Carbonyl Compounds and Carboxylic Acids: One Mark Questions, Book Back Answers and Notes

About Carbonyl Compounds and Carboxylic Acids

Hard ~180 min study

The carbonyl group, C=O, is polar. Oxygen pulls electron density towards itself, leaving the carbon slightly positive and open to attack by nucleophiles. Almost every reaction of aldehydes and ketones in this unit starts from that one fact. Carbonyl compounds are common outside the lab too: propanone is a solvent, methanal solution preserves biological specimens, and benzaldehyde gives the smell of bitter almonds.

This unit follows directly from alcohols. Oxidise a primary alcohol and you get an aldehyde; oxidise further and you get a carboxylic acid. You will also see how a hydrogen on the carbon next to C=O (the alpha-hydrogen) behaves as a weak acid, which makes carbon-carbon bond forming reactions such as aldol condensation possible. Aldehydes with no alpha-hydrogen take a different route, the Cannizzaro reaction.

Questions here tend to ask for the reagent in a named reaction, the product of a two- or three-step conversion, a test that separates an aldehyde from a ketone, or the order of acid strength in a set of substituted acids. Learn each named reaction with its reagent and one textbook example.

What you'll learn

• Explain the planar structure and polarity of the carbonyl group.
• Write IUPAC names for aldehydes, ketones and carboxylic acids.
• Describe the main methods of preparing aldehydes, ketones and carboxylic acids.
• Predict the products of nucleophilic addition with HCN, NaHSO3, alcohols and ammonia derivatives.
• Explain the mechanisms of aldol condensation and the Cannizzaro reaction.
• Arrange substituted carboxylic acids in order of acid strength and give the reason.
• Use Tollens' and Fehling's tests to tell an aldehyde from a ketone.

Before you start

• IUPAC nomenclature and hybridisation of carbon.
• Electronegativity and how it makes a bond polar.
• Preparation and oxidation of alcohols from Unit 11.

Carbonyl Compounds and Carboxylic Acids explained

Aldehydes, Ketones and Carboxylic Acids: What the Unit Covers

Structure and Naming of the Carbonyl Group

The carbonyl carbon is \(sp^{2}\) hybridised, so it and the three atoms attached to it lie in one plane with bond angles close to 120°. Oxygen is more electronegative than carbon, so the C=O bond has a large dipole moment and the carbon is electrophilic. In IUPAC names, aldehydes end in -al (ethanal) and ketones end in -one (propanone), and the carbonyl carbon gets the lowest possible number.

Preparation of Aldehydes and Ketones

Oxidation of primary alcohols gives aldehydes and oxidation of secondary alcohols gives ketones. In the Rosenmund reduction, an acid chloride is reduced with hydrogen over palladium supported on barium sulphate, and the partly deactivated catalyst stops the reaction at the aldehyde. Stephen's reaction reduces a nitrile with SnCl₂ and HCl to an imine, which is hydrolysed to an aldehyde. Hydration of alkynes and ozonolysis of alkenes are other routes.

Nucleophilic Addition

HCN, sodium bisulphite, alcohols and Grignard reagents all add across the C=O bond. Ammonia derivatives such as hydroxylamine, hydrazine and phenylhydrazine add and then lose water, giving oximes, hydrazones and phenylhydrazones. Aldehydes react faster than ketones for two reasons: there is less crowding around the carbonyl carbon, and there is only one electron-releasing alkyl group reducing its positive charge. HCN addition gives cyanohydrins; alcohols give hemiacetals and then acetals.

Alpha-Hydrogen: Aldol and Cannizzaro

A hydrogen on the alpha-carbon is weakly acidic because the carbanion left behind (the enolate ion) is stabilised by resonance with the C=O group. In dilute alkali, two molecules of an aldehyde or ketone with alpha-hydrogen combine to give a beta-hydroxy aldehyde or ketone, the aldol. On heating, the aldol loses water to give an alpha,beta-unsaturated carbonyl compound; ethanal gives but-2-enal. Aldehydes with no alpha-hydrogen, such as methanal and benzaldehyde, undergo the Cannizzaro reaction in concentrated alkali: one molecule is oxidised to the carboxylate salt and another is reduced to the alcohol.

Carboxylic Acids

The carboxyl group, –COOH, is more acidic than the –OH of alcohols or phenols because in the carboxylate ion the negative charge is shared equally between two oxygen atoms. Carboxylic acids are made by hydrolysing nitriles or by treating a Grignard reagent with CO₂ and then dilute acid. They form derivatives such as acid chlorides, esters, amides and anhydrides. Electron-withdrawing groups near the carboxyl group increase acid strength, so chloroethanoic acid is stronger than ethanoic acid.

Common mistakes to avoid

• Saying ketones are more reactive than aldehydes towards nucleophiles. Two alkyl groups make the ketone carbon both more crowded and less positive, so aldehydes react faster.
• Writing an aldol product for benzaldehyde or methanal. Neither has an alpha-hydrogen, so on their own they give the Cannizzaro reaction, not aldol condensation.
• Forgetting why the Rosenmund catalyst is deactivated. Ordinary palladium would reduce the aldehyde on to a primary alcohol.
• Giving one product for the Cannizzaro reaction. Two molecules react: benzaldehyde gives benzyl alcohol and sodium benzoate together.
• Ignoring electron-withdrawing groups when comparing acids. Trichloroethanoic acid is much stronger than ethanoic acid because the three chlorine atoms stabilise the carboxylate ion.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

Why are aldehydes more reactive than ketones?

There are two reasons. An aldehyde has a hydrogen on one side of the carbonyl carbon, so a nucleophile can approach more easily. A ketone has two electron-releasing alkyl groups, which reduce the positive charge on the carbonyl carbon more than the single alkyl group of an aldehyde does. Less crowding and a more positive carbon make aldehydes react faster.

Why is the palladium catalyst in the Rosenmund reduction supported on barium sulphate?

Barium sulphate lowers the activity of the palladium, and a little sulphur or quinoline is often added for the same purpose. A partly deactivated catalyst reduces the acid chloride to the aldehyde but is not active enough to reduce the aldehyde further to a primary alcohol.

How can I tell whether a compound will undergo aldol condensation?

Look at the carbon next to the C=O group. If it carries at least one hydrogen, the compound can form an enolate ion in dilute alkali and give an aldol. Ethanal and propanone do; methanal, benzaldehyde and 2,2-dimethylpropanal do not.

What is Tollens' reagent used for?

Tollens' reagent is ammoniacal silver nitrate. An aldehyde reduces the silver ions to metallic silver, which coats the inside of a clean test tube as a mirror, while the aldehyde is oxidised to the carboxylate. Simple ketones such as propanone do not give the test, so it separates aldehydes from ketones.

Why are carboxylic acids stronger acids than phenols?

Both anions are stabilised by resonance, but not equally. In the carboxylate ion the negative charge is shared between two oxygen atoms in two equivalent structures. In the phenoxide ion some structures put the charge on ring carbon atoms, which are less electronegative than oxygen, so the stabilisation is weaker.

What happens in the Hell-Volhard-Zelinsky reaction?

A carboxylic acid that has an alpha-hydrogen is treated with chlorine or bromine in the presence of a small amount of red phosphorus. The alpha-hydrogen is replaced by the halogen. Ethanoic acid gives chloroethanoic acid, and with excess chlorine, dichloro- and trichloroethanoic acids.