How can aldehydes and ketones be distinguished

A small amount of potassium dichromate VI solution is acidified with dilute sulphuric acid and a few drops of the aldehyde or ketone are added. If nothing happens in the cold, the mixture is warmed gently for a couple of minutes - for example, in a beaker of hot water. In turn the aldehyde is oxidized to the corresponding carboxylic acid. The electron-half-equation for the reduction of dichromate VI ions is:.

Combining that with the half-equation for the oxidation of an aldehyde under acidic conditions:. This is made from silver I nitrate solution. You add a drop of sodium hydroxide solution to give a precipitate of silver I oxide, and then add just enough dilute ammonia solution to redissolve the precipitate. To carry out the test, you add a few drops of the aldehyde or ketone to the freshly prepared reagent, and warm gently in a hot water bath for a few minutes.

Figure 1: Tollens' test for aldehyde: left side positive silver mirror , right side negative. Image used with permission from Wikipedia. Aldehydes reduce the diamminesilver I ion to metallic silver. Because the solution is alkaline, the aldehyde itself is oxidized to a salt of the corresponding carboxylic acid. The electron-half-equation for the reduction of of the diamminesilver I ions to silver is:. Combining that with the half-equation for the oxidation of an aldehyde under alkaline conditions:.

Fehling's solution and Benedict's solution are variants of essentially the same thing. Both contain complexed copper II ions in an alkaline solution. Both solutions are used in the same way. A few drops of the aldehyde or ketone are added to the reagent, and the mixture is warmed gently in a hot water bath for a few minutes. Figure 2 : Fehling's test. Next, the hydroxide reacts with the electrophilic carbonyl carbon center to form the tetrahedral intermediate with a negatively-charged oxygen, which is single bonded to the carbonyl.

Upon reformation of the C-O double bond, the trihalomethyl group leaves as a stabilized leaving group. Lastly, the formed carboxylic acid is deprotonated by the negativelycharged trihalomethyl goup to give the haloform — trihalomethane — and a carboxylate. If iodine is the halogen, the precipitate will have a characteristic yellow color. This reaction will only occur for methyl ketones, and it will give a negative result for any other carbonyl-containing compound.

The solid silver forms a film on the inner wall of the test tube, resembling a silver mirror. To learn more about our GDPR policies click here. If you want more info regarding data storage, please contact gdpr jove. Your access has now expired. Provide feedback to your librarian. If you have any questions, please do not hesitate to reach out to our customer success team.

Login processing Haloform Test Another test is the haloform test, which is used to determine whether a ketone is a methyl ketone. References Streitwieser, A. Introduction to Organic Chemistry. Fuson, R. The Haloform Reaction.

Chemical Reviews. Please enter your institutional email to check if you have access to this content. The ketones are less reactive to the oxidation process since it lacks the Hydrogen atom, unlike the aldehydes.

But once they're exposed to overheating, they can be oxidized with powerful oxidizing agents. It is because of this unique differentiability, that it can help to distinguish between aldehyde and ketone. Because of the difference in aldehyde vs ketone structure, the following tests shall only yield results for the reactive aldehydes. It is through these reactions or tests that one can tell the difference between aldehyde and ketone. They are both reagents containing complex copper II in an alkaline solution.

For Fehling's solution, the copper II ions are complexed with that of the tartrate ions in a sodium hydroxide solution. The complexing of the copper II ions with that of the tartrate ions restrict the formation of a precipitate - Copper II hydroxide.

For Benedict's solution, the copper II ions are complex with the citrate ions in a sodium-carbonate solution. Here the copper II ions don't lead to the formation of copper II carbonate. The alkaline solutions contain the complex Copper ions, the colour of the solution is blue.