Class 12 Chemistry · Chapter 8 NotesAldehydes, Ketones and Carboxylic Acids
Get complete Class 12 Chemistry notes on Aldehydes, Ketones and Carboxylic Acids. Learn nomenclature, preparation, properties, reactions, and uses with clear…
Aldehydes, ketones and carboxylic acids are organic compounds that contain the carbonyl group (>C=O), one of the most important functional groups in organic chemistry. In aldehydes and ketones, the carbonyl carbon is bonded to at least one hydrogen or two carbon atoms, while carboxylic acids contain a carboxyl group (–COOH). These compounds are widespread in nature and play vital roles in biochemical processes. They are used in the manufacture of fabrics, flavourings, plastics, drugs, solvents, perfumes and many other products. This chapter covers the nomenclature, structure, preparation, physical properties and chemical reactions of aldehydes, ketones and carboxylic acids. You will learn how to name them using common and IUPAC systems, how they are prepared from alcohols, hydrocarbons and other compounds, and how they undergo nucleophilic addition, oxidation, reduction, aldol condensation and other reactions. The chapter also explains the acidity of carboxylic acids and the effect of substituents on acid strength.
What you'll learn
1Write common and IUPAC names of aldehydes, ketones and carboxylic acids
2Describe the structure of the carbonyl and carboxyl groups
3Explain important methods of preparation of aldehydes, ketones and carboxylic acids
4Correlate physical properties with molecular structure
5Explain the mechanism of nucleophilic addition reactions of aldehydes and ketones
6Understand factors affecting the acidity of carboxylic acids
7Describe reactions of carboxylic acids involving cleavage of O–H, C–OH and –COOH bonds
8List uses of aldehydes, ketones and carboxylic acids
Chapter at a glance
01Aldehydes and Ketones: Structure and Nomenclature
02Preparation of Aldehydes and Ketones
03Physical and Chemical Properties of Aldehydes and Ketones
04Carboxylic Acids: Structure and Nomenclature
05Preparation and Properties of Carboxylic Acids
06Derivatives of Carboxylic Acids
Detailed chapter notes
01
Nomenclature and Structure of Carbonyl Group
Aldehydes and ketones are the simplest carbonyl compounds. In aldehydes, the carbonyl carbon is bonded to at least one hydrogen; in ketones, it is bonded to two carbon atoms. Common names of aldehydes are derived from the corresponding carboxylic acids by replacing –ic acid with –aldehyde. Common names of ketones are derived by naming the two alkyl or aryl groups attached to the carbonyl group. IUPAC names are derived from alkanes by replacing –e with –al for aldehydes and –one for ketones. The carbonyl carbon is sp2-hybridised and forms three sigma bonds and one pi bond with oxygen. The carbonyl group is polar due to the higher electronegativity of oxygen, making the carbonyl carbon electrophilic and the oxygen nucleophilic.
General formulaRCHO for aldehydes, RCOR' for ketones
Carbonyl carbonsp2 hybridised, bond angles ~120°
Polaritycarbonyl carbon is electrophilic, oxygen is nucleophilic
02
Preparation of Aldehydes and Ketones
Aldehydes and ketones can be prepared by several methods. Oxidation of primary alcohols gives aldehydes, while secondary alcohols give ketones. Dehydrogenation of alcohols over heated metal catalysts (Ag or Cu) also yields aldehydes and ketones. Ozonolysis of alkenes followed by hydrolysis gives aldehydes and/or ketones. Hydration of alkynes in the presence of H2SO4 and HgSO4 gives acetaldehyde from ethyne and ketones from other alkynes. Aldehydes can also be prepared by Rosenmund reduction of acyl chlorides, Stephen reaction of nitriles, and from aromatic hydrocarbons by Etard reaction, side-chain chlorination, or Gatterman-Koch reaction. Ketones are prepared from acyl chlorides with dialkylcadmium, from nitriles with Grignard reagents, and by Friedel-Crafts acylation of aromatic hydrocarbons.
Gatterman-Koch reactionbenzene + CO + HCl → benzaldehyde
03
Physical Properties of Aldehydes and Ketones
Methanal is a gas at room temperature, ethanal is a volatile liquid, and other aldehydes and ketones are liquids or solids. Their boiling points are higher than hydrocarbons and ethers of comparable molecular masses due to dipole-dipole interactions, but lower than alcohols because they cannot form intermolecular hydrogen bonds. Lower aldehydes and ketones (methanal, ethanal, propanone) are miscible with water in all proportions due to hydrogen bonding with water. Solubility decreases as the alkyl chain length increases. They are soluble in organic solvents. Lower aldehydes have sharp pungent odours; higher members have pleasant fragrances.
Boiling point orderalcohol > aldehyde/ketone > hydrocarbon/ether
Solubilitylower members miscible with water; decreases with chain length
Odourpungent for lower members, fragrant for higher members
04
Chemical Reactions of Aldehydes and Ketones
Aldehydes and ketones undergo nucleophilic addition reactions due to the polar carbonyl group. Nucleophiles such as HCN, NaHSO3, Grignard reagents, alcohols, and ammonia derivatives add to the carbonyl carbon. Aldehydes are generally more reactive than ketones due to less steric hindrance and greater electrophilicity. Reduction with NaBH4 or LiAlH4 gives alcohols; Clemmensen or Wolff-Kishner reduction gives hydrocarbons. Aldehydes are easily oxidised to carboxylic acids by mild oxidising agents like Tollens' reagent and Fehling's reagent, while ketones require vigorous conditions. Aldehydes and ketones with α-hydrogen undergo aldol condensation; those without α-hydrogen undergo Cannizzaro reaction. Aromatic aldehydes and ketones undergo electrophilic substitution at the ring.
Tollens' testsilver mirror with aldehydes
Fehling's testreddish brown precipitate with aldehydes (except aromatic)
Iodoform testmethyl ketones give yellow precipitate with I2/NaOH
05
Nomenclature and Structure of Carboxylic Acids
Carboxylic acids contain the carboxyl group (–COOH). Common names are derived from Latin or Greek names of natural sources (e.g., formic acid from ants, acetic acid from vinegar). IUPAC names are obtained by replacing –e of the corresponding alkane with –oic acid. For dicarboxylic acids, the suffix –dioic acid is used. The carboxyl carbon is sp2-hybridised and the bonds are in one plane. The carboxyl carbon is less electrophilic than the carbonyl carbon due to resonance. Carboxylic acids may be aliphatic (RCOOH) or aromatic (ArCOOH). Higher fatty acids (C12–C18) occur in natural fats as esters of glycerol.
General formulaRCOOH (aliphatic) or ArCOOH (aromatic)
IUPACreplace –e with –oic acid; for diacids, –dioic acid
Carboxyl groupresonance stabilised, less electrophilic than carbonyl
06
Preparation and Properties of Carboxylic Acids
Carboxylic acids are prepared by oxidation of primary alcohols and aldehydes, oxidation of alkylbenzenes, hydrolysis of nitriles and amides, reaction of Grignard reagents with CO2, hydrolysis of acyl halides and anhydrides, and hydrolysis of esters. Aliphatic carboxylic acids up to nine carbons are colourless liquids with unpleasant odours; higher acids are waxy solids. They have higher boiling points than alcohols due to more extensive hydrogen bonding and exist as dimers in vapour phase. Lower members are miscible with water; solubility decreases with increasing carbon chain. Carboxylic acids are more acidic than alcohols and phenols because the carboxylate ion is resonance stabilised. Electron-withdrawing groups increase acidity; electron-donating groups decrease it.
Carboxylic acids undergo reactions involving cleavage of O–H, C–OH, and –COOH bonds. They react with metals and alkalies to form salts, and with carbonates and hydrogencarbonates to evolve CO2. They form anhydrides on heating with P2O5, esters with alcohols in the presence of acid catalyst, acyl chlorides with PCl5, PCl3 or SOCl2, and amides with ammonia. Reduction with LiAlH4 or diborane gives primary alcohols. Decarboxylation with soda lime gives hydrocarbons. Hell-Volhard-Zelinsky reaction gives α-halocarboxylic acids. Aromatic carboxylic acids undergo electrophilic substitution at the ring, with the carboxyl group acting as a deactivating and meta-directing group.
HVZ reactionα-halogenation with Cl2 or Br2 in presence of red phosphorus
08
Uses of Aldehydes, Ketones and Carboxylic Acids
Aldehydes and ketones are used as solvents, starting materials, and reagents in the chemical industry. Formaldehyde (40% solution called formalin) is used to preserve biological specimens and to prepare bakelite, urea-formaldehyde glues, and other polymers. Acetaldehyde is used to manufacture acetic acid, ethyl acetate, vinyl acetate, polymers, and drugs. Benzaldehyde is used in perfumery and dye industries. Acetone and ethyl methyl ketone are common industrial solvents. Carboxylic acids also have many uses: methanoic acid in rubber, textile, dyeing, leather, and electroplating industries; ethanoic acid as a solvent and as vinegar in food; hexanedioic acid in the manufacture of nylon-6,6; esters of benzoic acid in perfumery; sodium benzoate as a food preservative; and higher fatty acids for soaps and detergents.
Formalin40% aqueous formaldehyde, used as preservative
Bakelitephenol-formaldehyde resin
Nylon-6,6from hexanedioic acid and hexamethylenediamine
Want the complete chapter resources?Topic notes, quizzes and flashcards for Aldehydes, Ketones and Carboxylic Acids.
What is the difference between aldehydes and ketones?
In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom (general formula RCHO). In ketones, the carbonyl carbon is bonded to two carbon atoms (general formula RCOR'). Aldehydes are more reactive than ketones in nucleophilic addition reactions due to less steric hindrance and greater electrophilicity of the carbonyl carbon.
Why are carboxylic acids more acidic than alcohols and phenols?
Carboxylic acids are more acidic because the carboxylate ion (conjugate base) is stabilised by two equivalent resonance structures where the negative charge is delocalised over two electronegative oxygen atoms. In phenoxide ion, the negative charge is delocalised over less electronegative carbon atoms, and in alkoxide ion, there is no resonance stabilisation.
What is the Tollens' test?
Tollens' test is used to distinguish aldehydes from ketones. When an aldehyde is warmed with freshly prepared ammoniacal silver nitrate solution (Tollens' reagent), a bright silver mirror is produced due to the formation of silver metal. Aldehydes are oxidised to carboxylate anions. Ketones do not give this test.
What is aldol condensation?
Aldol condensation is a reaction of aldehydes and ketones having at least one α-hydrogen in the presence of dilute alkali. They form β-hydroxy aldehydes (aldols) or β-hydroxy ketones (ketols), which readily lose water to give α,β-unsaturated carbonyl compounds. For example, two molecules of ethanal form 3-hydroxybutanal, which dehydrates to but-2-enal.
How is benzoic acid prepared from toluene?
Benzoic acid can be prepared by vigorous oxidation of toluene with chromic acid or acidic or alkaline potassium permanganate. The entire side chain is oxidised to the carboxyl group. Alternatively, side-chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde, which can be further oxidised to benzoic acid.
What is the Cannizzaro reaction?
Aldehydes that do not have an α-hydrogen atom undergo self oxidation and reduction (disproportionation) on heating with concentrated alkali. One molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt. For example, formaldehyde gives methanol and sodium formate.