Question
List all the methods to prepare alcohols from: (a) alkenes, (b) carbonyl compounds, (c) Grignard reagent, (d) esters, and (e) alkyl halides. For each, write the reagent and specify whether the product is a primary, secondary, or tertiary alcohol.
(CBSE 12 board + JEE Main recurring topic)
Solution — Step by Step
Method 1: Acid-catalysed hydration (Markownikoff addition)
Product follows Markownikoff’s rule — OH goes to the more substituted carbon. Gives secondary or tertiary alcohols.
Method 2: Hydroboration-oxidation (Anti-Markownikoff)
OH goes to the less substituted carbon. Gives primary alcohols. This is the go-to reaction when you need an anti-Markownikoff product.
Aldehydes Primary alcohols
Ketones Secondary alcohols
NaBH₄ is a mild reducing agent (works in aqueous/protic solvents). LiAlH₄ is stronger (needs dry ether). Both reduce C=O but do not touch C=C double bonds — a useful selectivity point for JEE.
| Carbonyl compound | Grignard product |
|---|---|
| Formaldehyde (HCHO) | Primary alcohol |
| Aldehyde (RCHO) | Secondary alcohol |
| Ketone (RCOR’) | Tertiary alcohol |
The Grignard reagent adds its alkyl group to the carbonyl carbon. More substituted carbonyl gives more substituted alcohol.
Esters Primary alcohols (reduces both the acid and alcohol parts)
Alkyl halides Alcohols (SN2 or SN1 depending on substrate)
For methyl and primary halides, SN2 dominates. For tertiary halides, SN1 dominates (and elimination competes — so this method is less clean for 3 degree substrates).
Alcohol Preparation Routes Flowchart
flowchart TD
A["Want to prepare an alcohol?"] --> B{"Starting material?"}
B -->|"Alkene"| C{"Which regiochemistry?"}
C -->|"Markownikoff (2°/3° OH)"| C1["H₃O⁺ / H₂SO₄ + H₂O"]
C -->|"Anti-Markownikoff (1° OH)"| C2["BH₃/THF then H₂O₂/NaOH"]
B -->|"Aldehyde/Ketone"| D["NaBH₄ or LiAlH₄ reduction"]
D --> D1["Aldehyde → 1° alcohol"]
D --> D2["Ketone → 2° alcohol"]
B -->|"Grignard + Carbonyl"| E{"Which carbonyl?"}
E -->|"HCHO"| E1["1° alcohol"]
E -->|"RCHO"| E2["2° alcohol"]
E -->|"R₂CO"| E3["3° alcohol"]
B -->|"Alkyl halide"| F["aq. NaOH / moist Ag₂O"]
B -->|"Ester"| G["LiAlH₄ → 1° alcohol"]Why This Works
Alcohols have the -OH group, and every preparation method is essentially a way to get -OH onto a carbon. The differences boil down to which carbon gets the -OH (regiochemistry) and how substituted the product is.
The Grignard method is the most powerful because by choosing the right combination of Grignard reagent and carbonyl compound, we can make any class of alcohol — primary, secondary, or tertiary. This flexibility makes it a favourite in JEE synthesis problems.
Common Mistake
Confusing Markownikoff and anti-Markownikoff products. Acid-catalysed hydration follows Markownikoff (OH on more substituted carbon). Hydroboration-oxidation gives anti-Markownikoff (OH on less substituted carbon). JEE questions love asking: “which reagent gives primary alcohol from propene?” Answer: hydroboration-oxidation, NOT acid-catalysed hydration.
For NEET and CBSE: focus on Grignard reactions and reduction of aldehydes/ketones — these appear almost every year. For JEE Main: hydroboration-oxidation and the selectivity of NaBH₄ vs LiAlH₄ are high-value topics.