Chapter 8 - Aldehydes, Ketones and Carboxylic Acids

Aldehydes, Ketones and Carboxylic Acids is a central organic chemistry chapter built around the carbonyl group. Aldehydes and ketones contain a carbon-oxygen double bond, while carboxylic acids combine a carbonyl group with a hydroxyl group. The arrangement of these groups determines reactivity, acidity and physical properties.

This chapter contains important preparation methods, nucleophilic addition reactions, oxidation and reduction, qualitative tests, named reactions and conversions. Understanding electron distribution in the carbonyl group makes the large reaction set much easier to organise.

Nomenclature and structure of the carbonyl group

In aldehydes, the carbonyl carbon is bonded to at least one hydrogen; in ketones it is bonded to two carbon groups. The carbonyl carbon is sp2 hybridised and approximately trigonal planar. Because oxygen is more electronegative, the bond is polar: carbon is electrophilic and oxygen is relatively electron rich.

This polarity explains why nucleophiles attack carbonyl carbon. Aldehydes are generally more reactive than comparable ketones because they have less steric crowding and fewer electron-donating alkyl groups.

Preparation of aldehydes and ketones

Aldehydes and ketones can be prepared by controlled oxidation or dehydrogenation of alcohols, ozonolysis of alkenes and hydration or other reactions of alkynes. Additional methods use acyl chlorides, nitriles or aromatic substitution, depending on the target compound.

Named preparations such as Rosenmund reduction, Stephen reaction, Etard oxidation and Friedel-Crafts acylation are best learned by writing the starting functional group, reagent and product change.

Physical properties

Carbonyl compounds have dipole-dipole attractions and generally boil at higher temperatures than comparable hydrocarbons but lower than hydrogen-bonded alcohols. Lower aldehydes and ketones dissolve in water by accepting hydrogen bonds from water; solubility decreases as the hydrocarbon portion becomes larger.

Nucleophilic addition

Nucleophiles attack the electrophilic carbonyl carbon and convert the planar carbonyl group into a tetrahedral addition product. Reactions with hydrogen cyanide, sodium hydrogen sulphite, alcohols, ammonia derivatives and Grignard reagents illustrate this general mechanism.

Aldehydes and ketones form oximes, hydrazones, semicarbazones and related derivatives with ammonia compounds. These reactions are useful for characterisation and demonstrate the same addition-followed-by-elimination pattern.

Reduction and oxidation

Reduction converts aldehydes and ketones into alcohols. Stronger deoxygenation methods such as Clemmensen and Wolff-Kishner reductions convert the carbonyl group into a methylene group under acidic and basic conditions respectively.

Aldehydes are readily oxidised to carboxylic acids, while ketones resist mild oxidation. Tollens' reagent gives a silver mirror with aldehydes, and Fehling's solution is reduced by many aliphatic aldehydes. These tests should be interpreted with their conditions and known exceptions, not used as universal slogans.

Methyl ketones and compounds oxidisable to them can give the haloform or iodoform reaction. Recognising the required structural unit is more useful than memorising a short list of positive compounds.

Reactions involving alpha hydrogen

Hydrogen atoms on a carbon adjacent to the carbonyl group are called alpha hydrogens. They are relatively acidic because the resulting species is resonance-stabilised.

Carbonyl compounds with alpha hydrogen can undergo aldol condensation. Aldehydes without alpha hydrogen may undergo the Cannizzaro reaction in concentrated base, in which one molecule is oxidised and another is reduced. Students should inspect the structure for alpha hydrogen before choosing between these reactions.

Preparation and properties of carboxylic acids

Carboxylic acids can be prepared by oxidation of primary alcohols or aldehydes, hydrolysis of nitriles and other carboxylic-acid derivatives, and reaction of Grignard reagents with carbon dioxide followed by hydrolysis.

They form strong hydrogen-bonded dimers and consequently have high boiling points. Lower members are soluble in water, but solubility decreases with increasing hydrocarbon chain length.

Acidity of carboxylic acids

Carboxylic acids are more acidic than alcohols and phenols because their conjugate-base carboxylate ion delocalises negative charge over two oxygen atoms. Electron-withdrawing groups stabilise the conjugate base and increase acidity, especially when close to the carboxyl group. Electron-donating groups usually decrease acidity.

Reactions of carboxylic acids

Carboxylic acids form salts, esters, acyl chlorides, anhydrides and amides. Esterification with an alcohol is reversible and acid-catalysed. Reduction produces primary alcohols, while decarboxylation removes carbon dioxide under appropriate conditions.

The Hell-Volhard-Zelinsky reaction introduces halogen at the alpha carbon of acids possessing alpha hydrogen. As with all named reactions, the structural requirement matters as much as the reagent.

What can students study on Teachoo?

• nomenclature and structure of carbonyl compounds;

• relative reactivity of aldehydes and ketones;

• preparation methods and named reactions;

• physical properties and solubility;

• nucleophilic-addition mechanisms;

• reactions with HCN, ammonia derivatives and Grignard reagents;

• oxidation, reduction and qualitative tests;

• aldol, Cannizzaro and haloform reactions;

• preparation and acidity of carboxylic acids;

• esterification, reduction and decarboxylation;

• the Hell-Volhard-Zelinsky reaction; and

• multi-step conversions and NCERT answers.

Common mistakes to avoid

• Do not assume ketones react as readily as aldehydes in nucleophilic addition.

• Check for alpha hydrogen before predicting aldol or Cannizzaro behaviour.

• Do not say every ketone gives the iodoform test; the correct structural unit is required.

• Distinguish carbonyl reduction to alcohol from complete reduction to a methylene group.

• Remember the different conditions for Clemmensen and Wolff-Kishner reductions.

• Compare acid strength by stability of the conjugate base, not by counting oxygen atoms alone.

Best way to study Aldehydes, Ketones and Carboxylic Acids

Begin with carbonyl polarity and the general nucleophilic-addition mechanism. Build a table of reagents and products instead of memorising disconnected equations. Create a decision tree based on the presence of alpha hydrogen, aldehyde versus ketone and methyl-carbonyl structure. Study carboxylic acids as a second module centred on conjugate-base stability and derivative formation. Finish with mixed conversion questions.

Frequently asked questions

Why are aldehydes generally more reactive than ketones?

Aldehydes have less steric hindrance and less electron donation from alkyl groups, so their carbonyl carbon is usually more accessible and more electrophilic.

What is nucleophilic addition?

A nucleophile attacks the electron-deficient carbonyl carbon, the carbon-oxygen pi bond opens and a tetrahedral product forms after proton transfer.

How do Tollens' and Fehling's tests help identify aldehydes?

Aldehydes can reduce Tollens' reagent to metallic silver. Many aliphatic aldehydes also reduce Fehling's solution to red copper(I) oxide under the test conditions.

What is the difference between aldol and Cannizzaro reactions?

Aldol reaction requires a carbonyl compound with alpha hydrogen. Cannizzaro reaction is shown by suitable aldehydes lacking alpha hydrogen in concentrated base.

Which compounds give the iodoform test?

Methyl ketones and compounds that can be oxidised to the required methyl-carbonyl arrangement give the test under suitable conditions.

Why are carboxylic acids stronger acids than phenols?

The carboxylate ion delocalises negative charge over two equivalent oxygen atoms and is more strongly stabilised than the phenoxide ion.

What is esterification?

It is the acid-catalysed reversible reaction of a carboxylic acid with an alcohol to form an ester and water.

Use Teachoo's chapter resources to understand carbonyl mechanisms, recognise named-reaction conditions and solve conversions one functional-group change at a time.