The structure and reactions of aldehydes and ketones, nucleophilic addition with hydrogen cyanide and reduction, oxidation of aldehydes, and the chemical tests that distinguish aldehydes from ketones.

What this dot point is asking

CCEA wants you to describe the carbonyl group, write the nucleophilic addition mechanism with HCN, reduce aldehydes and ketones, oxidise aldehydes, and choose tests (Tollens, Fehling, 2,4-DNP) to identify and distinguish carbonyl compounds.

The carbonyl group

The bond is polarised $\text{C}^{\delta+}=\text{O}^{\delta-}$ because oxygen is more electronegative, so the carbon is open to attack by nucleophiles. Unlike the $\text{C}=\text{C}$ bond of an alkene (which is electron-rich and attracts electrophiles), the $\text{C}=\text{O}$ bond is electron-poor at carbon, so carbonyls undergo nucleophilic, not electrophilic, addition. Aldehydes are generally more reactive towards nucleophiles than ketones because the single alkyl group on an aldehyde donates less electron density than the two alkyl groups on a ketone, leaving the aldehyde carbon more $\delta+$ and less sterically shielded.

Nucleophilic addition

The mechanism proceeds in two stages. First a curly arrow runs from the lone pair on $\text{CN}^-$ to the electron-deficient carbonyl carbon, while a second curly arrow moves the $\text{C}=\text{O}$ pi electrons onto the oxygen, generating a tetrahedral alkoxide intermediate. Second, the alkoxide oxygen is protonated by HCN (or water) to give the neutral hydroxynitrile and regenerate $\text{CN}^-$. A trace of base such as KCN speeds the reaction by raising the concentration of the $\text{CN}^-$ nucleophile.

$\text{CH}_3\text{CHO} + \text{HCN} \rightarrow \text{CH}_3\text{CH(OH)CN}$

The reaction is useful in synthesis: the nitrile group can be hydrolysed to a carboxylic acid ($\text{-COOH}$) or reduced to an amine ($\text{-CH}_2\text{NH}_2$), so adding HCN is a way of lengthening a carbon chain by one carbon. Because the planar carbonyl is attacked from either face with equal probability, an aldehyde or unsymmetrical ketone with no other chiral centre gives a racemic mixture of the hydroxynitrile.

Reduction and oxidation

Reduction supplies hydride ($\text{H}^-$) to the carbonyl carbon. CCEA uses sodium tetrahydridoborate(III), $\text{NaBH}_4$, in aqueous or alcoholic solution; the $\text{H}^-$ acts as the nucleophile. Aldehydes are reduced to primary alcohols and ketones to secondary alcohols. Reduction does not break the carbon skeleton.

$\text{CH}_3\text{CHO} + 2[\text{H}] \rightarrow \text{CH}_3\text{CH}_2\text{OH}$

$\text{CH}_3\text{COCH}_3 + 2[\text{H}] \rightarrow \text{CH}_3\text{CH(OH)CH}_3$

Oxidation distinguishes the two classes sharply. Only aldehydes are oxidised, to carboxylic acids, by acidified $\text{K}_2\text{Cr}_2\text{O}_7$ (orange to green) or by the mild reagents below. Ketones resist oxidation because there is no hydrogen on the carbonyl carbon to be removed; forcing conditions would only break a $\text{C}-\text{C}$ bond. This difference, oxidisable aldehyde versus inert ketone, is the basis of the Tollens and Fehling tests.

Tests to distinguish carbonyls

Two further points complete the picture. The orange 2,4-DNP precipitate is a crystalline derivative whose melting point, once recrystallised, is unique to the parent carbonyl, so historically it was used to identify the exact compound from data tables. In the Tollens test, the aldehyde reduces the silver(I) in the diamminesilver(I) complex $[\text{Ag(NH}_3)_2]^+$ to metallic silver while it is itself oxidised to a carboxylic acid; the half-equations are $[\text{Ag(NH}_3)_2]^+ + \text{e}^- \rightarrow \text{Ag} + 2\text{NH}_3$ and $\text{RCHO} + \text{H}_2\text{O} \rightarrow \text{RCOOH} + 2\text{H}^+ + 2\text{e}^-$. Fehling solution works the same way, with copper(II) ions in a blue alkaline tartrate complex reduced to brick-red copper(I) oxide.

Examples in context

Tollens reagent is the chemistry behind silvering mirrors and the inside of vacuum flasks: an aldehyde such as glucose reduces the silver(I) complex to a bright deposit of metallic silver. In a CCEA practical, students confirm an unknown is a carbonyl with 2,4-DNP, then warm a fresh sample with Tollens reagent; a silver mirror identifies it as an aldehyde, while no change with Tollens but an orange 2,4-DNP precipitate points to a ketone such as propanone.

The nucleophilic addition of HCN underpins industrial routes to important products: the addition of hydrogen cyanide to ethanal gives 2-hydroxypropanenitrile, which on hydrolysis yields 2-hydroxypropanoic acid (lactic acid), and the addition to propanone followed by hydrolysis and dehydration is a classic route to methyl 2-methylpropenoate, the monomer for perspex. Reduction chemistry matters too: the manufacture of menthol and many fragrance and pharmaceutical alcohols relies on the selective reduction of a ketone to a secondary alcohol with a hydride reagent, exactly the $\text{NaBH}_4$ step CCEA examines.

Try this

Q1. Name the product when propanal reacts with HCN. [1 mark]

• Cue. 2-hydroxybutanenitrile, $\text{CH}_3\text{CH}_2\text{CH(OH)CN}$.

Q2. State the reagent and observation that confirms a compound contains a carbonyl group. [2 marks]

• Cue. 2,4-DNP (Brady reagent); an orange/yellow precipitate forms.

Q3. Write the equation for the reduction of propanone to propan-2-ol using $[\text{H}]$. [1 mark]

• Cue. $\text{CH}_3\text{COCH}_3 + 2[\text{H}] \rightarrow \text{CH}_3\text{CH(OH)CH}_3$.