Alcohols, Phenols, Thiols, and Ethers – CHM 105 Chapter 13 Study Notes

Alcohols, Phenols, Thiols, and Ethers

Introduction

This chapter explores the structure, nomenclature, physical properties, and chemical reactions of alcohols, phenols, thiols, and ethers. These functional groups are fundamental in organic and biological chemistry, with wide-ranging applications in industry and health sciences.

13.1 Alcohols, Phenols, and Thiols

Functional Groups and Definitions

• Alcohols: Organic compounds containing a hydroxyl group (–OH) attached to a saturated carbon atom.

• Phenols: Compounds where a hydroxyl group (–OH) is bonded directly to a benzene ring.

• Thiols: Organic compounds containing a sulfhydryl group (–SH) attached to a carbon atom.

Example: Methanol (CH3OH), Phenol (C6H5OH), Ethanethiol (CH3CH2SH)

Naming Alcohols

• In the IUPAC system, replace the -e ending of the parent alkane with -ol.

• Common names use the alkyl group name followed by "alcohol" (e.g., methyl alcohol).

Formula	IUPAC Name	Common Name
CH4	methane
CH3OH	methanol	methyl alcohol
CH3CH3	ethane
CH3CH2OH	ethanol	ethyl alcohol

• For alcohols with three or more carbons, number the chain to give the lowest number to the –OH group.

• Alcohols with two –OH groups are called diols; with three, triols.

Example: 1,2-Ethanediol (ethylene glycol) is a diol.

Naming Cyclic Alcohols

• Cyclic alcohols are named as cycloalkanols.

• If substituents are present, numbering starts at the carbon attached to the –OH group.

• No number is needed for the –OH group if there are no other substituents.

Example: Cyclohexanol, 2-methylcyclopentanol

Naming Phenols

• The term phenol is the IUPAC name for a benzene ring with a hydroxyl group.

• Number the ring starting at the carbon bonded to –OH for additional substituents.

• Common names use ortho (1,2-), meta (1,3-), and para (1,4-) for disubstituted phenols.

• Cresol is the common name for methylphenols.

Example: 2-chlorophenol (ortho-chlorophenol), 4-methylphenol (para-cresol)

Thiols

• Contain a –SH group (sulfhydryl group).

• Often have strong, disagreeable odors (e.g., skunk spray, garlic, onions).

• Used to detect gas leaks due to their odor.

Naming Thiols

• In IUPAC, add thiol to the alkane name and number the chain from the end nearest the –SH group.

• Also known as mercaptans.

Example: Ethanethiol, 1,2-ethanedithiol

13.2 Ethers

Structure and Nomenclature

• An ether contains an oxygen atom connected by single bonds to two carbon groups (alkyl or aromatic).

• Common names list the alkyl groups in alphabetical order followed by "ether" (e.g., ethyl methyl ether).

• IUPAC names use the smaller alkyl group as an alkoxy substituent on the main chain (e.g., methoxypropane).

Example: CH3OCH2CH2CH3 is methoxypropane (IUPAC) or methyl propyl ether (common).

13.3 Physical Properties of Alcohols, Phenols, and Ethers

Classification of Alcohols

• Classified by the number of alkyl groups attached to the carbon bonded to the –OH group:

  • Primary (1°): One alkyl group

  • Secondary (2°): Two alkyl groups

  • Tertiary (3°): Three alkyl groups

Boiling Points

• Alcohols have higher boiling points than ethers and alkanes of similar mass due to hydrogen bonding between –OH groups.

• Ethers cannot form hydrogen bonds with themselves, so their boiling points are similar to alkanes.

Compound	Condensed Structural Formula	Boiling Point (°C)	Solubility in Water
Methanol	CH3OH	65	Soluble
Ethanol	CH3CH2OH	78	Soluble
Diethyl ether	CH3CH2OCH2CH3	35	Soluble
Butanol	CH3CH2CH2CH2OH	117	Slightly soluble
Hexanol	CH3(CH2)5OH	157	Insoluble

Additional info: Table values inferred from standard chemistry data.

Solubility in Water

• Alcohols with 1–3 carbons are soluble in water due to hydrogen bonding.

• Solubility decreases as the carbon chain length increases.

• Ethers are slightly soluble if they have fewer than four carbons; insoluble with five or more.

• Phenols are slightly soluble and can ionize in water to form phenoxide ions, acting as weak acids ($K_a = 1 \times 10^{-10}$).

Example: Methanol and ethanol are highly soluble; hexanol is not.

13.4 Reactions of Alcohols and Thiols

Combustion of Alcohols

• Alcohols combust in oxygen to produce carbon dioxide, water, and energy.

Example equation:

$\mathrm{CH_3CH_2OH(g) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g) + \text{energy}}$

Dehydration of Alcohols

• Alcohols lose –H and –OH from adjacent carbons (with acid and heat) to form alkenes and water.

• The major product is formed by removing hydrogen from the carbon with fewer hydrogens (Zaitsev's rule).

General equation:

$\text{Alcohol} \xrightarrow{\text{H}^+,\ \text{heat}} \text{Alkene} + H_2O$

Oxidation and Reduction

• Oxidation: Increases the number of C–O bonds (addition of O or loss of H).

• Reduction: Decreases the number of C–O bonds (addition of H or loss of O).

Oxidation of Alcohols

• Primary (1°) alcohols oxidize to aldehydes, then to carboxylic acids.

• Secondary (2°) alcohols oxidize to ketones.

• Tertiary (3°) alcohols do not oxidize under normal conditions.

Example equations:

$\text{1° Alcohol} \xrightarrow{[O]} \text{Aldehyde} \xrightarrow{[O]} \text{Carboxylic Acid}$ $\text{2° Alcohol} \xrightarrow{[O]} \text{Ketone}$

Oxidation of Thiols

• Two thiol (–SH) groups oxidize to form a disulfide (–S–S–) bond and water.

Example equation:

$2\ \mathrm{RSH} + [O] \rightarrow \mathrm{RSSR} + H_2O$

Disulfide bonds are important in protein structure (e.g., cysteine cross-links in hair).

Summary Table: Key Properties and Reactions

Class	Functional Group	Naming Suffix	Key Reaction
Alcohol	–OH	-ol	Oxidation, Dehydration, Combustion
Phenol	–OH on benzene	phenol	Weak acid, forms phenoxide
Thiol	–SH	-thiol	Oxidation to disulfide
Ether	R–O–R'	alkoxy- (IUPAC), ether (common)	Generally unreactive, used as solvents

Practice and Application

• Be able to name and draw structures for alcohols, phenols, thiols, and ethers using both IUPAC and common naming conventions.

• Classify alcohols as primary, secondary, or tertiary based on their structure.

• Predict solubility and boiling points based on molecular structure and functional groups.

• Write balanced equations for combustion, dehydration, and oxidation reactions.