5) Oxidation-reduction reaction of aldehydes. Dismutation of aldehydes. Reduction of Aldehydes and Ketones
Reduction: The reduction process is merely the reverse of oxidation. If two hydrogens are removed during oxidation, then two hydrogens should be added during reduction. Carbonyl compounds act as the oxidizing agent and hydrogen gas in the presence of a metal catalyst is the reducing agent. Other reducing agents which give up hydrogen may also be used in this reaction The definitions which are used in inorganic chemistry involving the gain or loss of electrons; or the increase or decrease in oxidation numbers are not very useful in organic chemistry. In organic chemistry, although changes in electrons are important, oxidation and reduction is more readily observed from changes in the number of hydrogen or oxygen atoms. The definitions are as follows: REDUCTION: gain of hydrogen, loss of oxygen, or gain of electrons. GENERAL EQUATION: REDUCTION OF ALDEHYDES AND KETONES aldehyde + H2 ---> primary alcohol ketone + H22 ---> secondary alcohol Reduction of an aldehyde: the reduction of ethanal In the first stage, there is a nucleophilic attack by the hydride ion on the slightly positive carbon atom. The lone pair of electrons on the hydride ion forms a bond with the carbon, and the electrons in one of the carbon-oxygen bonds are repelled entirely onto the oxygen, giving it a negative charge.
Reduction of ketone: The mechanism for the reduction of propanone The reaction starts with a nucleophilic attack by the hydride ion on the slightly positive carbon atom.
Again, what happens next depends on whether you add an acid or water to complete the reaction. Oxidations of aldehydes and ketones |
Aldehydes can be oxidized to carboxylic acid with both mild and strong oxidizing agents. However, ketones can be oxidized to various types of compounds only by using extremely strong oxidizing agents. Typical oxidizing agents for aldehydes include either potassium permanganate (KMnO4) or potassium dichromate (K2Cr2O7) in acid solution and Tollens reagent. Mild oxidising agents oxidise only aldehydes into carboxylic acids. Peroxy acids, such as peroxybenzoic acid:
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Glucose, fructose give positive test with Tollen's reagents and Fehling solution.
C5H11O5CHO + Cu2O (or) Ag2O ————→ C5H11O5COOH
Gluconic acid
Baeyer-Villiger oxidation is a ketone oxidation, and it requires the extremely strong oxidizing agent peroxybenzoic acid. For example, peroxybenzoic acid oxidizes phenyl methyl ketone to phenyl acetate (an ester).
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Disproportionation, also known as dismutation, is a specific type of redox reaction in which a species is simultaneously reduced and oxidized to form two different products.