| Feature | HVZ Reaction | Fischer Esterification | SOCl2 (Acyl Chloride) | LiAlH4 Reduction | Kolbe Electrolysis |
|---|---|---|---|---|---|
| Reagents | X2 / Red P | R'OH / H2SO4 | SOCl2 (thionyl chloride) | LiAlH4 in dry ether | Electrolysis of RCOO–(aq) |
| Product | alpha-Halocarboxylic acid | Ester (RCOOR') + H2O | Acyl chloride (RCOCl) | Primary alcohol (RCH2OH) | Alkane (R–R) + CO2 |
| Site of action | Alpha-carbon (C-2) | Carboxyl C–OH | C–OH of –COOH | Both C=O bonds | Anode (oxidation of RCOO–) |
| Reversibility | Irreversible | Reversible (K ≈ 1) | Irreversible | Irreversible | Irreversible (electrolytic) |
| Limitations | Needs alpha-H; not for HCOOH or ArCOOH | Equilibrium — need excess alcohol or H2O removal | — | Cannot stop at aldehyde stage | — |
| NaBH4 alternative? | N/A | N/A | N/A | NaBH4 fails for RCOOH | N/A |
| Byproducts | HX | H2O | SO2 + HCl (gaseous) | MgBrOH after workup | CO2 (gas) |
| NEET frequency | High | High | Medium | Very high | Medium |
NaBH4 vs LiAlH4 — Head-to-Head:
| Property | NaBH4 | LiAlH4 |
|---|---|---|
| Reducing power | Mild | Strong |
| Reduces RCOOH? | NO | YES → RCH2OH |
| Reduces RCHO? | YES → RCH2OH | YES → RCH2OH |
| Reduces RCOR'? | YES → R2CHOH | YES → R2CHOH |
| Reduces alkene? | NO | NO (under normal conditions) |
| Solvent | Protic (EtOH, MeOH) or ether | Dry ether only (anhydrous) |
| NEET trap | "No reaction" with RCOOH | Reduces everything except aromatic ring |