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!