Carbonyl compounds — OCR A-Level Chemistry

Carbonyl compounds react in ways that depend on the C=O bond, which is polar because oxygen is more electronegative than carbon, leaving the carbon δ⁺ and oxygen δ⁻. Nucleophiles therefore attack the carbon. Aldehydes and ketones undergo nucleophilic addition: with HCN (in the presence of a trace of alkali) the product is a hydroxynitrile, and acid hydrolysis of that nitrile gives a hydroxycarboxylic acid. Reduction with NaBH₄ or LiAlH₄ in a suitable solvent converts aldehydes to primary alcohols and ketones to secondary alcohols. Aldehydes are readily oxidised: Fehling's solution changes from blue to a red precipitate of copper(I) oxide, and Tollens' reagent gives a silver mirror. Ketones resist these mild oxidations, which distinguishes them.

Aldehydes are oxidised to carboxylic acids by acidified dichromate(VI), written Cr₂O₇²⁻/H⁺ and supplied practically as potassium dichromate(VI) in dilute sulfuric acid, K₂Cr₂O₇/H₂SO₄. The aldehyde's carbonyl carbon already carries a hydrogen, so the C–H bond is replaced by a C–O bond and the carbon is oxidised from +1 to +3. The dichromate ion is reduced, changing the solution from orange to green as chromium goes from +6 to +3. Heating under reflux with excess oxidising agent drives the reaction to completion. Ethanal, for example, gives ethanoic acid. Ketones lack the hydrogen on the carbonyl carbon and so are not oxidised in this way, which is why the test distinguishes the two families. Match the reagent, the structural change and the colour change when selecting an answer.

The C=O bond is polar, so the carbon carries a partial positive charge and is attacked by nucleophiles. In nucleophilic addition the nucleophile adds to the carbonyl carbon and the π bond opens, giving a tetrahedral intermediate that picks up a proton. With HCN, supplied as KCN acidified in situ, the product is a hydroxynitrile; acid hydrolysis of that nitrile then gives a hydroxycarboxylic acid. With hydride from NaBH₄, aldehydes are reduced to primary alcohols and ketones to secondary alcohols. The mechanism can be shown with curly arrows from the nucleophile to the δ⁺ carbon and from the C=O π bond to oxygen.

Sodium tetrahydridoborate, NaBH₄, is a source of hydride ions, H⁻, which act as the nucleophile in addition to a carbonyl group. The reagent is used in aqueous or alcoholic solution, and the reaction is a reduction because the organic compound gains hydrogen. An aldehyde reacts with NaBH₄ to form a primary alcohol, for example ethanal gives ethanol. A ketone reacts to form a secondary alcohol, for example propanone gives propan-2-ol. The mechanism follows nucleophilic addition: H⁻ attacks the δ⁺ carbonyl carbon, the C=O π bond breaks to leave a negative oxygen, and protonation by water or acid gives the OH group. NaBH₄ does not reduce C=C bonds, so it is a selective reducing agent for the carbonyl group.

Aldehydes and ketones react with hydrogen cyanide to give hydroxynitriles. Because HCN is a weak acid and hazardous as the pure gas or liquid, the reagent is supplied in practice as sodium cyanide solution followed by dilute acid: NaCN(aq) provides the cyanide ion CN⁻ nucleophile, and H⁺(aq) protonates the intermediate alkoxide. The carbonyl carbon is attacked by CN⁻, the C=O π bond breaks and the oxygen takes the electron pair, giving an alkoxide that is then protonated to an –OH group. The product therefore contains both a hydroxyl group and a nitrile group on the same carbon skeleton, which is why it is called a hydroxynitrile. For ethanal the product is CH₃CH(OH)CN, and for propanone it is (CH₃)₂C(OH)CN. The carbon chain is lengthened by one carbon, so this is a useful carbon–carbon bond-forming step.

Both NaBH₄ and HCN deliver a nucleophile to the electrophilic carbonyl carbon, and both follow the same nucleophilic addition sequence. With NaBH₄ the hydride ion H⁻ attacks the carbonyl carbon; the C=O π bond breaks and the oxygen becomes an alkoxide, which is protonated (for example by water or acid) to give a primary alcohol from an aldehyde or a secondary alcohol from a ketone. With HCN supplied as NaCN(aq)/H⁺(aq), the cyanide ion CN⁻ attacks the carbonyl carbon, the π bond breaks to give an alkoxide, and H⁺ protonates the oxygen to give a hydroxynitrile. In each case the mechanism is drawn with a curly arrow from the nucleophile's lone pair to the carbonyl carbon and a second arrow from the C=O π bond onto the oxygen, followed by protonation of the alkoxide.

Carbonyl compounds contain the C=O group, found in aldehydes (RCHO) and ketones (RCOR'). Characteristic tests distinguish them from other compounds and from each other. Adding 2,4-dinitrophenylhydrazine (Brady's reagent) gives a yellow-orange precipitate with both aldehydes and ketones, confirming a carbonyl group. Tollens' reagent (ammoniacal silver nitrate) is reduced by an aldehyde on warming to give a silver mirror, but not by a ketone. Fehling's solution, containing Cu²⁺ ions, is reduced by an aldehyde on heating to give a brick-red precipitate of copper(I) oxide, Cu₂O; a ketone gives no change. Acidified potassium dichromate(VI), K₂Cr₂O₇/H⁺, turns from orange to green with an aldehyde but not with a ketone. These tests rely on the aldehyde being readily oxidised to a carboxylic acid while the ketone is not.

2,4-Dinitrophenylhydrazine (Brady's reagent) reacts with the carbonyl group of aldehydes and ketones to form a yellow-orange crystalline precipitate called a 2,4-dinitrophenylhydrazone. This confirms the presence of a carbonyl compound. The precipitate can be filtered off, purified by recrystallisation and its melting point measured; comparing this melting point with known data identifies the specific aldehyde or ketone. The test does not distinguish an aldehyde from a ketone, because both react. The reagent is used in solution with methanol and sulfuric acid. The reaction is a condensation, releasing water as the C=O group is converted to C=N-NH-Ar.

A carbonyl group (C=O) is detected chemically by 2,4-dinitrophenylhydrazine (2,4-DNPH), which condenses with aldehydes and ketones to give an orange or yellow-orange crystalline precipitate of a 2,4-dinitrophenylhydrazone. The test confirms a carbonyl is present but does not by itself distinguish aldehyde from ketone. To distinguish, use Tollens' reagent (ammoniacal silver nitrate), where an aldehyde gives a silver mirror while a ketone does not, or Fehling's solution, where an aldehyde gives a brick-red precipitate of copper(I) oxide and a ketone does not. A carboxylic acid gives no such precipitate with 2,4-DNPH. In an MCQ, select the reagent and observation that together show a carbonyl group is present, and reject options that describe only a colour change in solution without a precipitate or that claim the test alone separates aldehyde from ketone.

A carbonyl compound is identified by converting it to a solid derivative, usually the 2,4-dinitrophenylhydrazone, then measuring the melting point of the purified solid and comparing it with published values. Each pure derivative has a sharp, characteristic melting point, so a measured value that matches a literature value within a narrow range identifies the original carbonyl compound. The solid is recrystallised to remove impurities, because impurities lower and broaden the melting range. A measured melting point close to a known value but a few degrees low, with a wide range, suggests impurity rather than a different compound. In an MCQ, choose the option that pairs a measured melting point with the matching literature value for the derivative, and reject options that compare the melting point of the original liquid carbonyl or that ignore purification.

Tollens’ reagent is ammoniacal silver nitrate: silver nitrate solution treated with ammonia until the grey-brown silver oxide just redissolves, leaving the colourless complex ion [Ag(NH₃)₂]⁺ in alkaline solution. It is a mild oxidising agent that tests for an aldehyde group. Warm the unknown with freshly prepared Tollens’ reagent in a clean test tube; an aldehyde reduces the silver complex, and silver metal deposits as a mirror on the glass or as a grey precipitate. A ketone gives no change. The reagent must be made fresh because it decomposes on standing, and the tube should be clean so the mirror is visible. This row covers the reagent and its use; the positive observation is developed in the linked detect-the-aldehyde row.

Aldehydes are carbonyl compounds in which the C=O carbon carries at least one hydrogen, giving the group –CHO. Two standard wet tests distinguish them from ketones. Fehling's solution (blue, containing Cu²⁺ complexes in alkaline tartrate) is warmed with the unknown; an aldehyde reduces Cu²⁺ to a brick-red precipitate of copper(I) oxide, Cu₂O, while a ketone leaves the blue colour unchanged. Tollens' reagent (colourless ammoniacal silver nitrate, [Ag(NH₃)₂]⁺) is warmed gently; an aldehyde reduces Ag⁺ to a silver mirror of metallic silver, whereas a ketone gives no change. Both tests work because the aldehyde is readily oxidised to the carboxylate, so the aldehyde acts as the reducing agent and is itself oxidised. A positive result therefore confirms an aldehyde group is present; a negative result with both reagents indicates no aldehyde group, consistent with a ketone.

Aldehydes and ketones both contain the carbonyl group, C=O, but only the aldehyde has a hydrogen atom attached to the carbonyl carbon. That hydrogen allows the aldehyde to be oxidised to a carboxylic acid, while the ketone, with two carbon groups on the carbonyl carbon, resists this oxidation under the same mild conditions. Tollens' reagent supplies silver ions in an alkaline ammonia solution; when warmed with an aldehyde, the aldehyde is oxidised to the carboxylate/carboxylic acid and the silver ions are reduced to silver metal, giving a silver mirror. A ketone gives no such change. So the test distinguishes them by whether reduction of Ag⁺ to Ag occurs, which happens only when the aldehyde is oxidised.