Aldehydes, Ketones, and Carboxylic Acids One Shot | CBSE Class 12th Chemistry Full Chapter Revision

Chapter Summary: Aldehydes, Ketones, and Carboxylic Acids

This lesson provides a broad revision of aldehydes, ketones, and carboxylic acids, covering their structures, naming, preparation, physical properties, characteristic reactions, tests, and uses. The instructor presents the revision as following NCERT and notes that the chapter is important for board exams.

1. Functional Groups and Their Importance

All three families contain a carbonyl group, C=O:

• Aldehydes: The carbonyl carbon is bonded to a hydrogen and a carbon group; general form R–CHO.
• Ketones: The carbonyl carbon is bonded to two carbon groups; general form R–CO–R′.
• Carboxylic acids: The carbonyl carbon is also bonded to a hydroxyl group; general form R–COOH.

Related compounds—including esters, amides, acid halides, and anhydrides—also contain carbonyl groups, though the chapter focuses on aldehydes, ketones, and carboxylic acids. These compounds are used in fragrances, flavorings, medicines, solvents, paints, plastics, and other industrial materials.

2. Naming

IUPAC Naming

• Choose the longest chain containing the principal functional group.
• Number the chain so the functional group receives the appropriate position.
• Name substituents alphabetically.
• Use:
  • -al for aldehydes
  • -one for ketones
  • -oic acid for carboxylic acids
• Use prefixes such as di- or tri- when multiple aldehyde or ketone groups are present.
• When an aldehyde group is attached directly to a ring, the lesson discusses using carbaldehyde in the name.

Common Names

Many common names are historical or derive from natural sources, properties, or related acids. Examples discussed include formaldehyde, acetaldehyde, benzaldehyde, acetone, acetophenone, and benzophenone. The instructor also reviews alpha, beta, and gamma positions, especially in common naming.

3. Carbonyl Structure and Reactivity

The carbonyl carbon is sp²-hybridized, with approximately trigonal-planar geometry and bond angles near 120°. The C=O bond is polar because oxygen is more electronegative than carbon:

• Oxygen has a partial negative charge.
• Carbon has a partial positive charge.

As a result, the carbonyl carbon behaves as an electrophile, while oxygen is electron-rich. This polarity explains why nucleophiles tend to attack the carbonyl carbon and underlies many reactions in the chapter.

4. Preparation of Aldehydes and Ketones

From Alcohols

• Primary alcohols can be oxidized to aldehydes under suitable conditions.
• Secondary alcohols can be oxidized to ketones.
• Heating alcohol vapours over copper or silver can remove hydrogen (dehydrogenation) and form aldehydes or ketones.
• Stronger oxidation can take primary alcohols beyond the aldehyde stage to carboxylic acids.

Other Preparation Methods

• Ozonolysis of alkenes: Ozone reacts with an alkene to form an ozonide. Reductive work-up with zinc and water breaks the ozonide into carbonyl compounds, which may be aldehydes, ketones, or both, depending on the alkene.
• Rosenmund reduction: Acyl chlorides are reduced to aldehydes using hydrogen and a poisoned palladium catalyst, described in the lesson as palladium over barium sulfate.
• Stephen reduction: Nitriles are reduced to an imine-related intermediate using stannous chloride and hydrochloric acid, then hydrolyzed to aldehydes. The lesson also mentions DIBAL-H as an alternative reagent for converting nitriles to aldehydes after work-up.
• Etard reaction: Oxidation of a methyl group on an aromatic ring, such as in toluene, can give an aromatic aldehyde using chromyl chloride or related chromium-based conditions.
• Gattermann–Koch reaction: Benzene or a substituted benzene reacts with carbon monoxide and hydrogen chloride in the presence of a Lewis-acid catalyst to introduce a formyl group.
• Other ketone preparations: Acyl chlorides can be treated with dialkyl cadmium reagents to form ketones. Nitriles can react with Grignard reagents and then undergo hydrolysis to form ketones. Friedel–Crafts acylation introduces an acyl group onto an aromatic ring to produce an aromatic ketone.

5. Physical Properties of Aldehydes and Ketones

• Small aldehydes may be gases; many low-molecular-mass aldehydes and ketones are volatile liquids. Larger compounds can be solids.
• Their boiling points are higher than those of comparable nonpolar hydrocarbons because carbonyl groups create dipole–dipole attractions.
• Their boiling points are generally below those of comparable alcohols, which can form intermolecular hydrogen bonds with each other.
• Lower aldehydes and ketones are relatively soluble in water. As the carbon chain grows, the nonpolar hydrocarbon portion becomes more influential and water solubility decreases.
• Many familiar fragrances and flavorings come from particular aldehydes and ketones.

6. Nucleophilic Addition to Aldehydes and Ketones

The central reaction pattern is:

1. A nucleophile attacks the positively polarized carbonyl carbon.
2. The C=O pi bond shifts onto oxygen, producing an alkoxide intermediate.
3. The oxygen is protonated, often during work-up, to form the final product.

Aldehydes generally undergo nucleophilic addition more readily than ketones because they have less steric hindrance and fewer electron-donating alkyl groups around the carbonyl carbon.

Reactions Covered

• HCN addition: Produces cyanohydrins.
• Grignard reagent addition: After acidic work-up, forms an alcohol. The class of alcohol depends on the carbonyl compound used: formaldehyde gives a primary alcohol, other aldehydes give secondary alcohols, and ketones give tertiary alcohols.
• Alcohol addition: Aldehydes can form hemiacetals and, with further alcohol addition under acid catalysis, acetals. The lesson notes that the analogous ketone products are ketals.
• Ammonia derivatives: Reactions with hydroxylamine, hydrazine, phenylhydrazine, and semicarbazide produce characteristic derivatives such as oximes, hydrazones, phenylhydrazones, and semicarbazones.

7. Reduction and Oxidation Reactions

Reduction to Hydrocarbons

• Clemmensen reduction: Reduces aldehydes or ketones to hydrocarbons using zinc amalgam under acidic conditions.
• Wolff–Kishner reduction: Accomplishes a similar conversion using hydrazine under strongly basic conditions, generally with heat.

Oxidation

Aldehydes are readily oxidized to carboxylic acids, including by relatively mild oxidizing agents. Ketones are more resistant to oxidation; strong conditions can break carbon–carbon bonds and produce smaller carboxylic acids.

Tests for Aldehydes

• Tollens’ test: Aldehydes reduce silver ions to metallic silver, producing a silver mirror or silver deposit. Ordinary ketones generally do not give this result.
• Fehling’s test: Aliphatic aldehydes can reduce the copper(II) solution to a reddish-brown precipitate of copper(I) oxide. Aromatic aldehydes generally do not give the same positive result.

8. Additional Reactions of Aldehydes and Ketones

• Sodium hydrogen sulfite addition: Forms addition compounds that can be useful for separating or purifying some carbonyl compounds.
• Acetal-related protection: The lesson discusses reaction with ethylene glycol to form cyclic derivatives under suitable conditions.
• Haloform and iodoform reactions: Methyl ketones give the haloform reaction; with iodine and base, the iodoform product is yellow. The iodoform test is characteristic of compounds with a suitable methyl-carbonyl arrangement and certain compounds that can be oxidized to one.

Reactions Involving Alpha Hydrogen

• Aldol condensation: Aldehydes or ketones with alpha hydrogens react in dilute base. The initial product is a beta-hydroxy carbonyl compound; further dehydration can produce an alpha,beta-unsaturated carbonyl compound. When two different carbonyl compounds react, crossed aldol condensation can produce a mixture of products.
• Cannizzaro reaction: Aldehydes without alpha hydrogens disproportionate in concentrated base. One molecule is reduced to an alcohol while another is oxidized to a carboxylate salt.
• Electrophilic substitution on aromatic rings: Aromatic aldehyde and ketone substituents are described as deactivating and meta-directing in electrophilic substitution reactions.

9. Carboxylic Acids

Structure, Naming, and Classification

Carboxylic acids contain –COOH, consisting of a carbonyl and hydroxyl group attached to the same carbon. They can be classified according to how many carboxyl groups they contain, such as mono-, di-, or polycarboxylic acids. The instructor reviews common names including formic, acetic, butyric, and benzoic acid.

The carboxylate ion is resonance-stabilized: its negative charge is shared between the two oxygen atoms. This stabilization helps explain why carboxylic acids are more acidic than alcohols and phenols, though they remain weak acids compared with common mineral acids.

Preparation Methods

• Oxidation of primary alcohols or aldehydes using reagents such as permanganate, dichromate, or Jones reagent.
• Oxidation of alkylbenzenes to aromatic carboxylic acids.
• Hydrolysis of nitriles or amides.
• Reaction of a Grignard reagent with carbon dioxide, followed by acidification.
• Hydrolysis of acyl halides, anhydrides, or esters.

Physical Properties

• Lower aliphatic carboxylic acids are typically colorless liquids with strong or unpleasant odors; higher members become waxy solids.
• Carboxylic acids often have higher boiling points than comparable aldehydes and ketones because they form hydrogen-bonded dimers.
• Their water solubility generally decreases as the carbon chain gets longer.

Chemical Reactions

• Acidic behavior: React with active metals to release hydrogen and form salts; react with bases to form salts and water.
• Carbonates and bicarbonates: Carboxylic acids release carbon dioxide, causing effervescence. The lesson presents this as a way to distinguish them from phenols.
• Anhydride formation: Dehydration of two carboxylic acid molecules can form an acid anhydride.
• Esterification: A carboxylic acid reacts reversibly with an alcohol, usually with acid catalysis, to form an ester and water. The lesson outlines protonation, nucleophilic attack by the alcohol, proton transfer, loss of water, and deprotonation.
• Amide formation: Reaction with ammonia first gives an ammonium salt; heating can lead to an amide.
• Reduction: Strong reducing agents such as lithium aluminium hydride can reduce carboxylic acids to primary alcohols; sodium borohydride does not normally do this.
• Decarboxylation: Heating a carboxylate salt with soda lime removes carbon dioxide and gives a hydrocarbon with one fewer carbon.
• Hell–Volhard–Zelinsky (HVZ) reaction: Halogenation replaces an alpha hydrogen of a carboxylic acid with a halogen under the specified conditions.
• Aromatic substitution: The carboxyl group is deactivating and meta-directing. The instructor also cautions that Friedel–Crafts reactions are generally not suitable in this context.

10. Uses of Carboxylic Acids

The lesson mentions uses in food preservation and flavoring, textile and leather industries, dyeing, electroplating, and polymer production. Examples include acetic acid in vinegar, sodium benzoate as a preservative, adipic acid in nylon manufacture, and higher fatty acids in soaps and detergents.

Speakers and Sources Featured

• Tapur Umar — Chemistry educator and the sole speaker in the subtitles; the name is auto-generated and may be imperfectly transcribed.
• Next Toppers — The channel introducing the lesson.
• NCERT — Referenced by the instructor as the basis for the chapter coverage.