Aldehydes and Ketones

Aldehydes and ketones have a carbonyl group (C=O) as a functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon (RCOR’). The simplest ketone is acetone, which has two methyl groups attached to the carbonyl carbon (CH₃COCH₃).

An aldehyde is similar to a ketone, except that instead of two side groups connected to the carbonyl carbon, they have at least one hydrogen (RCOH). The simplest aldehyde is formaldehyde (HCOH), as it has two hydrogens connected to the carbonyl group. All other aldehydes have one hydrogen bonded to the carbonyl group, like the simple molecule acetaldehyde, which has one hydrogen and one methyl group (HCOCH₃).

The carbonyl carbon in both aldehydes and ketones is electrophilic, meaning that it has a dipole due to the electronegativity of the attached oxygen atom. This makes the carbonyl carbon an ideal target for nucleophiles in a nucleophilic addition reaction. During this reaction, the nucleophile, or electron donor, attacks the carbonyl to form the tetrahedral intermediate. The negatively charged oxygen accepts a hydrogen ion to form a hydroxyl group.

Typically, nucleophiles possess a negative charge or lone pair on a heteroatom, which can take several forms (OH^-, RO^-, CN^-, R₃C^-, RNH₂, ROH). For primary amines (RNH₂), the reaction does not stop at the formation of the tetrahedral intermediate with a hydroxyl group. Rather, an elimination reaction occurs that produces a double-bonded carbon and nitrogen functional group known as an imine. Understanding the reactions that aldehydes and ketones can undergo provides a way to differentiate between these similar organic compound types.

DNPH Test

The formation of imines from ketones or aldehydes is exploited through the use of the reagent 2,4-dinitrophenylhydrazine (DNPH). In this addition-elimination reaction, the primary amino group of the DNPH attacks the carbonyl of an aldehyde or ketone in an acidic environment. The condensation reaction results in the formation of a hydrazone, which precipitates out of solution.

Precipitates that are yellow indicate non-conjugated ketones or aldehydes, whereas red-orange precipitates indicate conjugated systems. This test is used to differentiate ketones and aldehydes from alcohols and esters with which DNPH does not react, and thus no precipitate is formed. The ketone or aldehyde derivatives are crystalline materials with very well-defined melting points, which are reported in the literature and can be used to identify the specific compounds.

Haloform Test

Another test is the haloform test, which is used to determine whether a ketone is a methyl ketone. Methyl ketones have a methyl group connected to the carbonyl carbon (RCOCH₃). The R group can be either a hydrogen (methyl aldehyde), an alkyl group, or an aryl group.

The reaction mechanism for the haloform test occurs under basic conditions. First, the ketone undergoes a keto-enol tautomerization. Then the nucleophilic enolate attacks the iodine to give an iodine ion and a halogenated ketone. This repeats a total of three times until all hydrogens of the ketone methyl group are replaced by an iodine. 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.

Tollens Test

The Tollens’ test is a reaction that is used to distinguish aldehydes from ketones, as aldehydes are able to be oxidized into a carboxylic acid while ketones cannot. Tollens’ reagent, which is a mixture of silver nitrate and ammonia, oxidizes the aldehyde to a carboxylic acid. The silver ion Ag⁺ is reduced to solid silver, Ag_(s). The solid silver forms a film on the inner wall of the test tube, resembling a silver mirror. Ketones do not react with Tollen’s reagent, and therefore do not result in the formation of a silver mirror on the walls of the test tube.

References

1. Streitwieser, A., Heathcock, C.H., Kosower, E.M. (1998). Introduction to Organic Chemistry. Upper Saddle River, NJ: Prentice Hall.
2. Fuson, R.C., Bull, B.A. (1934). The Haloform Reaction. Chemical Reviews. 15(3), 275-309.