Question
How do we systematically approach organic chemistry reactions? Given a starting material and a target product, how do we figure out which reagents to use?
(JEE Main + NEET strategy)
Solution — Step by Step
Every organic reaction is a functional group transformation. Look at what functional group you have and what you need.
Common transformations:
- Alkene to alcohol (hydration)
- Alcohol to aldehyde/ketone (oxidation)
- Aldehyde to carboxylic acid (oxidation)
- Alkyl halide to alcohol (hydrolysis)
- Carboxylic acid to ester (esterification)
Organic chemistry has a clear oxidation ladder:
Moving right = oxidation (add O, remove H). Moving left = reduction (add H, remove O).
When the target is higher on the ladder than the starting material, you need an oxidizing agent. When lower, a reducing agent.
| Transformation | Reagent | Exam frequency |
|---|---|---|
| Alkene to alcohol | H₂O/H⁺ (Mark.) or BH₃/H₂O₂ (anti-Mark.) | Very high |
| Alcohol to aldehyde | PCC or Collins reagent | High |
| Alcohol to ketone | Jones reagent or CrO₃ | High |
| Aldehyde to acid | KMnO₄ or K₂Cr₂O₇ | Medium |
| Acid to ester | ROH + acid catalyst | Very high |
| Ester to alcohol | LiAlH₄ | High |
| Halide to amine | Gabriel synthesis or NH₃ | High |
Functional Group Interconversion Map
flowchart LR
A["Alkane"] -->|"Halogenation: X₂/hv"| B["Alkyl Halide"]
B -->|"aq. NaOH"| C["Alcohol"]
B -->|"alc. KOH"| D["Alkene"]
C -->|"PCC"| E["Aldehyde"]
C -->|"K₂Cr₂O₇/H⁺"| F["Carboxylic Acid"]
E -->|"KMnO₄"| F
D -->|"H₂O/H⁺"| C
D -->|"BH₃ then H₂O₂"| C
F -->|"ROH/H⁺"| G["Ester"]
G -->|"LiAlH₄"| C
B -->|"KCN"| H["Nitrile"]
H -->|"H₃O⁺"| FWhy This Works
Organic chemistry has hundreds of reactions, but they follow a pattern: each reaction converts one functional group into another. If you learn the map of which group converts to which, you can work backwards from the target product to figure out the route.
The strategy is: (1) identify start and end functional groups, (2) check if a direct reaction exists, (3) if not, find an intermediate that connects both. Most JEE synthesis problems need 2-3 steps at most.
Common Mistake
Students memorise reagents without understanding the functional group change. When you see PCC, think “alcohol to aldehyde (1 step oxidation, stops here).” When you see KMnO₄, think “strong oxidation — goes all the way to carboxylic acid.” Understanding the oxidation level change is more reliable than rote memorisation of 200 reactions.
For JEE: focus on named reactions — Aldol, Cannizzaro, Kolbe, Wurtz, Friedel-Crafts. Each named reaction has a specific pattern. Knowing 15-20 named reactions covers most synthesis problems.