Aldehydes & ketones – Flashcards

To distinguish between aldehydes and ketones when trying to determine the identity of a compound

The Tollens test, the Schiff test, and the Iodoform test.

To identify between aldehydes and ketones using purely chemical means. Aldehydes and ketones can be distinguished by rate of oxidation - aldehydes are easily oxidized to carboxylic acids. The oxidation occurs via silver nitrate. The silver nitrate is reduced to metallic silver which can be seen on the inside of the test tube as a silver mirror. If the silver mirror is present, the compound is positive for aldehydes. If no silver mirror is formed - the compound is probably a ketone, not an aldehyde.

The Schiffs test works because it is a solution of red dye, which becomes colorless when treated with sulfur dioxide. The colorless solution will turn magenta upon reaction with the aldehyde. If the solution remains colorless, the compound is most likely a ketone or some other type of compound - not an aldehyde.

The iodoform test is one that is specifically designed for ketones - it distinguishes ketones from methyl ketones. The substance will be dissolved in water or ethane, NaOH added along with iodine solution. The iodine solution is a brown colored reagent. If the substance is positive for methyl ketones, the brown reagent will disappear and the yellow iodoform will separate out.

The purpose of these tests are to use these stable imines to convert the unknown compound into a solid derivative to determine the melting point of the compound. Since it can be hard to determine the boiling point of a small amount of liquid, it is far easier to convert it to a solid derivative to determine boiling point.

The derivatives are created to help further identify the compound by determining the melting point of a solid. Since it is hard to determine the boiling point of a small amount of liquid, it is easier to determine the melting point of a solid. This is helpful when you are trying to determine the identity of multiple compounds that are producing the same positive tests and have similar melting points. Turning them into a derivative makes them different than other regular compounds (derivative MP is different than regular MP).

The two nitro groups increase the molecular weight of the compound which increases the melting point and makes it distinct. They are usually crystalline compounds/solids which make them better derivatives because you can do melting point with them.

Pentanal is an aldehyde so it would produce a positive test, resulting in a silver mirror for the Tollens test. Aldehydes have the ability to oxidize into acids, and will produce a silver mirror. Pentanal would also produce a positive test for the Schiff's test, which goes from colorless liquid to magenta in the presence of an aldehyde. It would be negative in the iodoform test which is a test for methyl ketones. Since both 2-pentanone and 3-pentanone are both ketones, the only way to distinguish them would be through the iodoform test which tests for methyl ketones. Since 2-pentanone is a methyl ketone, it would produce a positive test by removing the brown color of the iodine and forming a dark yellow color.