ChemistryMediumClass 12

Chemical Reactions — Oxidation, dehydration, esterification

Alcohols, Phenols & Ethers

JEE Qs

Hard

min

Master the specific reagents, reaction conditions, and potential rearrangements for each reaction type to accurately predict products for various alcohol substrates.

🧮 Key Formulas

1° alcohol oxidation: R-CH2-OH --[PCC/CrO3]--> R-CHO --[K2Cr2O7/KMnO4]--> R-COOH

2° alcohol oxidation: R2CH-OH --[K2Cr2O7/KMnO4]--> R2C=O

3° alcohol oxidation: No reaction under mild conditions; C-C bond cleavage under strong conditions.

Dehydration: R-CH(OH)-R' --[Conc. H2SO4/H3PO4, heat]--> Alkene(s) + H2O (via E1/E2 mechanisms)

Esterification: R-COOH + R'-OH <=> R-COO-R' + H2O (Acid-catalyzed, reversible)

Esterification (Acyl chloride): R-COCl + R'-OH --> R-COO-R' + HCl

Esterification (Acid Anhydride): (RCO)2O + R'-OH --> R-COO-R' + R-COOH

✅ Key Points for JEE

1Oxidation of alcohols depends critically on the type of alcohol (1°, 2°, 3°) and the oxidizing agent used (mild vs. strong) to control the product (aldehyde, ketone, carboxylic acid).
2Dehydration of alcohols is an elimination reaction (E1/E2) forming alkenes, often involving carbocation intermediates for 2° and 3° alcohols, which can undergo rearrangements.
3Saytzeff's rule dictates that the most substituted alkene is the major product in dehydration unless specific conditions favor the Hofmann product.
4Esterification is the reaction of an alcohol with a carboxylic acid (or its derivative) to form an ester; Fischer esterification is acid-catalyzed and reversible, requiring removal of water or excess reactant for good yield.
5Understand the mechanism of Fischer esterification to identify the bonds broken (C-OH of acid, H-O of alcohol) and the origin of water.

⚠️ Common Mistakes

✕Not differentiating between mild oxidizing agents (e.g., PCC, CrO3) that stop at aldehydes and strong ones (e.g., KMnO4, K2Cr2O7) that oxidize primary alcohols to carboxylic acids.
✕Failing to recognize or predict carbocation rearrangements during dehydration, leading to incorrect major products.
✕Confusing the role of temperature and acid concentration in dehydration vs. ether formation; higher temperature favors alkene formation.
✕Incorrectly identifying the atoms forming water during Fischer esterification (H from alcohol, OH from carboxylic acid).

NCERT Chapters

Class 12 Chemistry Ch 11: Alcohols, Phenols and Ethers
Class 12 Chemistry Ch 10: Haloalkanes and Haloarenes
Class 12 Chemistry Ch 12: Aldehydes, Ketones and Carboxylic Acids