Alkanals and Alkanones: Chemistry of Aldehydes and Ketones

Alkanals (aldehydes) and alkanones (ketones) are carbonyl compounds with the carbonyl group, >C=O. The carbonyl group is represented as RCHO in Alkanals and RR'CO in the Alkanones where there is an oxygen atom attached to the carbonyl functional group in alkanals, and none in alkanone. Examples of Alkanals includes methanal, ethanal, phenylmenthanal; while examples of alkanones are pentan-3-one, butanone and propan-2-one.

Alkanals and Alkanones are prepared from alkanols in the presence of excess oxidising agents. While they can be prepared from primary and secondary alkanol respectively, tertiary alkanols cannot be oxidised easily due to the presence of three alkyl and aryl groups substituting all the hydrogen atoms at the central carbon atom.

Preparation of Alkanals

Alkanals (aldehydes) are prepared by the oxidation of primary alkanols (alcohols) in the presence of either acidified potassium tetraoxomanganate(VII), KMnO₄, or acidified potassium heptaoxodichromate(VI), K₂Cr₂O₇.

The equation of reaction for the production of alkanal, R-CHO, from primary alkanols, R-CH₂-OH; is as follows:

R-CH₂-OH + [O] → R-CHO + H₂O

This can be expressed by the preparation of propanal, CH₃CH₂CHO, from propanol (propan-1-ol), CH₃CH₂CH-OH.

CH₃CH₂CH-OH + [O] → CH₃CH₂CHO + H₂O

Preparation of Alkanones

Alkanones (ketones) on the other hand, are prepared from the oxidation of secondary alkanols by acidified potassium tetraoxomanganate(VII) or potassium heptaoxodichromate(VI). The general equation of reaction of the preparation of alkanals, RR'C=O, from secondary alkanols, RR'CHOH; is expresses as:

RR'CHOH + [O] → RR'C=O + H₂O

An example of the preparation of Alkanones from secondary alkanols is the preparation of propan-2-one, CH₃(CH₃)C=O, from propan-2-ol, CH₃(CH₃)CH-OH .

CH₃(CH₃)CH-OH + [O] → CH₃(CH₃)C=O + H₂O

The two most important chemical reactions of the carbonyl compounds are oxidation and reduction reactions.

Oxidation

While alkanals are easily oxidised to carboxylic acids (alkanoic acids) by strong oxidising agents like acidified potassium tetraoxomanganate(VII) and acidified potassium heptaoxodichromate(VI):

CH₃CH₂CHO + [O] → CH₃CH₂COOH,

Alkanones on the other hand, are not easily oxidised because, like in tertiary alkanols, they do not have oxidizable hydrogen attached to the carbonyl group. But there is an advanced case where strong oxidizing agents, like hot concentrated trioxonitrate(V) acid, are used to oxidize the the alkanone molecule in a process that splits the molecule into two.

Reduction

Reduction of Alkanals and Alkanones are more of recovery reactions in that alkanals are reduced to primary alkanols and alkanols to secondary alkanols in the presence of reducing agents like lithium tetrahydridoaluminate(III), LiAlH₄, and sodium tetrahydridoborate(III), NaBH₄.

Here, ethanal can be reduced to ethanol as an example of reduction of Alkanal:

CH₃CHO + 2[H] → CH₃CH₂-OH,

and propan-2-one is reduced to propan-2-ol as an example of alkanone:

CH₃(CH₃)C=O + 2[H] → CH₃(CH₃)CH-OH.

Alkanals and alkanones respond differently to oxidizing and reducing agents. Fehling's solution and Tollen's reagent are good oxidising agents that are reduced by alkanals and not alkanones. Hence they are used to distinguish between alkanals and alkanones.

Fehling's solution test

Fehling's solution is only positive on alkanals, and not alkanones. Brick-red precipitate is formed when a few drops of fehling's solution is added and heated with 5 cm³of a sample containing alkanal. Fehling's solution contains copper(II) complexes that are reduced to copper(I) oxide, thus forming a brick-red precipitate.

RCHO + 2Cu²⁺+ NaOH + H₂O → RCOO^-Na⁺ + Cu₂O + 4H⁺

That is, blue Cu²⁺ → Cu⁺ red

Tollen's reagent test

Tollen's reagent contains Ag(NH)₂⁺complex ion that is reduced to metallic silver by Alkanals, and not Alkanones.

RCHO + 2Ag(NH₃)₂OH → RCO₂NH₄ + Ag + 3NH₃ + H₂O

The procedure for carrying out Tollen's test is as simplified below.

1. A few drops of dilute sodium hydroxide solution is added to 5 cm³of silver trioxonitrate(V) solution in a clean test tube to form a brown precipitate of silver oxide.

2. Make a dilute aqueous ammonia solution by diluting aqueous ammonia with three times its volume with water, and add it in drops to the test tube until the brown precipitate just dissolves.

3. Now, add two drops of the dilute test sample. A silver deposit will be formed as mirror on the walls of the test tube if alkanal is present in the sample.