Preparation and Reactions of Aldehydes – Class 12 Chemistry
Aldehydes form one of the most important chapters in Class 12 Organic Chemistry. Questions from aldehydes are frequently asked in CBSE board examinations, JEE, NEET, and practical viva. This article explains the preparation and reactions of aldehydes in a very simple, readable, and exam-oriented manner.
What are Aldehydes?
Aldehydes are organic compounds containing the functional group –CHO. In aldehydes, the carbonyl carbon is bonded to one hydrogen atom and one alkyl or aryl group.
General structural formula:
R – C (=O) – H
||
O
General molecular formula:
R – CHO
Examples:
Formaldehyde → H – CHO
Acetaldehyde → CH3 – CHO
Benzaldehyde → C6H5 – CHO
Preparation of Aldehydes
Several laboratory and industrial methods are used to prepare aldehydes. Only the most important methods required for CBSE and competitive exams are discussed below.
1. Preparation from Primary Alcohols (Controlled Oxidation)
Primary alcohols on controlled oxidation form aldehydes. Strong oxidation must be avoided, otherwise carboxylic acids are formed.
Reaction diagram:
R – CH2 – OH + [O]
↓
R – CHO + H2O
Oxidizing agents used:
PCC (preferred reagent)
Cu at 573 K
Acidified K2Cr2O7 (controlled)
Example:
CH3 – CH2 – OH → CH3 – CHO
2. From Acid Chlorides (Rosenmund Reduction)
Acid chlorides on reduction with hydrogen in the presence of a poisoned catalyst give aldehydes. This reaction is called Rosenmund reduction.
Reaction diagram:
R – COCl + H2
↓ (Pd / BaSO4)
R – CHO + HCl
The poisoned catalyst prevents further reduction of aldehyde to alcohol.
3. From Nitriles (Stephen Reaction)
Nitriles are converted into aldehydes using the Stephen reaction.
Reaction diagram:
R – C ≡ N
↓ (SnCl2 / HCl)
R – CH = NH
↓ (H2O)
R – CHO
4. From Alkynes (Hydroboration–Oxidation)
Terminal alkynes give aldehydes on hydroboration followed by oxidation.
Reaction diagram:
R – C ≡ CH
↓ (BH3 / THF)
↓ (H2O2 / OH−)
R – CH2 – CHO
5. From Alkenes (Ozonolysis)
Alkenes on ozonolysis followed by reduction form aldehydes.
Reaction diagram:
R – CH = CH – R
↓ (O3)
↓ (Zn / H2O)
R – CHO + R – CHO
Chemical Reactions of Aldehydes
Due to the presence of the polar carbonyl group, aldehydes are highly reactive. They undergo oxidation, reduction, nucleophilic addition, and condensation reactions.
1. Oxidation Reactions
Aldehydes are easily oxidized to carboxylic acids.
(a) Tollens’ Test (Silver Mirror Test)
Reaction diagram:
R – CHO + [Ag(NH3)2]+
↓
R – COO− + Ag ↓ (silver mirror)
This test is used to distinguish aldehydes from ketones.
(b) Fehling’s Test
Aliphatic aldehydes give a brick-red precipitate of Cu2O. Aromatic aldehydes do not respond to this test.
2. Reduction Reactions
Aldehydes on reduction give primary alcohols.
Reaction diagram:
R – CHO
↓ (NaBH4 / LiAlH4)
R – CH2 – OH
3. Nucleophilic Addition Reactions
(a) Reaction with Hydrogen Cyanide
Reaction diagram:
R – CHO + HCN
↓
R – CH(OH) – CN
(b) Reaction with Sodium Bisulphite
Reaction diagram:
R – CHO + NaHSO3
↓
R – CH(OH) – SO3Na
4. Aldol Condensation
Aldehydes containing α-hydrogen undergo aldol condensation.
Reaction diagram:
2 CH3 – CHO
↓ (dil. NaOH)
CH3 – CH(OH) – CH2 – CHO
5. Cannizzaro Reaction
Aldehydes without α-hydrogen undergo Cannizzaro reaction.
Reaction diagram:
2 H – CHO + NaOH
↓
H – COONa + CH3OH
Important CBSE Exam Points
- Aldehydes are more reactive than ketones
- Aldehhydes give Tollens’ test
- Aldehydes are easily oxidized
- Aldol reaction requires α-hydrogen
- Cannizzaro reaction occurs when α-hydrogen is absent
Conclusion:
Aldehydes are extremely important compounds in organic chemistry.
Mastering their preparation methods and reactions helps students score
high marks in CBSE boards and competitive examinations.
