Aldehydes, Ketones and Carboxylic Acids: common doubts, answered
The questions students ask most often about Aldehydes, Ketones and Carboxylic Acids, each with a short answer. For the full chapter, read the Aldehydes, Ketones and Carboxylic Acids notes.
Names and the carbonyl group
Read this section in the notes →Why is the carbonyl group polar and where do reagents attack it?
Oxygen is more electronegative than carbon, so the C=O electrons are pulled towards oxygen, leaving the carbon partly positive and the oxygen partly negative. Resonance places a full positive charge on carbon in one structure. So nucleophiles attack the carbonyl carbon, while electrophiles and protons go to the oxygen. The carbonyl carbon is sp² and the group is planar.
Preparing aldehydes and ketones
Read this section in the notes →What is the difference between Rosenmund reduction and Stephen reaction?
Both make aldehydes, but from different starting materials. Rosenmund reduction hydrogenates an acyl chloride over palladium on barium sulphate, a catalyst poisoned so it stops at the aldehyde. The Stephen reaction reduces a nitrile with stannous chloride and HCl to an imine, which hydrolysis turns into the aldehyde. DIBAL-H also converts nitriles and esters into aldehydes.
Why are dialkylcadmium compounds used to make ketones from acyl chlorides?
Dialkylcadmium, R₂Cd, adds an alkyl group to an acyl chloride and gives a ketone, but it is too weakly reactive to attack that ketone further. A Grignard reagent would keep reacting and turn the ketone into a tertiary alcohol. So the cadmium reagent lets the reaction stop at the ketone stage.
Aromatic aldehydes and ketones
Read this section in the notes →What is the Etard reaction?
It oxidises the methyl group of toluene to an aldehyde without going on to benzoic acid. Chromyl chloride, CrO₂Cl₂, forms a chromium complex with the methyl group, and hydrolysis releases benzaldehyde. Chromic oxide in acetic anhydride achieves the same end via a benzylidene diacetate, and side-chain chlorination followed by hydrolysis is another route.
Physical properties of aldehydes and ketones
Read this section in the notes →Why do aldehydes and ketones boil higher than hydrocarbons but lower than alcohols?
The polar C=O group gives strong dipole-dipole attractions between molecules, more than the weak van der Waals forces in hydrocarbons of similar mass. But the carbonyl compound has no hydrogen on oxygen, so its molecules cannot hydrogen bond with each other, which leaves the attractions weaker than in alcohols. Their lower members dissolve in water because they accept hydrogen bonds from it.
Nucleophilic addition
Read this section in the notes →Why are aldehydes more reactive than ketones in nucleophilic addition?
There are two reasons. Electronically, the two alkyl groups of a ketone push electron density towards the carbonyl carbon and make it less positive, while an aldehyde has only one. Sterically, the two groups of a ketone crowd the carbon and hinder the incoming nucleophile more. Benzaldehyde is less reactive than propanal because the ring's resonance reduces the carbonyl carbon's positive charge.
Why is a trace of base needed when HCN adds to a carbonyl compound?
HCN is a weak acid and gives few cyanide ions by itself, and the neutral molecule is a poor nucleophile. A base removes H⁺ from HCN and produces CN⁻, the strong nucleophile that attacks the carbonyl carbon. The resulting alkoxide then takes H⁺ to form a cyanohydrin, which can later be converted to hydroxy acids.
Ammonia derivatives and reduction
Read this section in the notes →Why is the addition of ammonia derivatives to carbonyl compounds done in weak acid?
A little acid protonates the carbonyl oxygen, making the carbon more positive and easier to attack. Too much acid, however, protonates the nitrogen of the ammonia derivative, which removes its lone pair and stops it acting as a nucleophile. So a pH of about 3.5 is chosen as the balance between the two effects.
What is the difference between Clemmensen and Wolff-Kishner reduction?
Both convert a carbonyl group into a CH₂ group, but under opposite conditions. Clemmensen reduction uses zinc amalgam and concentrated hydrochloric acid, so it suits compounds stable in acid. Wolff-Kishner reduction first forms a hydrazone with hydrazine, then heats it with KOH in ethylene glycol, so it suits compounds that would be damaged by acid.
Oxidation and the tests
Read this section in the notes →How do Tollens' and Fehling's tests distinguish aldehydes from ketones?
Aldehydes are easily oxidised and reduce these mild reagents, while ketones generally do not. Tollens' reagent, ammoniacal silver nitrate, is reduced to a silver mirror. Fehling's solution, an alkaline copper(II) tartrate complex, gives a red-brown precipitate of Cu₂O. Aromatic aldehydes such as benzaldehyde give the Tollens' test but not the Fehling's test.
Which compounds give the iodoform test?
Compounds containing a CH₃CO– group, or a group that can be oxidised to it, such as CH₃CH(OH)–, give yellow iodoform with sodium hypoiodite. So ethanal, propanone and ethanol give a positive test, as do propan-2-ol and acetophenone. Methanol, methanal, propanal and pentan-3-one do not. It is a common way to distinguish isomers such as pentan-2-one from pentan-3-one.
α-Hydrogen reactions and uses
Read this section in the notes →Why do α-hydrogens in aldehydes and ketones behave as acidic?
When a base removes a hydrogen from the carbon next to C=O, the negative charge left behind is delocalised onto the carbonyl oxygen by resonance, forming an enolate ion. This stabilisation makes those hydrogens far more acidic than ordinary C–H hydrogens. The enolate is the key intermediate in aldol and related reactions.
What is the difference between the aldol reaction and the Cannizzaro reaction?
The aldol reaction needs an aldehyde or ketone with at least one α-hydrogen and dilute alkali; two molecules join to give a β-hydroxy aldehyde or ketone, which loses water on heating. The Cannizzaro reaction needs an aldehyde with no α-hydrogen, such as methanal or benzaldehyde, and concentrated alkali; one molecule is oxidised to a carboxylate and another reduced to an alcohol.
What is a cross aldol reaction?
It is an aldol reaction between two different aldehydes or ketones. If both have α-hydrogens, four products can form, which is rarely useful. It becomes practical when one partner has no α-hydrogen, such as benzaldehyde, since that one can only accept attack. Benzaldehyde with acetophenone gives benzalacetophenone, a reaction known as Claisen-Schmidt condensation.
Carboxylic acids: names and structure
Read this section in the notes →Why don't carboxylic acids give typical carbonyl reactions like aldehydes do?
The lone pair on the –OH oxygen is shared with the C=O group by resonance, which makes the carbonyl carbon much less positive. So carboxylic acids do not form oximes or hydrazones and do not add HCN the way aldehydes and ketones do. Their chemistry is instead dominated by the acidic O–H and by substitution at the acyl carbon.
Preparing carboxylic acids
Read this section in the notes →How can a carboxylic acid be prepared from a Grignard reagent?
A Grignard reagent attacks carbon dioxide, usually as dry ice, to give a magnesium salt of a carboxylic acid, which mineral acid then converts to the free acid. The acid has one more carbon than the alkyl halide used to make the Grignard reagent. Hydrolysis of nitriles is the other main route that adds one carbon.
Physical properties and acidity
Read this section in the notes →Why do carboxylic acids boil higher than alcohols of similar mass?
Carboxylic acid molecules form two hydrogen bonds with each other, creating stable dimers in which the pair behaves like a heavier unit. These hydrogen bonds are not completely broken even in the vapour. More energy is therefore needed to boil an acid than an alcohol of similar mass, whose molecules form only single hydrogen bonds.
Why is a carboxylic acid more acidic than phenol?
Both anions are stabilised by resonance, but differently. In the carboxylate ion the negative charge is shared equally between two oxygen atoms, giving two equivalent structures. In the phenoxide ion the charge is spread onto carbon atoms of the ring, which are less electronegative, and the structures are not equivalent. The carboxylate is more stable, so the acid ionises more readily.
How do substituents change the acidity of carboxylic acids?
Electron-withdrawing groups stabilise the carboxylate ion and increase acidity, while electron-donating groups decrease it. So acidity rises in the order CH₃COOH < ClCH₂COOH < Cl₂CHCOOH < Cl₃CCOOH, with F₃CCOOH strongest. The effect weakens as the group moves further from –COOH. Benzoic acid, with pKa 4.19, is stronger than ethanoic acid, with pKa 4.76.
Reactions and uses of carboxylic acids
Read this section in the notes →What is the Hell-Volhard-Zelinsky reaction?
It replaces an α-hydrogen of a carboxylic acid with chlorine or bromine. The acid is treated with the halogen and a small amount of red phosphorus, followed by water, giving an α-halocarboxylic acid. The acid must have at least one α-hydrogen. The product is useful because the halogen can then be substituted by other groups.
Why can't NaBH₄ reduce a carboxylic acid to an alcohol?
Sodium borohydride is a mild hydride donor, and the carboxyl carbon is not electrophilic enough for it to attack. Lithium aluminium hydride, a much stronger reducing agent, converts carboxylic acids to primary alcohols, as does diborane. Because NaBH₄ leaves esters, amides and acids untouched, it can reduce an aldehyde or ketone in a molecule that also contains those groups.
Why does benzoic acid not undergo Friedel-Crafts reactions?
The –COOH group withdraws electrons from the ring and deactivates it, so the ring becomes too weak a nucleophile for Friedel-Crafts alkylation or acylation. The carboxyl group also binds to the Lewis acid catalyst, AlCl₃, using it up. Benzoic acid still undergoes nitration and halogenation, with the new group entering the meta position.
Lumi is not affiliated with or endorsed by NCERT. The official NCERT textbooks are free to read and download from NCERT's own website, ncert.nic.in. These notes and simulations are original work by Lumi (Aikolumi Software Pvt Ltd), © 2026, shared under CC BY-NC 4.0: copy, print, share and adapt them for any non-commercial use, with credit to Lumi and a link to lumineet.com.