Alcohols, Phenols, Ethers, Thiols, and Sulfides: Structure, Properties, Preparation, and Reactions

Alcohols, Phenols, Ethers, Thiols, and Sulfides

Introduction

This chapter covers the structure, nomenclature, physical properties, preparation, and reactions of alcohols, phenols, ethers, thiols, and sulfides. These functional groups are central to organic chemistry and biological systems.

Classification and Structure

Alcohols, Phenols, and Ethers

• Alcohols: Organic derivatives of water where one hydrogen is replaced by an alkyl group. General formula: R-OH.

• Phenols: Aromatic alcohols where the hydroxyl group is attached to a benzene ring. General formula: Ar-OH.

• Ethers: Compounds where both hydrogens of water are replaced by organic groups. General formula: R-O-R'.

Thiols and Sulfides

• Thiols: Sulfur analogs of alcohols, with the general formula R-SH.

• Sulfides: Sulfur analogs of ethers, with the general formula R-S-R'.

Alcohols: Types and Classification

Types of Alcohols

• Primary Alcohols: The carbon bonded to the -OH group is attached to one other carbon.

• Secondary Alcohols: The carbon bonded to the -OH group is attached to two other carbons.

• Tertiary Alcohols: The carbon bonded to the -OH group is attached to three other carbons.

Practice Problem

Classify the following alcohols as primary, secondary, or tertiary:

• A. CH3-CH2-CH(OH)-CH3 (Secondary)

• B. CH3-CH2-CH2-OH (Primary)

• C. CH3-CH2-C(OH)(CH3)-CH3 (Tertiary)

Nomenclature

IUPAC Naming of Alcohols

• Rule 1: Select the longest chain containing the -OH group and replace the -e ending of the corresponding alkane with -ol.

• Rule 2: Number the chain from the end nearer the -OH group.

• Rule 3: Number and list substituents in alphabetical order according to their position.

Accepted Common Names

• Some alcohols have widely accepted common names, such as isopropyl alcohol and ethyl alcohol.

Phenols

Definition and Examples

• Phenol: Benzene ring with a hydroxyl group. Used in antiseptics and disinfectants.

• Resorcinol: Benzene ring with two hydroxyl groups. Used in treatment of skin disorders and infections.

Diols

Types of Diols

• Vicinal Diols: Hydroxyl groups on adjacent carbons.

• Geminal Diols: Hydroxyl groups on the same carbon.

• Example: Ethylene glycol (HOCH2CH2OH) is a common vicinal diol used as antifreeze.

Biologically Important Alcohols

Examples

• Cholesterol, Estradiol, and Testosterone all contain alcohol functional groups and play vital roles in biological systems.

Physical Properties of Alcohols

Boiling Points

• Alcohols and phenols have higher boiling points than hydrocarbons due to hydrogen bonding.

• Example: Methanol (65°C) vs. Methane (-183°C); Ethanol (78°C) vs. Ethane (-164°C).

Solubility

• Low molecular weight (LMW) alcohols are soluble in water; high molecular weight (HMW) alcohols are less soluble.

• Example: Ethanol is completely miscible with water, while octanol is only slightly soluble.

• As the number of carbons increases, solubility in water decreases.

Preparation of Alcohols

Central Role in Organic Synthesis

• Alcohols are intermediates in the synthesis of alkenes, ketones, aldehydes, ethers, esters, carboxylic acids, and alkyl halides.

Methods of Preparation

1. Acid-catalyzed hydration of alkenes: Water adds across the double bond to form alcohols. $RCH=CH_2 + H_2O ightarrow RCH(OH)CH_3$

2. Reduction of carbonyl compounds:

   • Reduction of aldehydes and ketones forms alcohols. $RCHO + H_2 ightarrow RCH_2OH$ (Primary alcohol from aldehyde) $RCOR' + H_2 ightarrow RCH(OH)R'$ (Secondary alcohol from ketone)

   • Reduction of carboxylic acids and esters forms primary alcohols. $RCOOH + 4[H] ightarrow RCH_2OH + H_2O$ $RCOOR' + 4[H] ightarrow RCH_2OH + R'OH$ Reducing agent: LiAlH4 (Lithium aluminum hydride)

3. Diol formation: Water adds to carbonyl groups in the presence of acid or base to form diols. $O=C + H_2O ightarrow HO-C-OH$

Reactions of Alcohols

Overview

1. Acid/Base properties

2. Reaction with hydrogen halides

3. Dehydration (Zaitsev's rule)

4. Oxidation

5. Esterification

1. Acid/Base Properties

• Alcohols are slightly acidic compared to water.

• Alcohols can donate a proton to water, generating hydronium ion ($H_3O^+$) and an alkoxide ion ($RO^-$).

• Equation: $ROH + H_2O ightleftharpoons RO^- + H_3O^+$

2. Reaction with Hydrogen Halides

• Tertiary alcohols react readily with HCl and HBr to form alkyl halides.

• Primary and secondary alcohols react poorly under these conditions.

3. Dehydration (Elimination Reaction)

• Alcohols undergo elimination to form alkenes, losing H and OH from adjacent carbons.

• Catalyst: Concentrated $H_2SO_4$ or $H_3PO_4$.

• Equation: $RCH_2CH_2OH ightarrow RCH=CH_2 + H_2O$

• Zaitsev's Rule: When multiple alkene products are possible, the most substituted (more stable) alkene is the major product. This is called regioselectivity.

4. Oxidation

• Oxidation is the addition of oxygen or removal of hydrogen.

• Primary alcohols: Oxidized to aldehydes (moderate oxidizer, PCC) or carboxylic acids (strong oxidizer, chromic acid).

• Secondary alcohols: Oxidized to ketones.

• Tertiary alcohols: Do not oxidize (no reaction).

• Equations: Primary alcohol to aldehyde: $RCH_2OH ightarrow RCHO$ Aldehyde to carboxylic acid: $RCHO ightarrow RCOOH$ Secondary alcohol to ketone: $R_2CHOH ightarrow R_2CO$

5. Fischer Esterification

• Alcohol reacts with carboxylic acid in the presence of acid catalyst ($H_2SO_4$) to produce an ester and water.

• Equation: $ROH + R'COOH ightleftharpoons R'COOR + H_2O$

• Example: Benzoic acid + methanol $ightarrow$ methyl benzoate + water

Summary Table: Alcohol Oxidation

Alcohol Type	Oxidation Product (Moderate)	Oxidation Product (Strong)
Primary	Aldehyde	Carboxylic acid
Secondary	Ketone	Ketone
Tertiary	No reaction	No reaction

Additional info:

• Alcohol metabolism in the body occurs via oxidation in the liver, producing toxic intermediates that can cause illness when consumed in excess.

• Common reducing agents for laboratory preparation include LiAlH4 and NaBH4 (for milder reductions).