This reaction is carried out by adding a dilute mineral acid into a mixture of the carbonyl compound and an aqueous solution of sodium cyanide. Here the CN ion acts as the nucleophile.
In the first stage, there is a nucleophilic attack by the cyanide ion on the slightly positive carbon atom.
The negative ion formed then picks up a hydrogen ion from somewhere – for example, from a hydrogen cyanide molecule.
The hydrogen ion could also come from the water or the H3O+ions present in the slightly acidic solution
As before, the reaction starts with a nucleophilic attack by the cyanide ion on the slightly positive carbon atom.
It is completed by the addition of a hydrogen ion from, for example, a hydrogen cyanide molecule.
R group of Grignard reagent together with the electron pair of R-Mg bond reacts as a nucleophile with carbonyl carbon.
(i) Nucleophilic addition
(ii) Dehydration
The above intermediate product undergoes dehydration as soon as it is formed to give the final product, which is 2,4–dinitrophenylhydrozone.
This reaction is used as a test to identify aldyhydes and ketones.
Here, the aldehydes and ketones get reduced to alcohols.
Here, the aldehydes and ketones can be reduced to the corresponding hydrocarbons.
Aldehydes are oxidized to carboxylic acids even by mild oxidizing agents such as Tollen’s reagent and Fehling solution.
Tollen’s reagent, [Ag(NH3)2]+ is prepared by adding dilute ammonium hydroxide to the precipitate of silver oxide formed by the addition of a few drops of dilute sodium hydroxide to an aqueous solution of silver nitrate.
Oxidation by Tollen’s reagent or the silver mirror test is used to distinguish between aldehydes and ketones.
A solution of basic cupric tartarate is known as Fehling solution. This is a dark blue aquous solution. When a few drops of an aldehyde are added to this reagent and heated, the blue colour of the solution gradually disappears and a brick red precipitate of cuprous oxide is formed.
Aldehydes and ketones can be distinguished from each other by Fehling solution.
Aldehydes get oxidised to carboxylic acids by oxidizing agents such as acidified potassium dichromate or acidified chromic oxide or acidified potassium permanganate.
In the presence of aldehydes the pink colour of H+/KMnO4 solution become colourless. The orange colour of H+/Cr2O72- solution turns green. By using these reagents also aldehydes and ketones can be distinguished from each other.
• Due to the strong electron withdrawing nature of the carbonyl group, H atoms attached to the carbon atoms directly bound to the carbonyl carbon (the α- H) become acidic. The α- H can be abstracted as a proton by base (eg. OH–).The carbanion so formed is stabilized by resonance as shown below
The above carbanion attacks the carbon atom of the carbonyl group of an unionized aldehyde molecule as a nucleophile. Hence aldehydes and ketones with α hydrogens undergo self – condensation reactions.
Examples
Reaction of acetaldehyde in the presence of aqueous NaOH
Condensation of acetone
The addition products obtained above undergo dehydration easily.
Examples :
