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15.5: Oxidation of Aldehydes

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    86290
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    Learning Objectives

    • Objective 1
    • Objective 2

    ldehydes and ketones are much alike in many of their reactions, owing to the presence of the carbonyl functional group in both. They differ greatly, however, in one most important type of reaction: oxidation. Aldehydes are readily oxidized to carboxylic acids, whereas ketones resist oxidation.

    oxidation.jpg

    The aldehydes are, in fact, among the most easily oxidized of organic compounds. They are oxidized by oxygen (O2) in air to carboxylic acids.

    \[2RCHO + O_2 \rightarrow 2RCOOH \label{14.10.1} \]

    The ease of oxidation helps chemists identify aldehydes. A sufficiently mild oxidizing agent can distinguish aldehydes not only from ketones but also from alcohols. Tollens’ reagent, for example, is an alkaline solution of silver (Ag+) ion complexed with ammonia (NH3), which keeps the Ag+ ion in solution.

    \[H_3N—Ag^+—NH_3 \label{14.10.2} \]

    When Tollens’ reagent oxidizes an aldehyde, the Ag+ ion is reduced to free silver (Ag).

    A aldehyde group RCHO reacts with 2 Ag(NH subscript 3) subscript 2 superscript positive sign and 3 hydroxide ions to give COO superscript negative sign in addition to 2 silver in its solid state, 4 NH subscript 3 and 2 water.

    Deposited on a clean glass surface, the silver produces a mirror (Figure \(\PageIndex{1}\)). Ordinary ketones do not react with Tollens’ reagent.

    A person wearing gloves is holding a bunch of ornaments that she has just lifted from a basin containing a solution.
    Figure \(\PageIndex{1}\): Aldehyde Reactions. A reaction related to the Tollens’ reaction is often used to silver mirrors. These ornaments were silvered by such a reaction. Glucose, a simple sugar with an aldehyde functional group, is used as the reducing agent. Source: Photo courtesy of Krebs Glas Lauscha, commons.wikimedia.org/wiki/File:Silvering.jpg.

    Although ketones resist oxidation by ordinary laboratory oxidizing agents, they undergo combustion, as do aldehydes.


    15.5: Oxidation of Aldehydes is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by LibreTexts.

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