Alcohols: Structure, Classification, and Properties

Alcohols are organic compounds that contain a hydroxyl group (–OH) attached to a saturated carbon atom; they are classified as primary, secondary or tertiary according to the number of carbon atoms attached to the carbon that carries the –OH group. In brief: ethanol (CH₃CH₂OH) is a primary alcohol and boils at 78.4 °C, much higher than the ether CH₃OCH₃ of the same formula (−24.8 °C), because alcohols form hydrogen bonds.

Alcohols are among the most important organic compounds: they are solvents, fuels, disinfectants, drinks and starting materials for the synthesis of many other substances. The way they react depends strongly on their class. This article explains the structure and naming of alcohols, how to classify them, their physical properties with a data table, and their main reactions, with worked examples.

What is the structure of an alcohol?

The general formula of a saturated monohydric alcohol is R–OH, or C_nH_2n+1OH. The oxygen atom is sp³ hybridized with two lone pairs, and the C–O–H angle is about 108.9° in methanol. The O–H bond is polar, with the hydrogen carrying a partial positive charge, so alcohols can form hydrogen bonds.

In IUPAC nomenclature, the suffix -ol replaces the final -e of the alkane name, and the longest chain that contains the carbon bearing the –OH group is numbered to give that carbon the lowest number: propan-2-ol, 2-methylpropan-1-ol. Compounds with two or three –OH groups are diols and triols, for example ethane-1,2-diol (ethylene glycol) and propane-1,2,3-triol (glycerol). When the –OH group is a substituent in a compound with a higher-priority group, it is named as a hydroxy- prefix.

How are alcohols classified?

• Primary (1°): the carbon bearing the –OH is attached to one other carbon (or none, in methanol). Examples: ethanol, propan-1-ol.
• Secondary (2°): the carbon bearing the –OH is attached to two other carbons. Examples: propan-2-ol, butan-2-ol.
• Tertiary (3°): the carbon bearing the –OH is attached to three other carbons. Example: 2-methylpropan-2-ol.

The class determines how the alcohol is oxidized, how fast it reacts with acids and how easily it loses water.

What are the physical properties?

Boiling points are much higher than those of alkanes or ethers of similar molar mass, because of hydrogen bonding. They rise with chain length, and branched isomers boil lower than straight-chain ones. Solubility in water: the –OH group can form hydrogen bonds with water, so alcohols with 1–3 carbon atoms are miscible with water in all proportions. As the nonpolar chain gets longer, the solubility decreases: butan-1-ol dissolves to about 7 g per 100 mL, and hexan-1-ol to only about 0.6 g per 100 mL. Acidity: alcohols are very weak acids, with a pK_a of about 16 (ethanol 15.9), similar to water. They do not react with NaOH solution, but they react with active metals such as sodium.

Data for common alcohols

What are the main reactions?

Oxidation

With acidified potassium dichromate(VI) (orange to green) or permanganate:

• Primary alcohols give aldehydes (if the aldehyde is distilled off as it forms) and then carboxylic acids (with an excess of oxidant, under reflux): CH₃CH₂OH → CH₃CHO → CH₃COOH.
• Secondary alcohols give ketones: CH₃CH(OH)CH₃ → CH₃COCH₃.
• Tertiary alcohols are not oxidized under these conditions, since the carbon bearing the –OH has no hydrogen.

Dehydration

Heating with concentrated sulfuric or phosphoric acid removes water to give an alkene: CH₃CH₂OH → CH₂=CH₂ + H₂O at about 170 °C. Tertiary alcohols dehydrate most easily, and primary alcohols least.

Reaction with sodium

2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂. The reaction is slower than that of sodium with water, and gives sodium ethoxide and hydrogen.

Esterification

With a carboxylic acid and a little acid catalyst, an alcohol forms an ester and water: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O.

Substitution to haloalkanes

Alcohols react with hydrogen halides (HBr from NaBr and H₂SO₄), with PCl₅ or with SOCl₂ to give haloalkanes, for example C₂H₅OH + HBr → C₂H₅Br + H₂O. The Lucas test, with HCl and ZnCl₂, distinguishes the classes: tertiary alcohols make the solution cloudy at once, secondary ones in about 5 minutes and primary ones show no reaction at room temperature.

Worked examples

Example 1: classifying and predicting oxidation

Butan-2-ol, CH₃CH(OH)CH₂CH₃, has a carbon with two carbon neighbors, so it is secondary. Oxidation gives butanone, CH₃COCH₂CH₃. The compound 2-methylpropan-2-ol is tertiary and gives no product with warm acidified dichromate.

Example 2: comparing boiling points

Why does ethanol (78.4 °C) boil much higher than propane (−42.1 °C), although their molar masses are 46 and 44 g/mol? Ethanol molecules form hydrogen bonds with each other, which propane cannot, and a lot more energy is needed to separate them.

Example 3: hydrogen from sodium

What volume of hydrogen at 25 °C and 1 atm is produced when 2.30 g of sodium reacts with excess ethanol? n(Na) = 2.30 / 22.99 = 0.100 mol, and n(H₂) = 0.0500 mol, so V = 0.0500 × 24.47 = 1.22 L.

Where are alcohols used?

• Methanol: produced at over 100 million tonnes per year, as a raw material for formaldehyde, as a solvent and a fuel additive. It is toxic, since the body oxidizes it to formaldehyde and formic acid.
• Ethanol: in drinks, as a solvent, a disinfectant (70 % solutions) and, as bioethanol, a fuel.
• Propan-2-ol (isopropanol): rubbing alcohol and cleaning solvent.
• Ethane-1,2-diol: antifreeze and a monomer for polyesters.
• Glycerol: in cosmetics, foods, and the production of explosives and soaps.

How were alcohols discovered?

The fermentation of sugars to ethanol and its concentration by distillation have been known for centuries. Methanol was first isolated in 1661 by Robert Boyle from the distillation of wood, which gave it the name “wood alcohol”. The composition of ethanol was established in the early 19th century, and Alexander Williamson showed in the 1850s that ethers and alcohols are related through the water type of molecule. The term alcohol is of Arabic origin and originally meant a fine powder, such as kohl.

Which mistakes are common?

• Calling an alcohol with an –OH on a benzene ring an alcohol: it is a phenol.
• Forgetting the class when predicting oxidation: tertiary alcohols resist oxidation.
• Assuming that alcohols are basic or strongly acidic: they are neutral to litmus.
• Numbering from the wrong end: the –OH carbon gets the lowest number.
• Mixing up the oxidation products: primary gives aldehyde then acid, secondary gives a ketone.