

    Model answer

    Ketones cannot be oxidised by mild oxidising agents as they have no hydrogen atom attached to the carbonyl carbon

  6. 6

    What is the chemical basis for Tollens' reagent being able to distinguish aldehydes from ketones?

    Model answer

    Tollens' reagent is a mild oxidising agent that can oxidise aldehydes but not ketones

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