Preparation of Aldehydes - NEET Notes, Formula & Common Mistakes
Preparation of AldehydesAldehydes Ketones Carboxylic AcidsNEET ChemistryNEET 2025Oxidation of primary alcoholsStephen reaction
Preparation of Aldehydes - NEET Notes, Formula & Common Mistakes
Aldehydes, Ketones and Carboxylic Acids·2 min read·NEET 2026
What NEET Asks
Expect direct questions on reagents and products for specific reactions.
Mechanism-based questions are less frequent but knowing intermediates helps.
Often part of multi-step conversion problems, carrying 4 marks each.
Key Points
Primary alcohols oxidize to aldehydes, then to carboxylic acids.
To stop at the aldehyde stage, mild oxidizing agents like PCC or PDC are essential.
PCC (Pyridinium Chlorochromate) is CrO3.HCl.Pyridine (non-aqueous).
Stephen reaction converts nitriles to aldehydes using SnCl2/HCl followed by hydrolysis.
Nitriles can also be reduced to aldehydes using DIBAL-H (diisobutylaluminium hydride).
Must-Know Formula / Reaction
Oxidation of Primary Alcohols:
R-CH₂-OH [PCC, CH₂Cl₂] → R-CHO
R-CH₂-OH: Primary alcohol, the starting material.
PCC: Pyridinium Chlorochromate, a selective oxidizing agent.
R-CHO: Aldehyde, the desired product.
Stephen Reaction:
R-C≡N [1. SnCl₂/HCl, 2. H₃O⁺] → R-CHO
R-C≡N: Nitrile, the starting material.
SnCl₂/HCl: Stannous chloride and hydrochloric acid, reducing agent forming an imine.
H₃O⁺: Hydrolysis, converts the imine to an aldehyde.
Common Mistakes
Students often use strong oxidizing agents (KMnO₄, K₂Cr₂O₇) for primary alcohols, leading to carboxylic acids.
Don't forget the hydrolysis step after SnCl₂/HCl in Stephen reaction; it's crucial for aldehyde formation.
Confusing DIBAL-H reduction of esters/nitriles with LiAlH₄, which fully reduces to alcohol/amine respectively.
Rapid Revision
Mild oxidizers (PCC, PDC) stop primary alcohol oxidation at aldehydes. Stephen reaction reduces nitriles to imines with SnCl₂/HCl, then hydrolyzes to aldehydes. DIBAL-H also reduces nitriles to aldehydes. Remember specific reagents!
Frequently Asked Questions
Why are mild oxidizing agents preferred for converting primary alcohols to aldehydes?▾
Strong oxidizing agents like KMnO₄ or K₂Cr₂O₇ would further oxidize the aldehyde formed to a carboxylic acid. Mild agents like PCC or PDC are selective and stop the oxidation at the aldehyde stage, preventing over-oxidation.
What is the role of SnCl₂ and HCl in the Stephen reaction?▾
SnCl₂ and HCl act as a reducing agent system. They reduce the nitrile (R-C≡N) to an imine hydrochloride (R-CH=NH₂⁺Cl⁻), which is a partially reduced product that can then be hydrolyzed to an aldehyde.
Can DIBAL-H be used to prepare aldehydes from nitriles?▾
Yes, DIBAL-H (diisobutylaluminium hydride) is another excellent reagent for selectively reducing nitriles to aldehydes. It works under controlled conditions (low temperature) to stop the reduction at the aldehyde stage after hydrolysis.
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