Detection of aldehydes and ketones and ester

2,4-DNP Test for Aldehydes and Ketones

Chemical properties

Carbonyl Group

The carbonyl group in aldehydes and ketones reacts with hydrazine and its derivatives. The compound most frequently used is 2,4-dinitrophenylhydrazine because the product is always a solid. When 2,4-DNP is used as a solution it is known as Brady's Reagent. Addition of one or two drops of the suspected carbonyl compound to 1cm3 of Brady's reagent produces a yellow-orange solid

http://sustainability.sellafieldsites.com/resources/labmouse/chemistry_a2/2501.php

Physical Properties

Complications

Some ketones give oils which will not solidify

Some allylic alcohols are oxidized by the reagent to aldehydes and give a positive test

Some alcohols, if not purified, may contain aldehyde or ketone impurities

http://academics.wellesley.edu/Chemistry/chem211lab/Orgo_Lab_Manual/Appendix/ClassificationTests/aldehyde_ketone.html

Addition of sodium hydrogensulphite to aldehydes and ketones

Chemical properties

In the case of ethanal, the equation is

. . . and with propanone, the equation is

These compounds are rarely named systematically, and are usually known as "hydrogensulphite (or bisulphite) addition compounds"

Physical Properties

 Complications

Acetone phenone and benzophenone do not give this test

Aryl methyl ketones form the precipitate slowlyor not at all

Addition complex stable only in neutral solution

Tollen’s Test

Chemical properties

RCHO+3OH−⟶RCO2−+2H2O+2e−

which together yield the overall reaction

2Ag(NH3)2++RCHO+3OH−⟶2Ag0+RCO2−+4NH3+2H2O

Here is a diagram of the reaction mechanism. The carbonyl group is oxidized in the process and the Ag+

is reduced. The resultant oxidized aldehyde (now a radical cation) reacts with hydroxide to form a tetrahedral intermediate. A gem-diol like intermediate is formed via a hydrogen shift, which then continues on to the final carboxylate anion

Physical Properties

Complications

The test tube must be clean and oil-free if a silver mirror is to be observed

Easily oxidized compounds give a positive test. For example: aromatic amine and some phenols

http://academics.wellesley.edu/Chemistry/chem211lab/Orgo_Lab_Manual/Appendix/ClassificationTests/aldehyde_ketone.html

Schiff test

Chemical properties

The Schiff test named after is a for the detection of . An unknown sample is added to the decolorized Schiff reagent and when aldehyde is present a characteristic or purple color develops. The Schiff reagent is the reaction product of and . Human skin also contains aldehydes and gets stained as well.

Schiff reagents are used for various methods, eg. and periodic acid-Schiff stain.

Physical Properties

Complications

In this test the reagent should not be heated, and the solution tested should not be alkaline. When the test is used on an unknown, a simultaneous test on a known aldehyde and a known ketone should be performed for comparison

Note: With benzaldehyde the pink colour developes slowly 

http://documents.mx/documents/experiment-8b.html

Esters
Hydroxamic acid test

Chemical properties 

  R-CO-OR'  + H2N-OH  ->  R-CO-NH-OH + R'-OH

Esters react with hydroxylamine in the presence of sodium hydroxide to form the sodium salt of the corresponding hydroxamic acid. On acidification and addition of ferric chloride the magenta-coloured iron (III) complex of the hydroxamic acid is formed.

It is always advisable to ensure that an unknown compound does not give a colour with iron (III) chloride before carrying out the hydroxamic acid test

 Physical Properties