Phenols and Ethers: Structure and Reactions

Phenols are compounds in which a hydroxyl group is attached directly to a benzene ring, and ethers are compounds with an oxygen atom between two carbon groups (R–O–R′); phenols are much more acidic than alcohols, while ethers are rather unreactive. In brief: phenol has a pK_a of about 10 compared with about 16 for ethanol, so it dissolves in NaOH solution, and diethyl ether (boiling point 34.6 °C) is a common solvent that must be stored with care because it forms explosive peroxides.

Both families contain an oxygen atom bonded to carbon, but their chemistry is very different. Phenols are involved in antiseptics, plastics and medicines, and ethers range from industrial solvents to the first anesthetics. This article explains the structure and acidity of phenol, its reactions, the properties and synthesis of ethers, and how the two classes can be distinguished.

What is the structure of phenol?

Phenol is C₆H₅OH. It is a colorless crystalline solid (melting point 40.5 °C, boiling point 181.7 °C) with a characteristic smell, partly soluble in water (about 8 g per 100 mL at 20 °C) because of hydrogen bonding. The oxygen lone pair overlaps with the π system of the ring, so that the C–O bond is shorter (136 pm) than in alcohols (143 pm), and the ring is more reactive than benzene.

Why is phenol acidic?

When phenol loses a proton, the resulting phenoxide ion, C₆H₅O⁻, has its negative charge spread over the ring by resonance (onto the ortho and para carbons). This delocalization stabilizes the anion much more than the localized charge of an alkoxide ion, so phenol is a stronger acid than an alcohol by about six pK_a units. Electron-withdrawing groups such as –NO₂ on the ring stabilize the anion further and make the phenol more acidic.

Acidity of phenols compared with other compounds

Phenol reacts with sodium hydroxide, C₆H₅OH + NaOH → C₆H₅ONa + H₂O, but it is too weak an acid to release CO₂ from carbonates, which is a way to separate phenols from carboxylic acids.

What are the reactions of phenol?

• Electrophilic substitution is easier than in benzene: the –OH group activates the ring toward attack at the 2, 4 and 6 positions. Bromine water gives a white precipitate of 2,4,6-tribromophenol at once, without a catalyst. Dilute nitric acid gives 2- and 4-nitrophenol.
• Color test: aqueous iron(III) chloride gives a violet (purple) color with phenol.
• Ester formation: phenols form esters with acid chlorides more readily than with carboxylic acids.
• Kolbe–Schmitt reaction: sodium phenoxide with CO₂ under pressure gives salicylic acid, the precursor of aspirin.
• Condensation with methanal: phenol and formaldehyde form phenol–formaldehyde resins such as Bakelite.

What are ethers?

Ethers have the general formula R–O–R′ with a C–O–C angle of about 110°. They have no O–H bond, so they do not form hydrogen bonds with themselves, which gives them low boiling points compared with alcohols. They can accept hydrogen bonds from water, so small ethers are somewhat soluble. Ethers are named by listing the two groups alphabetically followed by “ether” (ethyl methyl ether), or by the IUPAC alkoxyalkane system (methoxyethane).

Ethers are chemically rather inert: they do not react with bases, dilute acids, reducing agents or most oxidizers, so they are good solvents. They are cleaved by strong acids such as HI or HBr. The main hazard is that, on exposure to air and light, they slowly form explosive hydroperoxides, so old bottles of diethyl ether can be dangerous.

Synthesis: the Williamson ether synthesis

An alkoxide reacts with a primary haloalkane by nucleophilic substitution: C₂H₅ONa + CH₃I → C₂H₅OCH₃ + NaI. Diethyl ether is made industrially by the dehydration of ethanol with sulfuric acid at about 140 °C: 2C₂H₅OH → C₂H₅OC₂H₅ + H₂O (at 170 °C ethene forms instead).

Properties of ethers

Worked examples

Example 1: distinguishing phenol and ethanol

Add a few drops of neutral iron(III) chloride solution to each: phenol gives a violet color, ethanol gives none. Or add bromine water: phenol gives a white precipitate (2,4,6-tribromophenol) and decolorizes the bromine, while ethanol does not react.

Example 2: ethanol and methoxymethane

Both have the formula C₂H₆O. Ethanol reacts with sodium to give bubbles of hydrogen, but methoxymethane does not, as it has no O–H bond. The boiling points are 78.4 °C and −24.8 °C.

Example 3: a Williamson synthesis

To make ethyl propyl ether, two routes are possible: sodium ethoxide with 1-bromopropane, or sodium propoxide with bromoethane. Both use a primary haloalkane and give the product. For tert-butyl methyl ether, the synthesis from sodium methoxide and 2-bromo-2-methylpropane would fail, because the tertiary halide gives elimination to an alkene; it is better to use sodium tert-butoxide and iodomethane.

Example 4: mass of tribromophenol

From 1.00 g of phenol (M = 94.11 g/mol, 0.01063 mol), the reaction with excess bromine water gives 2,4,6-tribromophenol (M = 330.8 g/mol): 0.01063 × 330.8 = 3.52 g at 100 % yield.

Where do they come from historically?

Phenol was discovered in coal tar by Friedlieb Ferdinand Runge in 1834. Joseph Lister used “carbolic acid” (phenol) from 1865–1867 to disinfect wounds and surgical instruments, which greatly reduced infections after operations. Diethyl ether was described in the 16th century, and its use as a surgical anesthetic was demonstrated publicly by William Morton in Boston in 1846. Alexander Williamson reported his synthesis of ethers in 1850. Charles Pedersen discovered crown ethers in 1967, for which he shared the Nobel Prize in Chemistry in 1987.

Which mistakes are common?

• Treating phenol as an alcohol: it is much more acidic and reacts with NaOH.
• Expecting phenol to release CO₂ from carbonate: it is too weak an acid.
• Assuming that ethers form hydrogen bonds with themselves: they cannot, which is why their boiling points are low.
• Using a tertiary halide in the Williamson synthesis: elimination wins.
• Storing ethers for years: peroxides can form.