Organic Chemistry  - Aldehydes, Ketones and Carboxylic Acids

SECTION 1 - Structure and Nomenclature

Q1. What are aldehydes and ketones? What is the carbonyl group ?

Both aldehydes and ketones contain the carbonyl group (C=O), which is the characteristic functional group of this chapter. In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom - the functional group is −CHO (formyl group). In ketones, the carbonyl carbon is bonded to two carbon atoms (two alkyl or aryl groups) - the functional group is −CO−. The carbonyl carbon is sp² hybridised and the C=O bond is shorter and stronger than a C−O single bond. The carbonyl group is planar with bond angles of approximately 120°.

Q2. How are aldehydes and ketones named by the IUPAC system ?

For aldehydes, the longest carbon chain containing the −CHO group is chosen as the parent chain. The terminal "e" of the corresponding alkane is replaced by "al." The carbonyl carbon is always C-1. For example, CH₃CHO is ethanal and CH₃CH₂CHO is propanal. For ketones, the longest chain containing the C=O group is chosen and the suffix "e" of the alkane is replaced by "one." The position of the carbonyl carbon is given the lowest possible number. For example, CH₃COCH₃ is propan-2-one and CH₃COCH₂CH₃ is butan-2-one.

Q3. What are the common names of important aldehydes and ketones ?

HCHO (methanal) is commonly called formaldehyde. CH₃CHO (ethanal) is called acetaldehyde. C₆H₅CHO (benzaldehyde) is called benzaldehyde or oil of bitter almonds. CH₃COCH₃ (propan-2-one) is called acetone. CH₃COC₆H₅ (methyl phenyl ketone) is called acetophenone. C₆H₅COC₆H₅ is called benzophenone.

SECTION 2 - Preparation of Aldehydes and Ketones

Q4. How are aldehydes prepared from primary alcohols ?

Primary alcohols are oxidised to aldehydes using mild oxidising agents. The most important reagent is PCC (pyridinium chlorochromate) in dichloromethane -it oxidises primary alcohols to aldehydes without further oxidation to carboxylic acid. Strong oxidising agents like KMnO₄ or K₂Cr₂O₇ oxidise primary alcohols all the way to carboxylic acids and must be avoided when the aldehyde is the desired product.

Q5. How are ketones prepared from secondary alcohols ?

Secondary alcohols are oxidised to ketones using oxidising agents like K₂Cr₂O₇/H₂SO₄, KMnO₄, or CrO₃. Since the ketone carbon has no hydrogen available for further oxidation under normal conditions, ketones are not easily oxidised further. For example, propan-2-ol on oxidation gives propan-2-one (acetone).

Q6. How are aldehydes and ketones prepared from alkynes ?

Alkynes undergo acid-catalysed hydration (Markovnikov addition of water) in the presence of H₂SO₄ and HgSO₄. Terminal alkynes (R−C≡CH) give methyl ketones because water adds according to Markovnikov's rule - except for ethyne (acetylene), which gives acetaldehyde (ethanal) because the only possible product is CH₃CHO. Internal alkynes give a mixture of ketones.

Q7. What is the Rosenmund reaction ?

In the Rosenmund reaction, an acyl chloride (RCOCl) is reduced by hydrogen gas over a palladium-on-barium sulphate catalyst (Pd/BaSO₄) -barium sulphate and a small amount of sulphur or quinoline is added to partially poison the catalyst and prevent over-reduction to alcohol. The product is always an aldehyde (RCHO). This is a specific method for preparing aldehydes from acyl chlorides. Note - formaldehyde cannot be made this way because formyl chloride (HCOCl) is unstable.

Q8. What is the Stephen reaction ?

In the Stephen reaction, a nitrile (RCN) is reduced with stannous chloride (SnCl₂) and hydrochloric acid to give an imine salt (aldimine), which on hydrolysis with water gives an aldehyde (RCHO). This reaction always gives an aldehyde with one more carbon than the corresponding starting material. It is useful for preparing aromatic aldehydes from aromatic nitriles.

Q9. What is the Etard reaction and Gatterman-Koch reaction ?

In the Etard reaction, toluene or a substituted toluene is treated with chromyl chloride (CrO₂Cl₂) in carbon disulphide to give a chromium complex, which on hydrolysis gives benzaldehyde or a substituted benzaldehyde. In the Gatterman-Koch reaction, benzene or its derivatives are treated with a mixture of carbon monoxide and hydrogen chloride (CO + HCl) in the presence of anhydrous AlCl₃ and cuprous chloride (CuCl) under pressure. A formyl group (−CHO) is directly introduced into the ring, giving benzaldehyde.

Q10. How are ketones prepared by Friedel-Crafts acylation ?

In Friedel-Crafts acylation, benzene or an arene is treated with an acyl chloride (RCOCl) or an acid anhydride ((RCO)₂O) in the presence of anhydrous aluminium chloride (AlCl₃) as Lewis acid catalyst. An acylium ion (RCO⁺) is generated as the electrophile, which attacks the benzene ring to give an aryl ketone. For example, benzene + acetyl chloride with AlCl₃ gives acetophenone.

SECTION 3 - Physical Properties

Q11. Why do aldehydes and ketones have higher boiling points than alkanes but lower than alcohols ?

Aldehydes and ketones are polar molecules because of the C=O group, which creates a permanent dipole moment. They have stronger dipole-dipole interactions than non-polar alkanes, so their boiling points are higher than alkanes of comparable molecular mass. However, they cannot form intermolecular hydrogen bonds with each other (no O−H or N−H bond), unlike alcohols. Alcohols have strong hydrogen bonding and so have higher boiling points than aldehydes or ketones of similar molecular mass.

Q12. Why are lower aldehydes and ketones soluble in water ?

Lower aldehydes and ketones (up to four carbons) are miscible with water because the oxygen of the carbonyl group forms hydrogen bonds with water molecules. As the carbon chain increases, the hydrophobic part of the molecule dominates and solubility decreases. All aldehydes and ketones are soluble in organic solvents.

SECTION 4 - Chemical Reactions of Aldehydes and Ketones

Q13. What is nucleophilic addition? Why do aldehydes and ketones undergo it?

The carbonyl carbon in aldehydes and ketones carries a partial positive charge (δ+) because oxygen is more electronegative and pulls electron density towards itself. This makes the carbonyl carbon electrophilic and susceptible to attack by nucleophiles (electron-rich species). The nucleophile attacks the carbonyl carbon, the pi bond breaks, and oxygen picks up the negative charge. This is called nucleophilic addition and it is the most characteristic reaction of aldehydes and ketones.

Q14. Why are aldehydes more reactive than ketones towards nucleophilic addition ?

Two factors explain this. First, steric effect - aldehydes have only one alkyl group on the carbonyl carbon while ketones have two. Two bulky alkyl groups in ketones create more steric hindrance, making it harder for a nucleophile to approach the carbonyl carbon. Second, electronic effect - alkyl groups have a positive inductive effect (+I) and push electron density onto the carbonyl carbon, reducing its electrophilicity. Ketones have two alkyl groups doing this, so their carbonyl carbon is less electron-deficient and less reactive towards nucleophiles than in aldehydes.

Q15. What is cyanohydrin formation ?

Aldehydes and ketones react with hydrogen cyanide (HCN) in the presence of a base catalyst (NaCN) to give cyanohydrins - compounds with an OH and a CN group on the same carbon. The reaction is: RCHO + HCN → RCH(OH)CN. Cyanohydrins are useful synthetic intermediates - the CN group can be hydrolysed to −COOH or reduced to −CH₂NH₂. Aldehydes react faster than ketones, and sterically hindered ketones react very slowly or not at all.

Q16. What is the reaction of aldehydes and ketones with sodium hydrogen sulphite ?

Aldehydes and methyl ketones (and cyclic ketones) react with saturated sodium hydrogen sulphite (NaHSO₃) solution to form white crystalline addition products called bisulphite addition compounds. The reaction is: RCHO + NaHSO₃ → RCH(OH)SO₃Na. This reaction is used to detect and purify aldehydes and methyl ketones because the bisulphite compound can be decomposed with dilute acid or alkali to regenerate the pure carbonyl compound. Ketones with bulky groups do not give this reaction due to steric hindrance.

Q17. What are the reactions of aldehydes and ketones with ammonia and its derivatives? What is the general product ?

Aldehydes and ketones react with ammonia (NH₃) and various nitrogen-containing compounds through nucleophilic addition followed by elimination of water. The general product contains a C=N bond and is called a Schiff's base or an imine. Important reactions are with hydroxylamine (NH₂OH) to give oximes, with hydrazine (NH₂NH₂) to give hydrazones, with 2,4-dinitrophenylhydrazine (2,4-DNP) to give 2,4-dinitrophenylhydrazones (orange/yellow precipitate -

used as a test for carbonyl compounds), and with semicarbazide (NH₂NHCONH₂) to give semicarbazones. These are called condensation reactions.

Q18. What is the 2,4-DNP test? What does it indicate ?

When a carbonyl compound (aldehyde or ketone) is treated with 2,4-dinitrophenylhydrazine (2,4-DNP) reagent in acidic medium, a yellow or orange precipitate of the corresponding 2,4-dinitrophenylhydrazone is formed. This test is positive for both aldehydes and ketones and confirms the presence of a carbonyl (C=O) group. Each compound gives a characteristic melting point, so this test can also be used to identify a specific aldehyde or ketone.

Q19. What is the aldol condensation ?

When an aldehyde or ketone with at least one alpha hydrogen (hydrogen on the carbon adjacent to the carbonyl group) is treated with dilute alkali (NaOH), the product is a beta-hydroxy aldehyde (called aldol) or beta-hydroxy ketone (called ketol). This is the aldol condensation. In the first step, the base removes an alpha hydrogen to form a carbanion (enolate ion), which acts as a nucleophile and attacks the carbonyl carbon of another molecule. On gentle heating, the aldol product undergoes dehydration to give an alpha-beta unsaturated carbonyl compound. For example, ethanal with dilute NaOH gives 3-hydroxybutanal (aldol) and on heating gives but-2-enal (crotonaldehyde).

Q20. What is cross aldol condensation ?

When aldol condensation is carried out between two different carbonyl compounds (both having alpha hydrogens), four products are possible - two self-condensation products and two cross-condensation products. Cross aldol condensation gives a predictable product only when one component has no alpha hydrogen and thus cannot self-condense. For example, benzaldehyde (no alpha-H) + acetaldehyde with NaOH gives cinnamaldehyde (C₆H₅CH=CHCHO) as the main product.

Q21. What is the Cannizzaro reaction? Which aldehydes undergo it?

The Cannizzaro reaction is a disproportionation reaction - one molecule of aldehyde is oxidised to a carboxylic acid (or its salt) while another molecule of the same aldehyde is reduced to an alcohol. It is undergone by aldehydes that have no alpha hydrogen (no alpha-H) when treated with concentrated alkali (NaOH). Examples are formaldehyde (HCHO), benzaldehyde (C₆H₅CHO), and trimethylacetaldehyde. For example, 2 HCHO + NaOH → HCOONa + CH₃OH (sodium formate + methanol).

Q22. What is the iodoform reaction? Which compounds give it ?

The iodoform test involves treating a compound with iodine and sodium hydroxide (I₂/NaOH) or with sodium hypoiodite (NaOI). A yellow precipitate of iodoform (CHI₃) with a characteristic antiseptic smell is formed. Compounds that give the iodoform test are acetaldehyde (CH₃CHO), all methyl ketones (CH₃COR), ethanol (CH₃CH₂OH - because it is oxidised to acetaldehyde), and secondary alcohols of the type CH₃CH(OH)R. The test confirms the presence of the CH₃CO− group or the CH₃CH(OH)− group. Acetone gives a positive iodoform test; other ketones do not.

Q23. How do aldehydes get oxidised? How are ketones different ?

Aldehydes are easily oxidised to carboxylic acids by mild oxidising agents such as Tollens' reagent (ammoniacal silver nitrate), Fehling's solution (alkaline copper sulphate with sodium potassium tartrate), and Benedict's solution. Ketones are resistant to mild oxidation because there is no hydrogen on the carbonyl carbon to be removed. Ketones can be oxidised only under vigorous conditions with strong oxidising agents, which break the carbon chain and give a mixture of carboxylic acids.

Q24. What are Tollens' test and Fehling's test? How do they distinguish aldehydes from ketones ?

Tollens' test -the compound is warmed with Tollens' reagent (silver ammonia complex, [Ag(NH₃)₂]⁺). If an aldehyde is present, the silver ions are reduced to metallic silver, which deposits as a shiny silver mirror on the inner wall of the test tube. Ketones do not give this reaction. Fehling's test - the compound is heated with Fehling's solution (blue). Aldehydes reduce the Cu²⁺ ions to Cu⁺, giving a brick-red precipitate of cuprous oxide (Cu₂O). Aromatic aldehydes like benzaldehyde do not give Fehling's test (they do give Tollens' test). Ketones give neither test.

Q25. What is the reduction of aldehydes and ketones ?

Aldehydes and ketones are reduced to alcohols by various reducing agents. LiAlH₄ (lithium aluminium hydride) or NaBH₄ (sodium borohydride) reduce aldehydes to primary alcohols and ketones to secondary alcohols. Catalytic hydrogenation (H₂/Ni or Pd) also reduces carbonyl compounds to alcohols. Clemmensen reduction (Zn-Hg/HCl) and Wolff-Kishner reduction (NH₂NH₂/KOH/ethylene glycol) reduce the carbonyl group all the way to a CH₂ group (complete deoxygenation), giving an alkane.

SECTION 5 - Carboxylic Acids

Q26. What are carboxylic acids? What is the carboxyl group ?

Carboxylic acids are organic compounds containing the carboxyl group (−COOH), which consists of a carbonyl group (C=O) and a hydroxyl group (−OH) on the same carbon. The general formula for aliphatic carboxylic acids is CₙH₂ₙO₂. Examples are formic acid (HCOOH), acetic acid (CH₃COOH), and benzoic acid (C₆H₅COOH). They are the most acidic among organic compounds and are used in foods, pharmaceuticals, and industrial chemistry.

Q27. How are carboxylic acids named ?

In the IUPAC system, the longest chain containing the −COOH group is chosen and the suffix "e" of the parent alkane is replaced by "oic acid." The carboxyl carbon is always C-1. For example, HCOOH is methanoic acid, CH₃COOH is ethanoic acid, and CH₃CH₂COOH is propanoic acid. Common names are still widely used - HCOOH is formic acid, CH₃COOH is acetic acid, C₆H₅COOH is benzoic acid.

Q28. How are carboxylic acids prepared ?

There are several important methods. Oxidation of primary alcohols and aldehydes with strong oxidising agents like K₂Cr₂O₇/H₂SO₄ or KMnO₄ gives carboxylic acids. Oxidation of alkylbenzenes with acidic or alkaline KMnO₄ gives benzoic acid regardless of the length of the side chain. Hydrolysis of nitriles (RCN) with dilute acid or base gives carboxylic acids with one more carbon. Hydrolysis of esters with dilute acid or alkali gives a carboxylic acid and an alcohol. Grignard reagents react with CO₂ and then with acid to give carboxylic acids - RMgX + CO₂ → RCOOMgX → RCOOH (after hydrolysis).

Q29. What are the physical properties of carboxylic acids? Why do they have unusually high boiling points ?

Carboxylic acids are liquids or solids at room temperature. They have very high boiling points - even higher than alcohols of comparable molecular mass- because they form stable dimers through two intermolecular hydrogen bonds (one on each side of the dimer). This strong hydrogen bonding requires much more energy to break. Lower carboxylic acids (up to four carbons) are completely miscible with water because of hydrogen bonding with water. As chain length increases, solubility decreases. Lower carboxylic acids have a sharp, pungent smell.

Q30. Why are carboxylic acids more acidic than alcohols and phenols ?

Carboxylic acids are more acidic (pKa ≈ 4–5) than alcohols (pKa ≈ 16) and phenols (pKa ≈ 10) because the carboxylate ion (RCOO⁻) formed after losing a proton is highly stabilised by resonance. In the carboxylate ion, the negative charge is equally delocalised over both oxygen atoms - both C−O bonds become equivalent and the negative charge rests on the electronegative oxygen atoms. In the phenoxide ion, the negative charge is delocalised partly onto less electronegative carbon atoms of the ring, making it less stable. In alkoxide ions (RO⁻), there is no resonance stabilisation at all, making alcohols the weakest acids of the three.

Q31. How does the structure of a carboxylic acid affect its acid strength ?

Electron-withdrawing groups (like −NO₂, −Cl, −F, −CN) near the carboxyl group increase acid strength by stabilising the carboxylate ion through inductive effect - they pull electron density away from the negatively charged oxygen, spreading the charge further and making the anion more stable. The closer the withdrawing group to the −COOH group, the stronger the acid. For example, chloroacetic acid (ClCH₂COOH) is a stronger acid than acetic acid (CH₃COOH). Electron-donating groups (like alkyl groups) decrease acid strength by destabilising the carboxylate ion. This is why formic acid (HCOOH) is a stronger acid than acetic acid (CH₃COOH) - the methyl group in acetic acid donates electrons and destabilises the carboxylate ion.

Q32. What are the important chemical reactions of carboxylic acids ?

The main reactions are as follows. Acidity - they react with metals (Na, Zn), bases (NaOH, NaHCO₃), and carbonates to form salts. They react with NaHCO₃ to give CO₂ gas -this distinguishes them from phenols (which do not react with NaHCO₃). Esterification - reaction with alcohols in the presence of a mineral acid catalyst to form esters and water (Fischer esterification). This is a reversible reaction. Acyl chloride formation - reaction with PCl₅, PCl₃, or SOCl₂ to replace the −OH of −COOH with −Cl, giving acyl chloride (RCOCl). Anhydride formation -two carboxylic acid molecules lose water on strong heating to give a carboxylic anhydride. Decarboxylation - heating the sodium or calcium salt of a carboxylic acid with soda lime (NaOH + CaO) removes CO₂ and gives an alkane with one less carbon -this is the Kolbe electrolysis precursor reaction.

Q33. What is esterification? Why is it a reversible reaction?

Esterification is the reaction of a carboxylic acid with an alcohol in the presence of a mineral acid catalyst (H₂SO₄) to give an ester and water: RCOOH + R'OH ⇌ RCOOR' + H₂O. It is reversible because the ester can be hydrolysed back to the acid and alcohol in the presence of water and acid or base. To shift the equilibrium towards ester formation, either the water is removed (using a dehydrating agent) or one reactant is taken in excess. The reverse reaction - hydrolysis of ester in alkali - is called saponification (complete and irreversible).

Q34. What is Hell-Volhard-Zelinsky (HVZ) reaction ?

The HVZ reaction involves the bromination of a carboxylic acid at the alpha carbon. When a carboxylic acid with an alpha hydrogen is treated with bromine in the presence of red phosphorus (which generates PBr₃ in situ), bromination occurs specifically at the alpha position to give an alpha-bromo carboxylic acid. For example, acetic acid gives bromoacetic acid (CH₂BrCOOH). The reaction proceeds through an acyl bromide intermediate. The product is a useful synthetic intermediate.

Q35. How can you distinguish between an aldehyde and a ketone using chemical tests ?

Three tests distinguish them. Tollens' test - aldehydes give a silver mirror; ketones do not.

Fehling's test - aliphatic aldehydes give a brick-red precipitate of Cu₂O; ketones and aromatic aldehydes do not. 

Schiff's test - aldehydes restore the pink colour of Schiff's reagent (decolourised fuchsin); ketones do not. Additionally, the iodoform test identifies acetaldehyde and methyl ketones specifically. In all other cases, both aldehydes and ketones give the 2,4-DNP test (positive for any carbonyl compound).