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Preparation of Alcohols | chemca

Preparation of Alcohols | chemca
Organic Chemistry

Preparation of Alcohols

Synthesis from Alkenes, Carbonyl Compounds, and Grignard Reagents.

By chemca Team • Updated Jan 2026

Alcohols ($R-OH$) are versatile compounds prepared primarily from alkenes (hydration) and carbonyl compounds (reduction or Grignard addition).

1. From Alkenes

A. Acid Catalyzed Hydration

Alkenes react with water in the presence of acid ($H_2SO_4$) to form alcohols according to Markovnikov's Rule.

$$ CH_3-CH=CH_2 + H_2O \xrightarrow{H^+} \underset{\text{Propan-2-ol}}{CH_3-CH(OH)-CH_3} $$
Mechanism involves Carbocation intermediate (Rearrangement possible).

B. Hydroboration-Oxidation

Treatment with Diborane ($B_2H_6$) followed by oxidation with $H_2O_2/OH^-$ gives alcohols formed by Anti-Markovnikov addition of water.

$$ 3CH_3-CH=CH_2 \xrightarrow{1. B_2H_6, 2. H_2O_2/OH^-} \underset{\text{Propan-1-ol}}{3CH_3-CH_2-CH_2-OH} $$
No Rearrangement. Syn-addition.

2. From Carbonyl Compounds (Reduction)

Reduction to Alcohols

A. Aldehydes & Ketones: Aldehydes reduce to $1^\circ$ Alcohols, Ketones reduce to $2^\circ$ Alcohols.
Reagents: $H_2/Pd$, $NaBH_4$, or $LiAlH_4$.
$$ R-CHO + H_2 \xrightarrow{Pd} R-CH_2OH $$ $$ R-CO-R' \xrightarrow{NaBH_4} R-CH(OH)-R' $$
B. Carboxylic Acids & Esters: Reduced to $1^\circ$ Alcohols. Requires strong reducing agent $LiAlH_4$ ($NaBH_4$ does not work).
$$ R-COOH \xrightarrow{1. LiAlH_4, 2. H_2O} R-CH_2OH $$ $$ R-COOR' \xrightarrow{H_2/Cat.} R-CH_2OH + R'OH $$

3. From Grignard Reagents

Nucleophilic Addition to Carbonyls

Reaction of Grignard reagent ($RMgX$) with aldehydes or ketones yields alcohols with increased carbon chain length.

1. With Formaldehyde (HCHO): Yields $1^\circ$ Alcohol.
$HCHO + RMgX \rightarrow R-CH_2OH$

2. With other Aldehydes (RCHO): Yields $2^\circ$ Alcohol.
$RCHO + R'MgX \rightarrow R-CH(OH)-R'$

3. With Ketones (RCOR): Yields $3^\circ$ Alcohol.
$R-CO-R + R'MgX \rightarrow R_2C(OH)R'$

$$ CH_3COCH_3 + CH_3MgBr \xrightarrow{H_2O} (CH_3)_3C-OH $$

4. From Alkyl Halides

Hydrolysis

Heating alkyl halides with aqueous KOH or moist silver oxide ($Ag_2O/H_2O$) leads to nucleophilic substitution forming alcohols.

$$ R-X + KOH(aq) \xrightarrow{\Delta} R-OH + KX $$
Caution: Alcoholic KOH causes elimination (Alkene formation). Use Aqueous KOH for substitution.

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