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Chemical Properties of Alcohols

Reactions involving O-H bond cleavage, C-O bond cleavage, and Oxidation.

By chemca Team • Updated Jan 2026

Alcohols are versatile compounds that can behave as nucleophiles (due to the lone pair on oxygen) or electrophiles (protonated alcohols). Their reactions are generally divided into those involving the cleavage of the O-H bond and those involving the cleavage of the C-O bond.

1. Reactions involving Cleavage of O-H Bond

A. Acidity (Reaction with Metals)

Alcohols are weak acids (weaker than water). They react with active metals like Sodium or Potassium to liberate Hydrogen gas and form alkoxides.

$$ 2R-O-H + 2Na \rightarrow 2R-O^-Na^+ + H_2 \uparrow $$

Acidity Order: $1^\circ > 2^\circ > 3^\circ$ (Electron releasing alkyl groups decrease acidity).

B. Esterification

Alcohols react with carboxylic acids, acid chlorides, or acid anhydrides to form esters.

$$ R-OH + R'-COOH \xrightleftharpoons{H^+} \underset{\text{Ester}}{R'-COOR} + H_2O $$

Mechanism Note: In esterification with carboxylic acids, the O-H bond of the alcohol and the C-OH bond of the acid are broken. The oxygen in the ester comes from the alcohol.

2. Reactions involving Cleavage of C-O Bond

A. Reaction with Hydrogen Halides (Lucas Test)

Alcohols react with HX ($HCl, HBr, HI$) to form Alkyl Halides. The difference in reactivity with Lucas Reagent ($HCl$ + Anhydrous $ZnCl_2$) is used to distinguish alcohols.

$$ R-OH + HCl \xrightarrow{ZnCl_2} R-Cl + H_2O $$

Tertiary ($3^\circ$): Reacts immediately (Turbidity appears instantly).

Secondary ($2^\circ$): Reacts within 5 minutes (Turbidity appears slowly).

Primary ($1^\circ$): No reaction at room temperature (Turbidity only on heating).

Reactivity Order: $3^\circ > 2^\circ > 1^\circ$ (Based on Carbocation stability)

3. Dehydration

Formation of Alkenes

Heating alcohols with protic acids ($H_2SO_4$ or $H_3PO_4$) causes the elimination of water ($\beta$-elimination) to form alkenes.

$$ CH_3CH_2OH \xrightarrow{H_2SO_4, 443K} CH_2=CH_2 + H_2O $$

Mechanism (E1):

1. Protonation: Alcohol accepts $H^+$ to form oxonium ion.
2. Carbocation Formation (Slow): Loss of water molecule.
3. Deprotonation (Fast): Loss of $H^+$ from $\beta$-carbon to form double bond.

Ease of Dehydration: $3^\circ > 2^\circ > 1^\circ$

4. Oxidation

Oxidation involves the formation of a $C=O$ double bond with cleavage of O-H and C-H bonds.

• Primary Alcohols: Oxidize to Aldehydes (with PCC) or Carboxylic Acids (with $KMnO_4$).
• Secondary Alcohols: Oxidize to Ketones (with $CrO_3$).
• Tertiary Alcohols: Resistant to oxidation. Under drastic conditions, C-C bond cleavage occurs forming mixed acids.

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