Aldehyde and ketone chemistry

Aldehydes and ketones are examples of carbonyl compounds. In an aldehyde the carbonyl group, C=O, is bonded to at least one hydrogen, hence it is always found at the end of the carbon chain.

In a ketone the carbonyl group is bonded to two alkyl or aryl groups, hence it is always found in the middle of the carbon chain.

The C=O bond is stronger and shorter than a C=C bond. It is reactive because the bond is polar.

The C=O bond is made up of a strong 𝜎-bond formed from the overlap of sp² hybrid orbitals on both the carbon and the oxygen and a weaker ∏-bond formed from the overlap of neighbouring p-orbitals on each atom, with electron density above and below the plane of the 𝜎-bond.

Testing for the presence of a carbonyl group

with 2,4-dinitrophenylhydrazine (which in practice is dissolved in methanol and sulphuric acid and better known as Brady’s agent). A yellow-orange precipitate indicates the presence of a carbonyl group in either an aldehyde or a ketone.

This is essentially a condensation reaction following the nucleophilic addition of the aldehyde or ketone to 2,4-dinitrophenylhydrazine.

Although the mechanism looks complicated, you should start to familiarise yourself with common organic steps such as lone pairs being used to attack 𝛅+ carbon atoms, electron pairs from the pi-bond in a C=O double bond moving onto an oxygen, oxygen using lone pairs of electrons to make bonds to H⁺ ions and N kicking out H⁺ to replace a lone pair of electrons it used earlier.

with Tollens’ reagent (the silver mirror test). The carbonyl group in an aldehyde can be oxidised to a carboxylic acid (ketones cannot be oxidised) so we can use this test to distinguish between the two functional groups. The oxidising agent is silver(I) in the form of a complex ion, [Ag(NH₃)₂]⁺, which is product of the reaction between aqueous solutions of silver nitrate and ammonia. When an aldehyde is warmed with Tollens’ reagent it is oxidised to a carboxylic acid and the silver(I) is reduced to metallic silver which coats the inside of the test tube giving a silver mirror like effect.

CH₃CHO + [O] ⇾ CH₃COOH and [Ag(NH₃)₂]⁺_(aq) + e^– ⇾ Ag_(s) + 2NH_3(aq)

with Fehling’s solution. Once again, this test will distinguish between an aldehyde and a ketone as Fehling’s solution contains copper(II) ions which will oxidise the carbonyl group in an aldehyde to a carboxylic acid (ketones cannot be oxidised). The copper(II) ions are reduced to copper(I) ions which form a brick red precipitate of copper (I) oxide.

Oxidation reactions of aldehydes

Just as aldehydes and ketones are the product of the oxidation of a primary and secondary alcohol respectively, aldehydes can be further oxidised to form carboxylic acids. Ketones, however, cannot be easily oxidised by common laboratory oxidising agents such as acidified potassium chromate(VII) (another way of distinguishing between the two).

The oxidising agent, [O], is itself reduced and there is a distinct colour change from orange (Cr₂O₇^2-) to green (Cr³⁺).

Nucleophilic addition reactions

In a nucleophilic addition reaction a nucleophile attacks the 𝛅+ carbon of the carbonyl group and a new bond is formed between the two.

Nucleophile: a negatively charged ion or neutral molecule that donates a pair of electrons to form a covalent bond e.g. CN^–, H^–, NH₃

Generally speaking, aldehydes react faster than ketones. The alkyl groups bonded to the C=O group can physically get in the way of the nucleophile approaching the 𝛅+ carbon (steric effect). Alkyl groups are also electron donating, helping to stabilise the charge on the carbon. As a result, they reduce the partial positive charge making the carbon less attractive to nucleophiles.

with cyanide ions to form a hydroxynitrile (cyanohydrin)

The CN^– ion is generated in situ when sodium cyanide is acidified with sulphuric acid.

This is an important reaction for synthesis purposes as a new C-C bond is formed and so another carbon is added to the chain. The nitrile group, CN, can be hydrolysed by heating with aqueous acid – in the first step of the reaction water is added to the nitrile forming an amide which is then further hydrolysed to a carboxylic acid.

with hydride ions (reduction)

The reaction of an aldehyde or ketone with sodium borohydride (sodium tetrahydridoborate(III)), NaBH₄, causes the C=O group to be reduced to an OH group forming a primary or secondary alcohol.

with water to form a diol

There is an additional step to the nucleophilic addition mechanism when the nucleophile is a neutral molecule. The first intermediate is an example of a zwitterion as the molecule has both a negative and a positive charge. The rate of the reaction is increased by the addition of an acid catalyst (see the final step below).

Practice questions

Describe the chemical tests (reagents and observations) that could be carried out to distinguish between the following molecules.

The carbonyl compound shown below reacts with cyanide ions.

(a) Give the systematic name for the molecule.

(b) Describe how the cyanide ions are generated for this reaction.

(c) The reaction takes place via a two step mechanism. Draw out both steps showing lone pairs of electrons, partial charges and curly arrows.

Answers

Molecule A is an aldehyde, molecule B is a ketone and molecule C is a carboxylic acid. Molecule C will not react with Brady’s agent / 2,4-dinitrophenylhydrazine (the solution will remain pale orange) but molecules A and B will react to give a yellow-orange precipitate.

We can distinguish between molecules A and B by shaking each with acidified potassium chromate(VI) because molecule A will be oxidised and the solution will change colour from orange to green but molecule B will not react. Alternatively, we could warm each solution with Tollen’s reagent / ammoniacal silver nitrate (Ag⁺ / NH₃) as molecule A will produce a silver mirror on the inside of the test tube, molecule B will give no reaction.