1. Introduction to Aldehydes and Ketones

2. Structure and Nature of the Carbonyl Group

The carbonyl carbon is approximately sp²-hybridized and the atoms around it are nearly trigonal planar. The C=O bond contains one σ bond and one π bond.

Oxygen is more electronegative than carbon, so the bond is strongly polarized:

This polarity explains why nucleophiles attack the carbonyl carbon.

3. Nomenclature and Isomerism

3.1 Aldehydes

Choose the longest chain containing –CHO and replace the “e” of the corresponding alkane with –al. The aldehyde carbon is carbon 1.

3.2 Ketones

Select the longest chain containing C=O and use the suffix –one. Number the chain to give the carbonyl carbon the lowest possible locant.

3.3 Isomerism

• Chain isomerism: different carbon skeletons.
• Position isomerism: important for ketones when C=O occupies different allowed positions.
• Functional isomerism: aldehydes and ketones can share the same molecular formula.

4. Preparation of Aldehydes and Ketones

4.1 From Alcohols by Oxidation

Primary alcohol gives an aldehyde on controlled oxidation; secondary alcohol gives a ketone.

4.2 By Catalytic Dehydrogenation of Alcohols

4.3 From Alkenes by Ozonolysis

Ozone cleaves a C=C bond. Reductive work-up gives aldehydes and/or ketones depending on alkene substitution.

4.4 From Acid Chlorides

Aldehyde by Rosenmund Reduction

Ketone from Acid Chloride

Acid chlorides can be converted into ketones using suitable organometallic reagents under controlled conditions; at Grade 12 level the key concept is that an acyl derivative can furnish a ketone without reducing all the way to an alcohol.

4.5 From Gem-Dihaloalkanes

Hydrolysis of geminal dihalides forms unstable gem-diols that lose water to produce carbonyl compounds.

4.6 From Alkynes by Catalytic Hydration

Acid-catalysed hydration in the presence of Hg²⁺ forms an enol that rapidly rearranges to a carbonyl compound.

5. Physical Properties

6. Identification and Distinction Tests

6.1 2,4-DNP Test for Carbonyl Group

2,4-Dinitrophenylhydrazine (2,4-DNP or Brady’s reagent) reacts with aldehydes and ketones to form yellow/orange crystalline hydrazones.

6.2 Tollens’ Test

Aldehydes reduce Tollens’ reagent, [Ag(NH₃)₂]⁺, to metallic silver and are oxidized to carboxylate.

Observation: silver mirror or grey silver deposit.

6.3 Fehling’s Test

Many aliphatic aldehydes reduce blue Cu²⁺ in alkaline Fehling’s solution to brick-red Cu₂O.

Observation: blue solution gives a brick-red precipitate.

7. Nucleophilic Addition Reactions

The electrophilic carbonyl carbon is attacked by nucleophiles. The π bond breaks and an addition product forms.

7.1 Addition of Hydrogen

7.2 Addition of HCN

The product is a cyanohydrin.

7.3 Addition of Sodium Hydrogen Sulphite

Many aldehydes and some ketones form crystalline bisulphite addition compounds; historically this reaction is useful for separation/purification.

8. Reactions with Ammonia Derivatives

Carbonyl compounds undergo condensation with compounds containing the group H₂N–Z. Water is eliminated and C=N derivatives form.

9. Aldol Condensation

Example: Ethanal

The product is 3-hydroxybutanal, historically called “aldol”.

10. Cannizzaro Reaction

Methanal

Benzaldehyde

11. Reduction of the Carbonyl Group to –CH₂–

11.1 Clemmensen Reduction

Clemmensen reduction is carried out in strongly acidic medium.

11.2 Wolff–Kishner Reduction

Wolff–Kishner reduction proceeds through a hydrazone and uses strongly basic, high-temperature conditions.

12. Action with PCl₅ and LiAlH₄

12.1 With PCl₅

Phosphorus pentachloride replaces carbonyl oxygen by two chlorine atoms.

Example

12.2 With Lithium Aluminium Hydride

LiAlH₄ is a strong reducing agent.

13. Important Reactions of Methanal (Formaldehyde)

13.1 With Ammonia

Methanal reacts with ammonia to form hexamethylenetetramine (urotropine/hexamine).

13.2 With Phenol

Methanal reacts with phenol to give hydroxymethyl phenols, which can condense further to phenol–formaldehyde resins.

14. Formalin and Its Uses

Uses

• Preservation and fixation of biological specimens and tissues in controlled laboratory/medical settings.
• Raw material for formaldehyde-based resins and polymers.
• Industrial disinfectant/biocide applications under regulated conditions.
• Manufacture of chemicals and synthetic materials.

15. Aromatic Aldehydes and Ketones

15.1 Benzaldehyde from Toluene

Controlled side-chain oxidation of toluene can produce benzaldehyde. A classical school-level method uses chromyl chloride (Etard reaction).

15.2 Acetophenone from Benzene

Friedel–Crafts acylation of benzene with ethanoyl chloride gives acetophenone.

16. Properties and Important Reactions of Benzaldehyde

Benzaldehyde is an aromatic aldehyde. It undergoes typical aldehyde reactions such as oxidation, reduction and nucleophilic addition, but it lacks an α-hydrogen and therefore shows Cannizzaro rather than ordinary self-aldol condensation.

Oxidation

Reduction

Cannizzaro

17. Perkin Condensation

Example: Cinnamic Acid

The product is cinnamic acid.

18. Benzoin Condensation

19. Electrophilic Substitution in Aromatic Carbonyl Compounds

The –CHO and –COR groups withdraw electron density from the aromatic ring by inductive and resonance effects. They are therefore deactivating and meta directing.

Nitration of Benzaldehyde

Halogenation

Sulphonation

20. High-Yield Reaction Summary

21. Common Exam Mistakes

22. Worked Examples

23. Important Exam Questions

Short-Answer Questions

1. Define aldehyde and ketone with general structures.
2. Explain the polarity and geometry of the carbonyl group.
3. Why are aldehydes generally more reactive than ketones toward nucleophilic addition?
4. Give IUPAC names of HCHO, CH₃CHO, CH₃COCH₃ and CH₃COCH₂CH₃.
5. Give an example of functional isomerism between an aldehyde and a ketone.
6. How is an aldehyde prepared from a primary alcohol?
7. How is a ketone prepared from a secondary alcohol?
8. What products are formed in ozonolysis of propene?
9. What is Rosenmund reduction?
10. How can a gem-dihalide give a carbonyl compound?
11. What is the role of HgSO₄/H₂SO₄ in hydration of an alkyne?
12. What observation is obtained with 2,4-DNP reagent?
13. Describe Tollens’ test for an aldehyde.
14. Describe Fehling’s test for an aliphatic aldehyde.
15. What is a cyanohydrin?
16. What is the product of carbonyl compound + NH₂OH?
17. State the condition required for aldol condensation.
18. State the condition required for Cannizzaro reaction.
19. Differentiate Clemmensen and Wolff–Kishner reductions.
20. What does LiAlH₄ do to an aldehyde or ketone?
21. What does PCl₅ do to a carbonyl group?
22. Write the reaction of methanal with ammonia.
23. What is formalin?
24. How is benzaldehyde prepared from toluene?
25. How is acetophenone prepared from benzene?
26. What is Perkin condensation?
27. What is benzoin condensation?
28. Why is –CHO meta directing?

Long-Answer Questions

1. Explain structure and reactivity of the carbonyl group.
2. Describe five syllabus methods of preparing aldehydes and ketones.
3. Compare physical properties of aldehydes, ketones, hydrocarbons and alcohols.
4. Explain how 2,4-DNP, Tollens’ and Fehling’s reagents are used in carbonyl identification.
5. Describe nucleophilic addition of H₂, HCN and NaHSO₃ to carbonyl compounds.
6. Describe reactions with hydroxylamine, hydrazine, phenylhydrazine and semicarbazide.
7. Explain aldol condensation of ethanal with equation.
8. Explain Cannizzaro reaction of methanal or benzaldehyde.
9. Compare Clemmensen, Wolff–Kishner and LiAlH₄ reductions.
10. Describe reactions of methanal with ammonia and phenol.
11. Define formalin and state its uses and safety concerns.
12. Explain preparation of benzaldehyde from toluene and acetophenone from benzene.
13. Explain Perkin condensation and benzoin condensation.
14. Explain electrophilic substitution orientation in benzaldehyde.

Conversion Questions

1. Ethanol → ethanal.
2. Propan-2-ol → propanone.
3. Ethene / propene → carbonyl compounds by ozonolysis.
4. Ethanal → ethanol.
5. Propanone → propan-2-ol.
6. Propanone → propane.
7. Benzaldehyde → benzyl alcohol.
8. Benzaldehyde → benzoic acid.
9. Toluene → benzaldehyde.
10. Benzene → acetophenone.
11. Benzaldehyde → cinnamic acid.
12. Benzaldehyde → benzoin.

Diagram / Flowchart Questions

1. Draw aldehyde vs ketone structures.
2. Draw the polar trigonal-planar carbonyl group.
3. Draw a preparation map for carbonyl compounds.
4. Draw a test flowchart using 2,4-DNP, Tollens’ and Fehling’s tests.
5. Draw the general nucleophilic-addition mechanism concept.
6. Draw aldol condensation of ethanal.
7. Draw Cannizzaro disproportionation.
8. Draw Clemmensen vs Wolff–Kishner reduction.
9. Draw methanal reaction routes with ammonia and phenol.
10. Draw preparation of benzaldehyde and acetophenone.
11. Draw Perkin condensation.
12. Draw the meta-directing effect of –CHO.

24. One-Minute Revision

• Aldehyde = R–CHO; ketone = R–CO–R′.
• The carbonyl carbon is sp², trigonal planar and δ⁺.
• Aldehydes are generally more reactive than comparable ketones toward nucleophilic addition.
• Primary alcohol oxidation/dehydrogenation gives aldehyde; secondary alcohol gives ketone.
• Ozonolysis cleaves C=C to carbonyl compounds.
• Rosenmund reduction converts an acid chloride to an aldehyde.
• Gem-dihalide hydrolysis can produce an aldehyde or ketone.
• HgSO₄/H₂SO₄ hydration of alkynes gives carbonyl compounds after tautomerization.
• 2,4-DNP gives yellow/orange carbonyl derivative with both aldehydes and ketones.
• Tollens’ positive aldehyde test gives silver mirror.
• Fehling’s gives brick-red Cu₂O with many aliphatic aldehydes.
• HCN adds to C=O to form cyanohydrin.
• NH₂OH gives oxime; NH₂NH₂ gives hydrazone.
• Aldol requires α-H.
• Cannizzaro is characteristic of aldehydes without α-H.
• Clemmensen: Zn(Hg)/HCl; Wolff–Kishner: NH₂NH₂/KOH/heat.
• Clemmensen/Wolff–Kishner convert C=O to CH₂.
• LiAlH₄ converts aldehyde to 1° alcohol and ketone to 2° alcohol.
• PCl₅ replaces carbonyl oxygen by two chlorine atoms.
• 6HCHO + 4NH₃ gives hexamethylenetetramine.
• Formalin is aqueous formaldehyde solution.
• Toluene can be converted to benzaldehyde; benzene + CH₃COCl/AlCl₃ gives acetophenone.
• Perkin reaction of benzaldehyde can give cinnamic acid.
• Benzoin condensation joins two benzaldehyde molecules.
• –CHO and –COR deactivate the aromatic ring and direct electrophiles mainly meta.

25. Diagram Practice

Students should practice these diagrams for the NEB examination:

1. Aldehyde vs ketone structure.
2. Carbonyl polarity and trigonal-planar geometry.
3. Preparation routes of aldehydes and ketones.
4. 2,4-DNP / Tollens’ / Fehling’s test flowchart.
5. General nucleophilic addition to carbonyl.
6. Aldol condensation of ethanal.
7. Cannizzaro disproportionation.
8. Clemmensen vs Wolff–Kishner reduction.
9. Methanal reactions with ammonia and phenol.
10. Benzaldehyde and acetophenone preparation.
11. Perkin condensation.
12. Meta-directing effect of –CHO / –COR.