Overview of Aldehydes, Ketones and Carboxylic Acids

The functional group >C=O present in aldehydes and ketones is called the carbonyl group.

Organic compounds containing the carbonyl group (>C=O) are collectively called carbonyl compounds.

Organic compounds in which the carbonyl group is attached to at least one hydrogen atom are called aldehydes (–CHO group).

Organic compounds in which the carbonyl group is attached to two alkyl or aryl groups are called ketones (>C=O).

Aldehydes in which the –CHO group is attached to an alkyl group or hydrogen are called aliphatic aldehydes.

Aldehydes in which the –CHO group is attached to an aryl group are called aromatic aldehydes.

Ketones in which both groups attached to the carbonyl carbon are alkyl groups.

Ketones in which one or both groups attached to the carbonyl carbon are aryl groups.

Ketones having identical groups attached to the carbonyl carbon.

Ketones having different groups attached to the carbonyl carbon.

1. Physical state:
   Lower aldehydes and ketones are colourless liquids (formaldehyde is a gas), while higher members are solids.
2. Odour:
   Lower aldehydes have an unpleasant odour which becomes pleasant with increase in molecular mass; ketones generally have pleasant odour and some are used in perfumery.
3. Polarity:
   Aldehydes and ketones are polar compounds due to the presence of the carbonyl (C=O) group and have high dipole moments.
4. Solubility:
   Lower members (up to C₄) are soluble in water due to hydrogen bonding with water; solubility decreases with increase in alkyl group size.
5. Boiling points:
   They have higher boiling points than hydrocarbons and ethers but lower than alcohols and acids, due to dipole–dipole interactions and absence of intermolecular hydrogen bonding.

1. General reactivity:
   Aldehydes and ketones are highly reactive due to the presence of the polar carbonyl (C=O) group.
2. Relative reactivity:
   Aldehydes are more reactive than ketones because aldehydes have at least one hydrogen atom attached to the carbonyl carbon.
3. Nucleophilic addition reactions:
   Both aldehydes and ketones undergo nucleophilic addition reactions as the carbonyl carbon is electron-deficient.
4. Addition of hydrogen cyanide:
   They react with HCN to form cyanohydrins, which are useful intermediates in organic synthesis.
5. Addition of Grignard reagents:
   Reaction with Grignard reagents (RMgX) followed by hydrolysis gives alcohols (1° from formaldehyde, 2° from other aldehydes, 3° from ketones).
6. Reduction reactions:
   Aldehydes and ketones are reduced to alcohols using reducing agents like LiAlH₄, NaBH₄, or hydrogen in presence of catalysts.
7. Oxidation reactions:
   Aldehydes are easily oxidised to carboxylic acids, whereas ketones resist oxidation under mild conditions.

The unstable addition product formed when aldehydes react with ammonia is called aldehyde ammonia.

A compound containing the functional group –C=NH formed by dehydration of aldehyde ammonia is called an aldimine (Schiff base).

The compound formed by the reaction of aldehydes or ketones with hydroxylamine having the group >C=NOH is called an oxime.

The compound formed by the reaction of aldehydes or ketones with hydrazine having the group >C=NNH₂ is called a hydrazone.

The derivative formed when aldehydes or ketones react with phenylhydrazine is called a phenylhydrazone.

The compound formed by the reaction of aldehydes or ketones with semicarbazide is called a semicarbazone.

The reduction of aldehydes and ketones to hydrocarbons using zinc amalgam and concentrated hydrochloric acid is called Clemmensen reduction.

The reduction of aldehydes and ketones to hydrocarbons using hydrazine and strong alkali at high temperature is called Wolff–Kishner reduction.

The test in which aldehydes reduce Tollens’ reagent to metallic silver producing a silver mirror is called Tollens’ test.

The test in which aliphatic aldehydes reduce Fehling’s solution to a red precipitate of cuprous oxide is called Fehling’s test.

The haloform reaction carried out using iodine that gives a yellow precipitate of iodoform is called the iodoform test.

The dehydration of aldol to form an α,β-unsaturated carbonyl compound is called crotonisation.

The disproportionation reaction of aldehydes without α-hydrogen atoms in concentrated alkali is called the Cannizzaro reaction.

The condensation reaction of aldehydes or ketones containing α-hydrogen atoms in dilute alkali to form β-hydroxy compounds is called aldol condensation.

The test in which the pink colour of Schiff’s reagent is restored by aldehydes but not by ketones is called Schiff’s test.

The chemical combination of a large number of similar or identical small molecules (monomers) to form a high molecular mass compound is called polymerisation.

The self-condensation reaction of aromatic aldehydes in the presence of alcoholic potassium cyanide to form α-hydroxy ketones is called benzoin condensation.

The reaction in which an aromatic aldehyde reacts with an acid anhydride in the presence of sodium salt of the corresponding acid to form an α,β-unsaturated carboxylic acid is called Perkin reaction.

The substitution reaction in which aromatic aldehydes or ketones react with halogens to form halo-substituted products is called halogenation.

The electrophilic substitution reaction in which aromatic aldehydes or ketones react with nitric acid in the presence of sulphuric acid to form nitro compounds is called nitration.

The electrophilic substitution reaction in which aromatic aldehydes or ketones react with concentrated sulphuric acid to introduce a sulphonic acid group is called sulphonation.

1. Physical nature: Methanal (formaldehyde) is a colourless gas with a strong, pungent smell and is highly soluble in water; its 40% aqueous solution is called formalin.
2. Chemical behaviour: Formaldehyde shows reactions typical of aldehydes but is more reactive and gives addition products with ammonia, bisulphite and hydroxylamine.
3. Reaction with ammonia: Formaldehyde reacts with ammonia to form hexamethylenetetramine (urotropine), which is used as a urinary antiseptic.
4. Polymerisation: Formaldehyde undergoes polymerisation to form paraformaldehyde, polyoxymethylene, and trioxane (metafomaldehyde) under different conditions.
5. Uses: Formaldehyde (as formalin) is used as a disinfectant and germicide, for preserving biological specimens, and in the manufacture of bakelite, resins, and polymers.

1. Physical nature: Acetaldehyde is a colourless, volatile liquid with a pungent smell and boils at 294 K; it is completely miscible with water, alcohol and ether.
2. Chemical behaviour: Acetaldehyde shows all the general chemical reactions characteristic of aldehydes.
3. Polymerisation (paraldehyde): When treated with a few drops of concentrated sulphuric acid at room temperature, acetaldehyde undergoes polymerisation to form a cyclic trimer called paraldehyde.
4. Polymerisation (metaldehyde): When acetaldehyde is treated with concentrated sulphuric acid at 273 K, it polymerises to form a cyclic tetramer called metaldehyde.
5. Uses: Acetaldehyde is used in the manufacture of acetic acid, ethyl acetate and n-butyl alcohol, and in silvering of mirrors; paraldehyde is used in medicine as a hypnotic.

1. Physical nature: Acetone is a colourless, volatile and highly inflammable liquid with a pleasant smell; it boils at 329 K and is completely miscible with water, alcohol and ether.
2. Chemical behaviour: Acetone shows all the general chemical reactions characteristic of ketones.
3. Condensation with dry HCl: In the presence of dry HCl gas, two molecules of acetone condense to form mesityl oxide, which further condenses with another acetone molecule to form phorone.
4. Condensation with conc. H₂SO₄: When acetone is distilled with concentrated sulphuric acid, it undergoes condensation to form mesitylene (1,3,5-trimethylbenzene).
5. Uses: Acetone is widely used as a solvent for cellulose acetate, cellulose nitrate, varnishes and lacquers, in the manufacture of chemicals like chloroform and iodoform, and for extraction of essential oils.

1. Physical nature: Benzaldehyde is a colourless oily liquid with an almond-like odour, boiling at 452 K, and is only slightly soluble in water but soluble in organic solvents.
2. General behaviour: Benzaldehyde shows most reactions of aldehydes, but its reactivity is lower than that of aliphatic aldehydes due to resonance with the benzene ring.
3. Nucleophilic addition: Benzaldehyde undergoes nucleophilic addition reactions with reagents like HCN, sodium bisulphite, and Grignard reagents to form addition products.
4. Condensation with ammonia derivatives: Benzaldehyde reacts with hydroxylamine, hydrazine, phenylhydrazine and 2,4-dinitrophenylhydrazine to form oxime, hydrazone, phenylhydrazone and 2,4-DNPH derivative respectively.
5. Reduction: On reduction with sodium amalgam and water or with zinc and dilute HCl, benzaldehyde is converted into benzyl alcohol.
6. Oxidation and tests: Benzaldehyde is oxidised to benzoic acid by strong oxidising agents and gives a silver mirror with Tollens’ reagent but does not reduce Fehling’s solution.
7. Special reactions: Benzaldehyde undergoes characteristic reactions such as Tischenko reaction, Cannizzaro reaction, Perkin reaction, benzoin condensation, and Claisen–Schmidt condensation.
8. Uses: Benzaldehyde is used as a flavouring agent, in the manufacture of dyes like malachite green, and as a starting material for compounds such as cinnamaldehyde, cinnamic acid and benzoyl chloride.

An organic compound containing the carboxyl group (–COOH) as its functional group is called a carboxylic acid.

The functional group formed by the combination of a carbonyl group (–C=O) and a hydroxyl group (–OH) is called the carboxyl group.

A carboxylic acid in which the –COOH group is attached to a hydrogen atom or an alkyl group is called an aliphatic carboxylic acid.

A carboxylic acid in which the –COOH group is attached directly to an aryl group (benzene ring) is called an aromatic carboxylic acid.

A carboxylic acid containing only one carboxyl group in its molecule is called a monocarboxylic acid.

A carboxylic acid containing two carboxyl groups in its molecule is called a dicarboxylic acid.

A carboxylic acid containing three carboxyl groups in its molecule is called a tricarboxylic acid.

Saturated aliphatic monocarboxylic acids having 12–18 carbon atoms and occurring naturally in fats and oils are called fatty acids.

The process of converting primary alcohols or aldehydes into carboxylic acids using oxidising agents is called oxidation.

Compounds containing the –C≡N (cyanide) group are called nitriles.

The preparation of carboxylic acids by hydrolysis of acid chlorides or acid anhydrides is called acyl hydrolysis.

The oxidation of alkyl side chains of aromatic hydrocarbons to –COOH group is called side-chain oxidation.

1. Physical state: Lower aliphatic carboxylic acids are colourless liquids with pungent odour, while higher members are waxy solids due to increase in molecular mass.
2. Solubility: Lower carboxylic acids are highly soluble in water due to hydrogen bonding, but solubility decreases as the alkyl chain length increases.
3. Hydrogen bonding: Carboxylic acids form strong intermolecular hydrogen bonds and exist as cyclic dimers, especially in non-polar solvents.
4. Boiling points: Carboxylic acids have very high boiling points compared to alcohols and hydrocarbons of similar molecular mass due to strong intermolecular hydrogen bonding.
5. Melting points: The melting points of aliphatic carboxylic acids show an irregular (odd–even) pattern, where acids with even number of carbon atoms have higher melting points than adjacent odd members.

1. Reactions due to H-atom of –COOH group:
   The reactions which occur due to the replacement of the acidic hydrogen atom of the carboxyl (–COOH) group are called reactions due to H-atom of the carboxyl group.
2. Acidic character of carboxylic acids:
   The ionisation of carboxylic acids in aqueous solution to give carboxylate ions (RCOO⁻) and hydronium ions (H₃O⁺) is called the acidic character of carboxylic acids.
3. Reactions involving –OH part of –COOH group:
   The reactions in which the –OH part of the carboxyl group is replaced by other atoms or groups such as –Cl, –OR, –NH₂ or –OCOR are called reactions involving the –OH part of the carboxyl group.
4. Reactions involving –COOH group as a whole:
   The reactions in which the entire –COOH group participates and undergoes change are called reactions involving the carboxyl group as a whole.
5. Decarboxylation:
   The removal of carbon dioxide (CO₂) from a carboxylic acid or its salt on heating is called decarboxylation.
6. Reduction of carboxylic acids:
   The conversion of the –COOH group into a –CH₂OH group or into a –CH₃ group by suitable reducing agents is called reduction of carboxylic acids.
7. Reactions involving alkyl group (HVZ reaction):
   The halogenation of the α-carbon atom of an aliphatic carboxylic acid in the presence of red phosphorus to form α-halo acids is called Hell–Volhard–Zelinsky reaction.
8. Reactions due to benzene ring in aromatic carboxylic acids:
   The electrophilic substitution reactions occurring in the benzene ring of aromatic carboxylic acids (mainly at meta position due to –COOH group) are called reactions due to the benzene ring present in aromatic carboxylic acids.

1. Physical nature:
   Ethanoic acid is a colourless liquid with a pungent vinegar-like odour and sour taste.
2. Glacial acetic acid:
   On cooling below 289.5 K, pure ethanoic acid forms ice-like crystals and is called glacial acetic acid.
3. Solubility:
   It is completely miscible with water, alcohol and ether, and dissolves in water with evolution of heat.
4. Chemical behaviour:
   Ethanoic acid is a typical aliphatic carboxylic acid and shows all characteristic reactions of carboxylic acids.
5. Important uses:
   It is used in manufacture of dyes, plastics, perfumes, acetates, esters and as table vinegar.
6. Acidic test:
   Its aqueous solution turns blue litmus red and gives effervescence with sodium bicarbonate due to CO₂ formation.
7. Special tests and reactions:
   It gives a wine-red colour with ferric chloride and forms ethyl acetate (fruity smell) when heated with ethanol and conc. H₂SO₄.