Chapter 13: Alcohols, Phenols, Thiols, and Ethers – Study Guide

Compounds with Hydroxyl Group

Alcohols: Features and Classification

Alcohols are organic compounds containing one or more hydroxyl (-OH) groups attached to a carbon atom. Their structure, naming, and classification are fundamental to understanding their chemical behavior.

• Features of Alcohols: Alcohols have the general formula R-OH, where R is an alkyl group.

• Naming Alcohols: The name is based on the longest carbon chain containing the hydroxyl group, with the suffix -ol. Number the chain to give the hydroxyl group the lowest possible number.

• Multiple Hydroxyl Groups: Compounds with more than one hydroxyl group use prefixes such as diol or triol (e.g., ethylene glycol is ethane-1,2-diol).

• Classification: Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of alkyl groups attached to the carbon bearing the hydroxyl group.

Type	Structure	Oxidation Ability
Primary (1°)	R-CH2-OH	Can be oxidized
Secondary (2°)	R2CH-OH	Can be oxidized
Tertiary (3°)	R3C-OH	Cannot be oxidized

Example: Isopropanol (2-propanol) is a secondary alcohol.

Oxidation of Alcohols

Oxidation reactions of alcohols depend on their classification. Primary and secondary alcohols can be oxidized, while tertiary alcohols cannot.

• Primary Alcohols: Oxidize to form aldehydes, then carboxylic acids.

• Secondary Alcohols: Oxidize to form ketones.

• Tertiary Alcohols: Resistant to oxidation due to lack of hydrogen on the carbon with the hydroxyl group.

General Oxidation Equations:

• Primary: $\mathrm{R{-}CH_2OH \xrightarrow{[O]} R{-}CHO \xrightarrow{[O]} R{-}COOH}$

• Secondary: $\mathrm{R_2CH{-}OH \xrightarrow{[O]} R_2C{=}O}$

Structure, Properties, and Reactions of Alcohols

Physical and Chemical Properties

The physical properties of alcohols are influenced by the presence of the hydroxyl group, which enables hydrogen bonding.

• Hydrogen Bonding: Alcohols can form strong hydrogen bonds, especially primary alcohols, leading to higher boiling and melting points compared to similar alkanes.

• Effect of Molecular Size: As the size and mass of the alcohol molecule increase, so do the melting and boiling points.

• Solubility: Small alcohols (e.g., methanol, ethanol) are soluble in water; larger alcohols become less soluble.

Example: Methanol (CH3OH) is highly soluble in water due to its small size and strong hydrogen bonding.

Reactions of Alcohols

• Dehydration: Alcohols can lose water to form alkenes, typically in the presence of acid and heat.

• Oxidation: As described above, primary and secondary alcohols undergo oxidation to form carbonyl-containing compounds.

Dehydration Equation:

• $\mathrm{R{-}CH_2{-}CH_2OH \xrightarrow{H_2SO_4,\ heat} R{-}CH{=}CH_2 + H_2O}$

Phenols: Structure, Properties, and Reactions

Physical and Chemical Properties of Phenols

Phenols are compounds where the hydroxyl group is attached directly to an aromatic ring. They exhibit unique properties compared to alcohols.

• Acidity: Phenols are more acidic than alcohols and can react with strong bases to form phenoxide ions.

• Hydrogen Bonding: Phenols also form hydrogen bonds, but their aromatic ring affects solubility and reactivity.

• Reactivity: Phenols undergo acid-base reactions and can participate in electrophilic aromatic substitution.

Example: Phenol reacts with sodium hydroxide to form sodium phenoxide and water.

Ethers: Structure, Properties, and Reactions

Naming and Properties of Ethers

Ethers are compounds with an oxygen atom connected to two alkyl or aryl groups (R-O-R'). Their naming and properties are distinct from alcohols and phenols.

• Naming: Name each alkyl group attached to the oxygen, followed by "ether" (e.g., ethyl methyl ether).

• Physical Properties: Ethers have lower boiling points than alcohols due to lack of hydrogen bonding between molecules.

• Chemical Properties: Ethers are generally unreactive, but can be cleaved by strong acids.

Example: Diethyl ether (CH3CH2-O-CH2CH3) is a common solvent.

Preparation and Reactions of Ethers

• Preparation: Ethers can be synthesized by dehydration of alcohols or by Williamson ether synthesis.

• Reactions: Ethers are stable but can react with strong acids to form alcohols and alkyl halides.

Williamson Ether Synthesis Equation:

• $\mathrm{R{-}X + R'{-}O^- \rightarrow R{-}O{-}R' + X^-}$

Thiols and Sulfides: Structure, Properties, and Reactions

Naming and Properties of Thiols

Thiols are sulfur analogs of alcohols, containing a -SH group. Sulfides are sulfur analogs of ethers.

• Naming Thiols: Add "thiol" to the name of the parent hydrocarbon (e.g., ethanethiol).

• Physical Properties: Thiols have lower boiling points than alcohols and often possess strong odors.

• Chemical Properties: Thiols can be oxidized to form disulfides.

Example: Ethanethiol (CH3CH2SH) is used as an odorant in natural gas.

Compound	Functional Group	Properties
Alcohol	-OH	Hydrogen bonding, higher boiling point
Phenol	-OH on aromatic ring	Acidic, reacts with bases
Ether	R-O-R'	Lower boiling point, unreactive
Thiols	-SH	Strong odor, oxidized to disulfides

Additional info: Sulfides (R-S-R') are similar to ethers but contain sulfur instead of oxygen. They are less common in biological systems but important in some industrial processes.