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

Alcohols, Phenols, Thiols, and Ethers

Introduction to Functional Groups

Organic molecules are often classified by their functional groups, which are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In this chapter, we focus on four important functional groups: alcohols, phenols, thiols, and ethers.

• Alcohols contain a hydroxyl group (–OH) attached to a saturated carbon atom (sp3 hybridized).

• Phenols have a hydroxyl group attached directly to an aromatic benzene ring.

• Thiols contain a sulfhydryl group (–SH), analogous to alcohols but with sulfur replacing oxygen.

• Ethers have an oxygen atom connected to two alkyl or aryl groups (R–O–R').

These functional groups are common in biological molecules, industrial chemicals, and household products.

Overview of Functional Groups

The following table summarizes some common functional groups relevant to this chapter:

Functional Group	Classification	Example	Structure
Alcohol	R–OH	Methanol	CH3–OH
Phenol	Aromatic –OH	Phenol
Thiols	R–SH	Methanethiol	CH3–SH
Ether	R–O–R'	Diethyl ether	CH3CH2–O–CH2CH3

Additional info: Table structure inferred from provided image and standard textbook tables.

Structure and Classification

Alcohols

In alcohols, the –OH group replaces a hydrogen atom in a hydrocarbon. Alcohols are classified based on the number of alkyl groups attached to the carbon bearing the –OH group:

• Primary (1°) alcohol: The carbon with the –OH is attached to one other carbon.

• Secondary (2°) alcohol: The carbon with the –OH is attached to two other carbons.

• Tertiary (3°) alcohol: The carbon with the –OH is attached to three other carbons.

Phenols

Phenols have the –OH group directly bonded to a benzene ring. This structure imparts unique chemical properties, such as increased acidity compared to alcohols.

Thiols

Thiols are similar to alcohols but contain a sulfur atom in place of oxygen. The –SH group gives thiols strong, often unpleasant odors (e.g., garlic, onions).

Ethers

Ethers have an oxygen atom bonded to two carbon atoms (alkyl or aryl groups). The general formula is R–O–R'. Ethers are relatively unreactive and often used as solvents.

Nomenclature

Naming Alcohols

• Replace the -e ending of the parent alkane with -ol.

• Number the carbon chain to give the –OH group the lowest possible number.

• For multiple –OH groups, use diol (two) or triol (three).

• Cyclic alcohols are named as cycloalkanols; number the ring from the carbon attached to the –OH group.

Example: CH3CH2OH is named ethanol.

Naming Phenols

• The parent name is phenol.

• Number the ring starting from the carbon bonded to the –OH group.

• Use ortho- (o-), meta- (m-), and para- (p-) for common names when two substituents are present.

Example: 2-bromophenol (o-bromophenol)

Naming Thiols

• Add thiol to the alkane name.

• Number the chain from the end nearest the –SH group.

Example: CH3SH is methanethiol.

Naming Ethers

• Common names: List the alkyl groups attached to oxygen in alphabetical order, followed by "ether" (e.g., methyl propyl ether).

• IUPAC names: Name the smaller group as an alkoxy substituent (e.g., methoxy, ethoxy) and the larger group as the parent alkane.

Example: CH3OCH2CH2CH3 is 1-methoxypropane.

Physical Properties

Hydrogen Bonding and Boiling Points

• Alcohols and phenols can form hydrogen bonds due to the –OH group, resulting in higher boiling points than alkanes and ethers of similar molar mass.

• Ethers cannot hydrogen bond with themselves, so their boiling points are lower than alcohols but higher than alkanes.

• Thiols have lower boiling points than alcohols due to weaker S–H bonds compared to O–H bonds.

Solubility in Water

• Alcohols and ethers with up to four carbon atoms are soluble in water due to hydrogen bonding with water molecules.

• Solubility decreases as the hydrocarbon chain length increases.

• Phenol is slightly soluble in water and can ionize slightly to form phenoxide ions.

Chemical Reactions

Combustion

Alcohols combust in oxygen to produce carbon dioxide and water:

$\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$

Dehydration of Alcohols

Alcohols can lose a water molecule (dehydration) in the presence of acid and heat to form alkenes:

$\text{R–CH}_2\text{CH}_2\text{OH} \xrightarrow{\text{H}_2\text{SO}_4,\ 180^\circ\text{C}} \text{R–CH}=\text{CH}_2 + \text{H}_2\text{O}$

Oxidation of Alcohols and Thiols

• Primary alcohols oxidize to aldehydes, then to carboxylic acids:

• $\text{R–CH}_2\text{OH} \xrightarrow{[O]} \text{R–CHO} \xrightarrow{[O]} \text{R–COOH}$

• Secondary alcohols oxidize to ketones:

• $\text{R}_2\text{CH–OH} \xrightarrow{[O]} \text{R}_2\text{C=O}$

• Tertiary alcohols do not oxidize under mild conditions.

• Thiols oxidize to form disulfides:

• $2\ \text{R–SH} \xrightarrow{[O]} \text{R–S–S–R} + 2\ \text{H}^+$

Applications and Health Connections

• Methanol is toxic and can cause blindness or death if ingested.

• Ethanol is the alcohol found in beverages and is also used as a solvent and fuel.

• Ethylene glycol (antifreeze) and glycerol (in cosmetics) are important polyalcohols.

• Phenol was historically used as an antiseptic but is now replaced by safer compounds.

• Thiols are added to natural gas for leak detection due to their strong odor.

• Ethers such as diethyl ether were used as anesthetics but have largely been replaced due to flammability.

Summary Table: Properties of Alcohols, Phenols, Thiols, and Ethers

Compound Type	Functional Group	Hydrogen Bonding	Boiling Point	Water Solubility
Alcohol	–OH	Yes (strong)	High	High (short chains)
Phenol	–OH on benzene	Yes	High	Slightly soluble
Thiols	–SH	No (weak S–H)	Low	Low
Ether	R–O–R'	No (with self), Yes (with water)	Intermediate	Moderate (short chains)

Additional info: Table content inferred and expanded for clarity and completeness.