18.5 Reactions of Alcohols

Alcohols are versatile organic compounds that undergo various chemical transformations. Their reactivity is primarily due to the presence of the polar hydroxyl (-OH) group. Reactions generally involve either the cleavage of the C-OH bond or the O-H bond.

---

(a) Combustion

Alcohols are highly flammable compounds that undergo complete combustion in the presence of excess oxygen to produce carbon dioxide, water, and heat. Due to their high octane rating, they are used as fuels, especially in high-performance engines.

• Methanol Combustion: $2\mathrm{C}{\mathrm{H}}_3\mathrm{OH}+3{\mathrm{O}}_2\to 2\mathrm{C}{\mathrm{O}}_2+4{\mathrm{H}}_2\mathrm{O}+\text{heat}$

• Ethanol Combustion: ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+3{\mathrm{O}}_2\to 2\mathrm{C}{\mathrm{O}}_2+3{\mathrm{H}}_2\mathrm{O}+\text{heat}$

---

(b) Formation of Alkyl Halides

Alcohols can be converted to alkyl halides through reactions with various halogenating agents. The hydroxyl (-OH) group is substituted by a halogen atom. These reactions involve C-OH bond cleavage.

Reaction with Thionyl Chloride ($\mathrm{SOC}{\mathrm{l}}_2$)

When alcohols react with thionyl chloride ($\mathrm{SOC}{\mathrm{l}}_2$), typically in the presence of pyridine, they form alkyl chlorides. The by-products $\mathrm{S}{\mathrm{O}}_2$ and $\mathrm{HCl}$ are gases and escape, driving the reaction to completion.

$R\text{-}\mathrm{OH}+\mathrm{SOC}{\mathrm{l}}_2\stackrel{\text{Pyridine}}{\to }R\text{-}\mathrm{Cl}+\mathrm{S}{\mathrm{O}}_{2(\mathrm{g})}+\mathrm{HC}{\mathrm{l}}_{(\mathrm{g})}$

Reaction with Phosphorus Halides ($\mathrm{P}{\mathrm{X}}_3$ and $\mathrm{P}{\mathrm{X}}_5$)

Alcohols react with phosphorus trihalides and pentahalides to yield the corresponding alkyl halides.

• With Phosphorus Trichloride ($\mathrm{PC}{\mathrm{l}}_3$): $3R\text{-}\mathrm{OH}+\mathrm{PC}{\mathrm{l}}_3\to 3R\text{-}\mathrm{Cl}+{\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_3$

• With Phosphorus Pentachloride ($\mathrm{PC}{\mathrm{l}}_5$): $R\text{-}\mathrm{OH}+\mathrm{PC}{\mathrm{l}}_5\to R\text{-}\mathrm{Cl}+\mathrm{POC}{\mathrm{l}}_3+\mathrm{HCl}$

Reaction with Hydrogen Halides ($\mathrm{HX}$)

The -OH group of an alcohol can be replaced by a halogen atom by reacting with a hydrogen halide ($\mathrm{HCl}$, $\mathrm{HBr}$, $\mathrm{HI}$):

$R\text{-}\mathrm{OH}+\mathrm{HX}\to R\text{-}\mathrm{X}+{\mathrm{H}}_2\mathrm{O}$

The Lucas Test

The Lucas test distinguishes between primary ($1^{\circ }$), secondary ($2^{\circ }$), and tertiary ($3^{\circ }$) alcohols based on their reaction rate with Lucas reagent — a solution of anhydrous $\mathrm{ZnC}{\mathrm{l}}_2$ in concentrated $\mathrm{HCl}$. The reaction produces an alkyl chloride, which is insoluble and appears as a cloudy suspension or oily layer.

$R\text{-}\mathrm{OH}+\mathrm{HCl}\stackrel{\mathrm{ZnC}{\mathrm{l}}_2}{\to }R\text{-}\mathrm{Cl}+{\mathrm{H}}_2\mathrm{O}$

Examples:

• $(\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{COH}+\mathrm{HCl}\stackrel{\mathrm{ZnC}{\mathrm{l}}_2}{\to }(\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{CCl}+{\mathrm{H}}_2\mathrm{O}$ (immediate)
• $(\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{CHOH}+\mathrm{HCl}\stackrel{\mathrm{ZnC}{\mathrm{l}}_2}{\to }(\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{CHCl}+{\mathrm{H}}_2\mathrm{O}$ (5–10 min)
• $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OH}+\mathrm{HCl}\stackrel{\mathrm{ZnC}{\mathrm{l}}_2,\Delta }{\to }\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{Cl}+{\mathrm{H}}_2\mathrm{O}$ (heating required)

Reaction with Potassium Bromide ($\mathrm{KBr}$)

Bromoalkanes can be prepared by heating an alcohol with potassium bromide and concentrated sulfuric acid:

$R\text{-}\mathrm{OH}+\mathrm{KBr}+{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4\stackrel{\Delta }{\to }R\text{-}\mathrm{Br}+\mathrm{KHS}{\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}$

---

(c) Reaction with Active Metals (O-H Bond Cleavage)

Alcohols react with active metals such as sodium ($\mathrm{Na}$) and potassium ($\mathrm{K}$) to release hydrogen gas and form metal alkoxides. This reaction involves O-H bond cleavage and demonstrates the weak acidic character of alcohols.

$2R\text{-}\mathrm{OH}+2\mathrm{Na}\to 2R\text{-}\mathrm{ONa}+{\mathrm{H}}_{2(\mathrm{g})}$

Example: $2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+2\mathrm{Na}\to 2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{ONa}+{\mathrm{H}}_2$

The reactivity order follows the acidity order: Primary > Secondary > Tertiary.

---

(d) Oxidation of Alcohols

Alcohols can be oxidized using oxidizing agents such as acidified potassium dichromate (${\mathrm{K}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7/{\mathrm{H}}^{+}$) or acidified potassium permanganate ($\mathrm{KMn}{\mathrm{O}}_4/{\mathrm{H}}^{+}$). The product depends on the class of alcohol:

• Primary alcohols ($1^{\circ }$): Oxidized first to aldehydes, then further to carboxylic acids. $\mathrm{RC}{\mathrm{H}}_2\mathrm{OH}\stackrel{[O]}{\to }\mathrm{RCHO}\stackrel{[O]}{\to }\mathrm{RCOOH}$

  Example: Ethanol → Ethanal → Ethanoic acid $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OH}\stackrel{[O]}{\to }\mathrm{C}{\mathrm{H}}_3\mathrm{CHO}\stackrel{[O]}{\to }\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}$

• Secondary alcohols ($2^{\circ }$): Oxidized to ketones only. ${\mathrm{R}}_2\mathrm{CHOH}\stackrel{[O]}{\to }{\mathrm{R}}_2\mathrm{C}=\mathrm{O}$

  Example: Propan-2-ol → Propanone $(\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{CHOH}\stackrel{[O]}{\to }(\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{C}=\mathrm{O}$

• Tertiary alcohols ($3^{\circ }$): Resist oxidation under normal conditions (no $\alpha$-hydrogen on the carbon bearing -OH).

---

(e) Dehydration of Alcohols

Dehydration is the elimination of a water molecule from an alcohol. The product depends on the reaction conditions. Common dehydrating agents include concentrated ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ or $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$.

• Formation of an Alkene (Intramolecular Dehydration): Occurs at higher temperatures (~170–180°C). $\underset{\text{Ethanol}}{\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OH}}\underset{170\text{-}180^{\circ }C}{\overset{\text{conc. }{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4}{\to }}\underset{\text{Ethene}}{\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2}+{\mathrm{H}}_2\mathrm{O}$

• Formation of an Ether (Intermolecular Dehydration): Occurs at lower temperatures (~140°C) with excess alcohol. $2\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OH}\underset{140^{\circ }C}{\overset{\text{conc. }{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4}{\to }}\underset{\text{Diethyl ether}}{\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3}+{\mathrm{H}}_2\mathrm{O}$

---

(f) Formation of Esters (Esterification)

Alcohols react with carboxylic acids in the presence of a strong acid catalyst (conc. ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$) to form esters and water. This reversible reaction is known as esterification. It involves O-H bond cleavage of the alcohol. Esters often have pleasant, fruity smells.

$\underset{\text{Carboxylic acid}}{\mathrm{RCOOH}}+\underset{\text{Alcohol}}{R^{\prime }\mathrm{OH}}\underset{\Delta }{\stackrel{\text{conc. }{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4}{\rightleftharpoons }}\underset{\text{Ester}}{R\mathrm{COO}{\mathrm{R}}^{\prime }}+{\mathrm{H}}_2\mathrm{O}$

Example: Ethanol + Ethanoic acid → Ethyl ethanoate (fruity smell) $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}+{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\underset{\Delta }{\stackrel{\text{conc. }{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4}{\rightleftharpoons }}\mathrm{C}{\mathrm{H}}_3\mathrm{COO}{\mathrm{C}}_2{\mathrm{H}}_5+{\mathrm{H}}_2\mathrm{O}$

---

Summary: Bond Cleavage in Alcohol Reactions