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

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Organic Compounds Containing Oxygen and Sulfur

Introduction to Alcohols, Phenols, Thiols, and Ethers

Organic compounds containing oxygen and sulfur are fundamental in both biological and industrial chemistry. This chapter focuses on alcohols, phenols, thiols, and ethers, exploring their structures, nomenclature, properties, and reactions.

Alcohols

Definition and Structure

Alcohols are organic compounds in which a hydroxyl group (–OH) is bonded to a saturated, alkane-like carbon atom. The general formula is R–OH, where R is an alkyl group.

Primary (1°) alcohol: –OH group attached to a carbon bonded to one other carbon.
Secondary (2°) alcohol: –OH group attached to a carbon bonded to two other carbons.
Tertiary (3°) alcohol: –OH group attached to a carbon bonded to three other carbons.

Ball-and-stick models of ethanol, diethyl ether, ethanethiol

Common Alcohols and Their Uses

Methanol (CH3OH): Also known as wood alcohol; toxic and used as a solvent and fuel.
Ethanol (CH3CH2OH): Found in alcoholic beverages and used as a solvent and fuel.
Isopropyl alcohol ((CH3)2CHOH): Used as rubbing alcohol and antiseptic.
Ethylene glycol (HOCH2CH2OH): Used as antifreeze.
Glycerol (HOCH2CH(OH)CH2OH): Used in cosmetics and food.

Methyl alcohol (methanol) ball-and-stick model Ethyl alcohol (ethanol) ball-and-stick model Isopropyl alcohol ball-and-stick model Ethylene glycol ball-and-stick model Glycerol ball-and-stick model

Nomenclature of Alcohols

Alcohols are named by replacing the -e ending of the parent alkane with -ol. The position of the –OH group is indicated by a number. For common names, the alkyl group is named first, followed by 'alcohol'.

Number the longest carbon chain containing the –OH group from the end nearest the –OH.
Indicate the position of the –OH group and any substituents.

Guide to Naming Alcohols Examples of IUPAC names for alcohols

Physical Properties of Alcohols

Alcohols are more polar than hydrocarbons due to the electronegative oxygen atom. They can form hydrogen bonds, resulting in higher boiling points and greater solubility in water compared to alkanes of similar molecular mass.

Hydrogen bonding increases boiling point and water solubility.
Alcohols with more –OH groups are even more soluble and have higher boiling points.

Comparison of boiling points: alcohol, ether, thiol

Phenols

Definition and Structure

Phenols are compounds in which a hydroxyl group is directly bonded to an aromatic benzene ring. Phenol itself (C6H5OH) is a well-known antiseptic and precursor to many important chemicals.

Phenols are more acidic than alcohols.
Substituted phenols are named by indicating the position and type of substituent.

Thymol, a phenol in Listerine

Nomenclature of Phenols

Number the ring to give the substituents the lowest possible numbers.
Use prefixes o- (ortho), m- (meta), and p- (para) for common names.

Phenol, 3-chlorophenol, 3-methylphenol structures

Properties and Uses of Phenols

Phenols are used as antiseptics and disinfectants.
They are found in essential oils and flavorings (e.g., thymol, vanillin).

Ethers

Definition and Structure

Ethers are compounds in which an oxygen atom is bonded to two organic groups (R–O–R'). Ethers are less polar than alcohols and do not form hydrogen bonds with themselves.

Common ethers: diethyl ether (CH3CH2OCH2CH3), used as an anesthetic.

Ball-and-stick models of ethanol, diethyl ether, ethanethiol

Nomenclature of Ethers

Common names: Name each R group alphabetically, followed by 'ether'.
IUPAC names: The larger group is the parent alkane; the smaller group plus oxygen is named as an alkoxy substituent.

Physical Properties of Ethers

Ethers have lower boiling points than alcohols of similar mass.
They are more soluble in water than alkanes but less than alcohols.

Thiols and Disulfides

Definition and Structure

Thiols (mercaptans) are sulfur analogs of alcohols, containing an –SH group. Disulfides contain an S–S bond and are formed by oxidation of thiols.

Thiols are named by adding '-thiol' to the parent hydrocarbon name.
They are known for their strong, often unpleasant odors.

Ball-and-stick models of ethanol, diethyl ether, ethanethiol

Biological Importance

Disulfide bonds (S–S) are crucial for protein structure (e.g., in hair and enzymes).
Thiols are used as odorants in natural gas for leak detection.

Reactions of Alcohols

Dehydration

Alcohols can undergo dehydration (loss of water) in the presence of a strong acid and heat to form alkenes. The major product is determined by Zaitsev’s Rule: the more substituted alkene is favored.

General reaction:

Oxidation

Alcohols can be oxidized to different products depending on their classification:

Primary alcohols → Aldehydes → Carboxylic acids
Secondary alcohols → Ketones
Tertiary alcohols do not oxidize easily

Common oxidizing agents: potassium permanganate (KMnO4), dichromate (K2Cr2O7).

Acidity of Alcohols and Phenols

Alcohols are very weak acids, but phenols are much more acidic due to resonance stabilization of the phenoxide ion. Phenols can react with bases to form phenoxide salts.

Summary Table: Comparison of Alcohols, Ethers, and Thiols

Class	General Structure	Key Properties	Example
Alcohol	R–OH	Hydrogen bonding, high boiling point, polar	Ethanol
Ether	R–O–R'	Lower boiling point, less polar, no H-bonding between molecules	Diethyl ether
Thiol	R–SH	Strong odor, less polar than alcohols	Ethanethiol

Concept Map

This chapter connects the structure, nomenclature, properties, and reactions of alcohols, phenols, thiols, and ethers, providing a foundation for understanding their roles in chemistry and biology.