Chapter 11: Reactions of Alcohols – Oxidation, Substitution, and Dehydration

Reactions of Alcohols

Overview

This chapter covers the major chemical reactions of alcohols, including oxidation, formation of tosylates, dehydration, and substitution reactions with hydrohalic acids, phosphorus halides, and thionyl chloride. Understanding these transformations is essential for organic synthesis and functional group interconversion.

• Oxidation of Alcohols

• Formation of Tosylates

• Dehydration of Alcohols

• Reactions with Hydrohalic Acids

• Reactions with Phosphorus Halides

• Reactions with Thionyl Chloride

Oxidation of Alcohols

Oxidation and Reduction: Definitions and Overview

Oxidation and reduction in organic chemistry involve changes in electron density, typically through the addition or removal of oxygen, hydrogen, or halogens.

• Oxidation: Loss of electron density (addition of O, O2, X2 (halogens); loss of H2).

• Reduction: Gain of electron density (loss of O, O2, X2; addition of H2 or H-).

Alcohol Oxidation Pathways:

• Primary alcohols: Can be oxidized to aldehydes, then further to carboxylic acids.

• Secondary alcohols: Oxidized to ketones; no further simple oxidation.

• Tertiary alcohols: No simple oxidation occurs.

Common Oxidizing Agents

• Strong oxidizers: Na2Cr2O7 (sodium dichromate), CrO3 (chromium trioxide), H2CrO4 (chromic acid), KMnO4 (potassium permanganate)

• Mild oxidizer: Pyridinium chlorochromate (PCC)

Key Reactions:

• Primary alcohol + strong oxidizer → carboxylic acid

• Primary alcohol + PCC → aldehyde

• Secondary alcohol + oxidizer → ketone

Example Equations:

• $\ce{R-CH2OH} \xrightarrow{[O]} \ce{R-CHO} \xrightarrow{[O]} \ce{R-COOH}$

• $\ce{R2CHOH} \xrightarrow{[O]} \ce{R2CO}$

Mechanism of Alcohol Oxidation (Chromic Acid)

• Chromic acid is generated in situ from sodium dichromate and sulfuric acid:

• $\ce{Na2Cr2O7 + H2SO4 + H2O \rightarrow 2 H2CrO4 + 2 NaHSO4}$

• Alcohol reacts with chromic acid to form a chromate ester intermediate.

• Chromate ester undergoes elimination to yield the carbonyl compound (aldehyde or ketone).

Formation of Tosylates

Improving the Leaving Group Ability of Alcohols

The hydroxyl group (OH) is a poor leaving group in nucleophilic substitution and elimination reactions. It can be converted into a tosylate (a sulfonate ester) to improve its leaving group ability.

• Tosylation: Alcohol reacts with p-toluenesulfonyl chloride (TsCl) to form a tosylate ester (ROTs).

• Reaction: $\ce{R-OH + TsCl \rightarrow R-OTs + HCl}$

• Tosylate esters are good leaving groups in subsequent substitution or elimination reactions.

Dehydration of Alcohols

Conversion to Alkenes

Alcohols can be dehydrated to form alkenes, typically using a strong acid catalyst such as sulfuric acid.

• General Reaction: $\ce{R-CH2CH2OH} \xrightarrow{cat.\ H2SO4} \ce{R-CH=CH2} + H2O$

• The major product is usually the more substituted (Zaitsev) alkene.

• If a carbocation intermediate forms, rearrangement may occur to yield a more stable carbocation and thus a more substituted alkene.

Reactions of Alcohols with Hydrohalic Acids

Substitution to Form Alkyl Halides

Alcohols react with hydrohalic acids (HBr, HCl, HI) to form alkyl halides. The reaction mechanism and yield depend on the alcohol and acid used.

• General Reactions:

• $\ce{R-OH + HBr \rightarrow R-Br + H2O}$

• $\ce{R-OH + HCl/ZnCl2 \rightarrow R-Cl + H2O}$ (Lucas reagent: HCl + ZnCl2)

• $\ce{R-OH + HI \rightarrow R-I + H2O}$

• Elimination and rearrangement may occur, especially with secondary and tertiary alcohols.

Reactions of Alcohols with Phosphorus Halides

Conversion to Alkyl Halides

Alcohols can be converted to alkyl halides using phosphorus halides (PCl3, PBr3, PI3). These reactions are effective for primary and secondary alcohols.

• General Reactions:

• $\ce{R-OH + PCl3 \rightarrow R-Cl}$

• $\ce{R-OH + PBr3 \rightarrow R-Br}$

• $\ce{R-OH + PI3 \rightarrow R-I}$

• PI3 is generated in situ from P and I2.

Reactions of Alcohols with Thionyl Chloride

Conversion to Alkyl Chlorides

Thionyl chloride (SOCl2) is commonly used to convert alcohols to alkyl chlorides. The reaction produces alkyl chloride, sulfur dioxide, and hydrogen chloride as byproducts.

• General Reaction: $\ce{R-OH + SOCl2 \rightarrow R-Cl + SO2 + HCl}$

• This method is efficient and often used in organic synthesis.

Summary Table: Alcohol Reactions

Reaction Type	Reagents	Product	Notes
Oxidation	Na2Cr2O7, CrO3, PCC, KMnO4	Aldehyde, Ketone, Carboxylic Acid	PCC for aldehyde; strong oxidizers for carboxylic acid
Tosylation	TsCl	Tosylate ester (ROTs)	Improves leaving group ability
Dehydration	H2SO4 (cat.)	Alkene	Major product is more substituted alkene
Substitution (Hydrohalic Acid)	HBr, HCl/ZnCl2, HI	Alkyl halide	Lucas reagent for HCl; elimination/rearrangement possible
Substitution (Phosphorus Halide)	PCl3, PBr3, PI3	Alkyl halide	Works well for primary/secondary alcohols
Substitution (Thionyl Chloride)	SOCl2	Alkyl chloride	Efficient, clean reaction

Example Syntheses

• Convert 1-butanol to butanoic acid: Use a strong oxidizer such as Na2Cr2O7/H2SO4.

• Convert 1-butanol to butanal: Use PCC as the oxidizing agent.

Additional info: Rearrangement and elimination can occur during substitution and dehydration reactions, especially with secondary and tertiary alcohols. The choice of reagent and reaction conditions is crucial for controlling the product outcome.