Alcohols & Phenols: Oxidation, Reactions, and Biological Importance

Alcohols and Phenols: Oxidation and Reactions

Introduction

This study guide covers the oxidation of alcohols and phenols, their chemical reactions, biological significance, and their role as antimicrobial agents. These topics are central to organic chemistry and pharmaceutical applications.

Oxidation of Alcohols

Types of Alcohols and Their Oxidation Products

• Primary Alcohols (1°): Oxidation yields aldehydes and, with further oxidation, carboxylic acids.

• Secondary Alcohols (2°): Oxidation yields ketones.

• Tertiary Alcohols (3°): Generally resistant to oxidation due to lack of hydrogen atoms on the carbon bearing the hydroxyl group.

Common Oxidizing Agents

• Pyridinium Chlorochromate (PCC): Selectively oxidizes primary alcohols to aldehydes.

• Chromic Acid (H2CrO4): Oxidizes primary alcohols to carboxylic acids and secondary alcohols to ketones.

• Sodium Dichromate and Sulfuric Acid: Used for strong oxidation.

Summary Table: Alcohol Oxidations

To Oxidize	Product	Reagent
1° alcohol	aldehyde	PCC
1° alcohol	acid	chromic acid
2° alcohol	ketone	chromic acid (or PCC)

Oxidation Equations

• Primary alcohol to aldehyde: $\ce{RCH2OH} \xrightarrow{[O]} \ce{RCHO}$

• Primary alcohol to carboxylic acid: $\ce{RCH2OH} \xrightarrow{[O]} \ce{RCOOH}$

• Secondary alcohol to ketone: $\ce{R2CHOH} \xrightarrow{[O]} \ce{R2CO}$

Biological Oxidation of Alcohols

Hepatic Enzymatic Oxidation

• Alcohol Dehydrogenase (ADH): Converts ethanol to acetaldehyde in the liver.

• Nicotinamide Adenine Dinucleotide (NAD+): Acts as an oxidizing agent, reduced to NADH.

• Aldehyde Dehydrogenase: Converts acetaldehyde to acetic acid.

Equations:

• $\ce{CH3CH2OH} + \ce{NAD^+} \xrightarrow{ADH} \ce{CH3CHO} + \ce{NADH} + \ce{H^+}$

• $\ce{CH3CHO} + \ce{NAD^+} \xrightarrow{Aldehyde\ Dehydrogenase} \ce{CH3COOH} + \ce{NADH} + \ce{H^+}$

Toxicity and Poisoning of Alcohols

Toxic Effects

• High blood alcohol concentration (>0.4%): Can be fatal.

• Methanol Poisoning: Metabolized to formic acid, causing blindness and death.

• Ethylene Glycol Poisoning: Metabolized to oxalic acid, causing renal failure.

Treatment of Alcohol Poisoning

• Intravenous diluted ethanol: Used as an antidote for methanol poisoning by competitive inhibition of alcohol dehydrogenase.

• Enhanced excretion: Methanol is excreted when ethanol is administered.

• Chronic alcoholics: Require higher doses of ethanol due to enzyme induction.

Oxidation of Phenols

Products of Phenol Oxidation

• Benzoquinone: Formed from hydroquinone.

• Catechols: Oxidized to ortho-quinones (e.g., 3-methyl-1,2-benzoquinone).

Equation:

• $\ce{Hydroquinone} \xrightarrow{[O]} \ce{p-Benzoquinone} + 2\ce{H^+}$

Biologically Important Quinones

• Ubiquinone (Coenzyme Q): Involved in electron transport.

• Vitamin K: Essential for blood clotting.

Reactions of Phenols

Bromination

• Ortho, para-directing: Phenol directs substitution to ortho and para positions.

• 2,4,6-tribromophenol: Formed with bromine in aqueous NaOH.

• Para-bromophenol: Formed with carbon disulfide at low temperature.

Nitration

• o- and p-nitrophenol: Formed by nitration; ortho is more volatile due to intramolecular H-bonding.

• Nitration of 4-methyl-phenol: Yields 4-methyl-2-nitrophenol (ortho product favored by electron-donating groups).

• Nitration of 4-hydroxy-benzoic acid: OH group directs ortho/para, COOH directs meta.

Sulfonation

• Ortho product: Favored at low temperature (kinetic control).

• Para product: Favored at high temperature (thermodynamic control).

Phenolic Antioxidants

Role and Examples

• Prevent rancidity: Used in oils, fats, and pharmaceutical products.

• Structures: Include butylated hydroxytoluene (BHT) and tocopherols (Vitamin E).

Naturally Occurring Phenols

Types and Functions

• Phenolic phytochemicals: Found in plants, often as polyphenols.

• Applications: Antiseptics, thyroid gland regulation, antibiotics.

Alcohols and Phenols as Antimicrobial Agents

Modes of Action

• Protein denaturation: High concentrations of phenol and alcohol disrupt bacterial proteins.

• Membrane disruption: Alcohols and phenols increase membrane permeability, leading to cell death.

• Effective concentrations: 70%–75% ethyl or isopropyl alcohol commonly used as disinfectants.

External Learning Resources

• Khan Academy: Biological Oxidation of Alcohols

• ScienceDirect: Biological Oxidation of Alcohols

• LibreTexts: Electrophilic Substitution of Phenols

Summary

• Alcohols and phenols undergo characteristic oxidation and substitution reactions.

• Biological oxidation is crucial for metabolism and toxicity.

• Phenols play important roles as antioxidants and antimicrobials.

Additional info: Some chemical structures and mechanistic details were inferred for completeness and clarity.