ChemistryMediumClass 12

Preparation of Alcohols — From alkenes, carbonyl, Grignard

Alcohols, Phenols & Ethers

JEE Qs

Hard

100

min

Master the regioselectivity and stereoselectivity of alkene reactions, understand the strength and scope of different reducing agents, and meticulously remember Grignard reagent reactions with various carbonyl compounds and their limitations.

🧮 Key Formulas

R-CH=CH2 + H2O --(H+/H2SO4)--> R-CH(OH)-CH3 (Acid-catalyzed hydration, Markovnikov)

R-CH=CH2 --(1. BH3.THF, 2. H2O2, NaOH)--> R-CH2-CH2-OH (Hydroboration-oxidation, Anti-Markovnikov, syn addition)

R-CH=CH2 --(1. Hg(OAc)2, H2O; 2. NaBH4)--> R-CH(OH)-CH3 (Oxymercuration-demercuration, Markovnikov)

R-CHO + [H] --(NaBH4 or LiAlH4)--> R-CH2-OH (Aldehyde to 1° alcohol)

R-CO-R' + [H] --(NaBH4 or LiAlH4)--> R-CH(OH)-R' (Ketone to 2° alcohol)

R-COOH + [H] --(LiAlH4 only)--> R-CH2-OH (Carboxylic acid to 1° alcohol)

R-COOR' + [H] --(LiAlH4 only)--> R-CH2-OH + R'-OH (Ester to 1° alcohol + alcohol from R')

HCHO + R-MgX --(1. Dry Ether; 2. H3O+)--> R-CH2-OH (Formaldehyde + Grignard to 1° alcohol)

R'-CHO + R-MgX --(1. Dry Ether; 2. H3O+)--> R'-CH(OH)-R (Aldehyde + Grignard to 2° alcohol)

R'-CO-R'' + R-MgX --(1. Dry Ether; 2. H3O+)--> R'-C(OH)(R)-R'' (Ketone + Grignard to 3° alcohol)

R'-COOR'' + 2 R-MgX --(1. Dry Ether; 2. H3O+)--> R'-C(OH)(R)2 (Ester + Grignard to 3° alcohol)

✅ Key Points for JEE

1Alkene hydration methods (acid-catalyzed, oxymercuration-demercuration) follow Markovnikov's rule, while hydroboration-oxidation follows Anti-Markovnikov's rule.
2Acid-catalyzed hydration can lead to carbocation rearrangements, while oxymercuration-demercuration and hydroboration-oxidation avoid them, making them suitable for specific alkene structures.
3NaBH4 is a milder reducing agent, selectively reducing aldehydes and ketones, whereas LiAlH4 is stronger and can reduce aldehydes, ketones, carboxylic acids, and esters.
4Grignard reagents are highly reactive nucleophiles. Their reaction with formaldehyde yields primary alcohols, with other aldehydes yields secondary alcohols, and with ketones or esters yields tertiary alcohols.
5Grignard reagents react readily with any acidic proton (e.g., from water, alcohols, carboxylic acids, terminal alkynes), hence must be prepared and used in anhydrous conditions.

⚠️ Common Mistakes

✕Confusing the regioselectivity (Markovnikov vs. Anti-Markovnikov) or stereoselectivity (syn vs. anti) of alkene hydration methods.
✕Incorrectly identifying which reducing agent (NaBH4 vs. LiAlH4) is appropriate for specific carbonyl functional groups (e.g., attempting to reduce a carboxylic acid with NaBH4).
✕Failing to account for the highly basic and nucleophilic nature of Grignard reagents, leading to side reactions with acidic protons or protic solvents instead of the desired carbonyl reaction.

NCERT Chapters

Class 11 Chemistry Part II Ch 13: Hydrocarbons
Class 12 Chemistry Part II Ch 11: Alcohols, Phenols and Ethers
Class 12 Chemistry Part II Ch 12: Aldehydes, Ketones and Carboxylic Acids