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Aldehydes and ketones - AL only (3.3.8)
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Aldehydes are oxidised to carboxylic acids.
The oxidatising reagent is an acidified dichromate (VI) solution.
The mixture is boiled under reflux, using the apparatus below:
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The volatile aldehyde is prevented from escaping the oxidation reaction as it is condensed and returned to the reaction flask. The product is a carboxylic acid.
The reaction is associated with the colour change from orange to green. This indicates oxidation has occurred, as the chromium species has been reduced from orange to green .
Using to represent the oxidising agent, the oxidation of ethanal to ethanoic acid would be:
The oxidation number of the carbon increases by 1 therefore only 1 mole of is required per mole of ethanal.
or can both be used to reduce aldehydes and ketones.
The carbonyl group is reduced by two ions, forming the primary alcohol from the aldehyde, and the secondary alcohol from the ketone group.
It is common to use the symbol in equations to represent the hydrogen from the reducing agent in a reduction reaction.
Reduction of aldehydes and ketones is written as follows:
For an aldehyde:
For a ketone.
There is a 1:2 mole ratio of carbonyl : .
One hydrogen is from the nucleophilic attack of and the second is from a protic solvent.
Sodium borohydride, , in aqueous ethanolic solution, is a good reducing agent, and can reduce both aldehydes and ketones, but cannot reduce carboxylic acids.
The mechanism for carbonyl reduction is drawn in two key stages:
- Attack by the nucleophile, forming a new bond to the of the carbonyl group, alongside movement of pi bond electrons to the carbonyl oxygen.
- The electrons from of the intermediate ion attack a proton from a water molecule from the aqueous solution.
When drawing this mechanism, ensure the dipole is added to the carbonyl group and that all arrows clearly start from electrons (lone pairs or bonds) and go to atoms.
When heated under reflux with acidified solution, carbonyl compounds undergo nucleophilic addition reactions. This reaction is very useful in extending a carbon chain in organic synthesis reactions.
The lone pair of electrons on the carbon atom of the nitrile group, forms a new carbon-carbon bond with the carbonyl carbon. An ion then binds to the carbonyl oxygen forming an alcohol group. The reaction saturates the carbonyl bond. The product is a hydroxynitrile.
The product is 2-hydroxypropanenitrile.
The product is 2-hydroxy, 2-methylpropanenitrile.
(Note the nitrile group is a higher priority than the alcohol group, so is named in the suffix.)
is a highly toxic gas, so is not used directly.
The nucleophile is generated in situ by reacting or with a dilute acid, such as .
The reaction must be managed in a fume cupboard to avoid contact with the generated.
The mechanism for nucleophilic addition of to a carbonyl is drawn in two key stages:
1. Nucleophilic attack by the lone pair nitrile ion, , forming a new bond to the of the carbonyl group, resulting in movement of pi bond electrons to the carbonyl oxygen.
2. The of the intermediate ion gaining a proton from the solvent water, or from the .
The organic products are hydroxynitriles which have both the group and the group now bonded to the same carbon.
During nucleophilic addition to a carbonyl, the nucleophile can approach the carbonyl carbon from above the plane, or below the plane with equal probability.
When the product is chiral, an equimolar mixture of both optical isomers (a racemic mixture) is produced.
The reaction of the nucleophile with an aldehyde, or an unsymmetrical ketone, will always produce a product with four different groups bonded to the same carbon atom; a chiral product.
The trigonal planar functional group makes attack equally likely from above or below the group.
Where a chiral hydroxynitrile product is formed, it will always exist as a racemic mixture of two non-superimposable mirror images, known as optical isomers, or enantiomers.
The Tollen’s reagent test can be used to distinguish aldehydes from ketones.
The reagent is prepared on the day of use, as it is unstable. It consists of a colourless solution of ammoniacal silver nitrate, which is made from an alkaline silver nitrate reacted with concentrated ammonia solution.
The observation for aldehydes is the formation of a ‘silver mirror’. Ketones DO NOT give a silver mirror – the solution remains colourless.
The silver mirror is formed as a redox reaction occurs with the silver ions reduced to silver metal, as the aldehyde is oxidised to a carboxylic acid salt.
Ketones cannot be further oxidised so do not support the reduction of Tollen’s reagent.
Fehling’s solution and Benedict’s solution are used to identify aldehydes.
Both solutions rely upon the oxidation of the aldehyde to a carboxylic acid salt, by a solution of a blue complex of copper(II) ions, which is subsequently reduced to a brick-red precipitate of copper(I) oxide in alkaline conditions.
The ligands on the copper complex in Fehling’s and Benedict’s solutions are different but both exhibit a change from a blue solution to a brick-red precipitate on heating with a reducing agent (e.g. an aldehyde).
Felling’s test and Tollens’ test can be used to distinguish between aldehydes and ketones. Both tests give a positive result for an aldehyde but not for a ketone.
They are also useful for distinguishing primary and secondary alcohols. The alcohols can be oxidised and the products distilled as they are formed then tested. Only primary alcohols produce aldehydes that give a positive result.