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Aldehydes and Ketones

Distinguishing aldehydes from ketones, and reduction with lithal

  • 3 things to know
  • Unit A2 1

Overview

Observation with acidified potassium dichromate(VI) that identifies an aldehyde, appearance of Fehling's solution before it is used and why ketones are not oxidised by these mild reagents.

What you need to know

Answer each one before you reveal it.

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Key term

State the observation with acidified potassium dichromate(VI) that identifies an aldehyde.

Key term

Name the product formed when an aldehyde is oxidised by acidified potassium dichromate(VI).

Key term

State what is observed when an aldehyde is warmed with Fehling's solution.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State the observation with acidified potassium dichromate(VI) that identifies an aldehyde.

    Model answer

    On warming, the orange solution turns green as the aldehyde is oxidised and the dichromate(VI) is reduced to chromium(III).

  2. 2

    State what is observed when a ketone is warmed with acidified potassium dichromate(VI).

    Model answer

    There is no change and the solution stays orange, because a ketone is not oxidised.

  3. 3

    Name the product formed when an aldehyde is oxidised by acidified potassium dichromate(VI).

    Model answer

    The corresponding carboxylic acid is formed.

  4. 4

    Describe the appearance of Fehling's solution before it is used.

    Model answer

    It is a clear deep blue solution containing copper(II) ions held in an alkaline complex.

  5. 5

    State what is observed when an aldehyde is warmed with Fehling's solution.

    Model answer

    A brick-red precipitate of copper(I) oxide forms and the blue colour fades.

  6. 6

    State the change undergone by the copper species when Fehling's solution reacts with an aldehyde.

    Model answer

    The copper is reduced from Cu²⁺ to the copper(I) state.

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