Introduction

Replacing one or more hydrogen atoms in a hydrocarbon with a different atom or group of atoms is one of the most productive ideas in organic chemistry — it turns a fairly unreactive starting material into an entirely new class of compound, with its own set of properties and its own uses. Alcohols and phenols are exactly this kind of substitution product: a hydrogen atom of a hydrocarbon is swapped out for a hydroxyl ($-\text{OH}$) group. When that hydrocarbon is aliphatic (an alkyl chain), the result is an alcohol — methanol, $\text{CH}_3\text{OH}$, is the simplest example. When the hydroxyl group sits directly on an aromatic ring carbon instead, the result is a phenol — $\text{C}_6\text{H}_5\text{OH}$ is the parent compound.

These two families of compounds are everywhere in daily life, not just in the laboratory. The spirit used to polish wooden furniture is chiefly ethanol, an alcohol. Sugar, the cotton in fabric, and the cellulose that makes up paper are all built from molecules carrying $-\text{OH}$ groups. Every notebook, newspaper, currency note and certificate exists because of compounds in this family — a genuine illustration of how widely a single functional group can show up across ordinary materials.

A related substitution gives a third family of compounds. Instead of replacing a hydrocarbon's hydrogen with a hydroxyl group, replace it with an alkoxy or aryloxy group ($\text{R}-\text{O}-$ or $\text{Ar}-\text{O}-$), and the product is an ether — dimethyl ether, $\text{CH}_3-\text{O}-\text{CH}_3$, is a simple example. Ethers can also be pictured the other way round: as an alcohol or phenol whose hydroxyl hydrogen has itself been replaced by an alkyl or aryl group, linking two carbon-containing groups through a single oxygen atom.

This chapter takes up all three families together — alcohols, phenols and ethers — starting with how each is classified.