Qualitative test for carbonyl compounds are essential in organic chemistry for identifying and distinguishing between different types of carbonyl-containing compounds, such as aldehydes, ketones, carboxylic acids, esters, and amides.
Qualitative test exploit the reactivity of the carbonyl group to provide visual cues or reactions that can help in identifying the presence and sometimes the specific type of carbonyl compound present in a sample.
Below are some commonly used qualitative tests in the context of carbonyl compounds:
1. Tollen’s Test (Silver Mirror Test)
Purpose:
To distinguish between aldehydes and ketones.
Principle:
Aldehydes are oxidized by Tollen’s reagent (ammoniacal silver nitrate solution) to carboxylic acids, reducing Ag++ ions to metallic silver, which deposits as a mirror-like coating on the inner walls of the test tube. Ketones do not react.
Positive Result for:
Aldehydes
2. Fehling’s Test
Purpose:
Another test to differentiate aldehydes from ketones.
Principle:
Fehling’s solution (a mixture of copper(II) sulfate, sodium potassium tartrate, and sodium hydroxide) is reduced by aldehydes to insoluble copper(I) oxide, which precipitates as a red to brick-red solid. Ketones, except alpha-hydroxy ketones, do not react.
Positive Result for:
Aldehydes
3. Benedict’s Test
Purpose:
Similar to Fehling’s test, used to identify reducing sugars (which are aldehydes or ketones capable of being oxidized) in aqueous solutions.
Principle:
Benedict’s solution (copper(II) sulfate, sodium citrate, and sodium carbonate) is reduced by aldehydes to copper(I) oxide, yielding a red to orange precipitate.
Positive Result for:
Reducing sugars (aldehydic or ketonic groups).
4. Schiff’s Test
Purpose:
To distinguish aldehydes from ketones.
Principle:
Schiff’s reagent (rosaniline hydrochloride decolorized by sulfurous acid) turns magenta (pink) in the presence of aldehydes due to the re-formation of the colored rosaniline dye upon reaction with aldehydes. Ketones generally do not give this reaction.
Positive Result for:
Aldehydes
5. Seliwanoff’s Test
Purpose:
To distinguish ketoses from aldoses (specific types of carbohydrates).
Principle:
Seliwanoff’s reagent (resorcinol and hydrochloric acid) yields a red color with ketoses due to the formation of furfural derivatives faster than with aldoses.
Positive Result for:
Ketoses (a type of carbonyl compound in carbohydrates).
6. Iodoform Test
Purpose:
To identify methyl ketones (ketones with a CH33CO- group).
Principle:
Iodoform test involves the reaction of iodine and sodium hydroxide with methyl ketones to produce a yellow precipitate of iodoform (CHI33). Some secondary alcohols that can be oxidized to methyl ketones also give a positive result.
Positive Result for:
Methyl ketones and some secondary alcohols.
7. 2,4-Dinitrophenylhydrazine (DNPH) Test
Purpose:
To detect carbonyl compounds in general.
Principle:
DNPH reacts with both aldehydes and ketones to form yellow, orange, or red precipitates of the corresponding hydrazones.