Compounds in which a hydroxy group, -\mathrm{OH}, is bonded to a carbon atom of a hydrocarbon group are called alcohols. Compounds in which the hydroxy group is bonded directly to a benzene ring are called phenols, and their properties are different.
Ethanol, \mathrm{C_2H_5OH}, is the alcohol in alcoholic drinks. It is made when yeast ferments sugars such as glucose.
\mathrm{C_6H_{12}O_6} \longrightarrow 2\mathrm{C_2H_5OH} + 2\mathrm{CO_2}
Some alcohols have more than one hydroxy group. Ethane-1,2-diol (ethylene glycol), which has two, is used in antifreeze, and propane-1,2,3-triol (glycerol), which has three, is used in cosmetics and medicines.
Alcohols have higher boiling points than hydrocarbons with a similar relative molecular mass. Ethanol, with a relative molecular mass of 46, boils at 78 °C, while propane, with 44, boils at -42 °C.
This is because the hydrogen atom of the hydroxy group of one molecule is attracted to the oxygen atom of another molecule, forming a hydrogen bond.
Methanol and ethanol mix with water in any proportion, because they also form hydrogen bonds with water molecules. The larger the hydrocarbon group of an alcohol, the less it dissolves in water.
Like water, alcohols react with sodium to give off hydrogen, because the hydrogen atom of the hydroxy group is replaced by sodium. Ethanol forms sodium ethoxide.
2\mathrm{C_2H_5OH} + 2\mathrm{Na} \longrightarrow 2\mathrm{C_2H_5ONa} + \mathrm{H_2}
The reaction is much gentler than the reaction with water. Sodium is denser than ethanol, so it sinks and gives off bubbles slowly. Ethers, which have no hydroxy group, do not react, so this reaction is used to tell alcohols from ethers.
When an alcohol is warmed with potassium dichromate solution acidified with sulfuric acid, the alcohol is oxidized, and the orange dichromate ions are reduced to green \mathrm{Cr}^{3+}.
Alcohols are classified by the number of carbon atoms bonded to the carbon atom that carries the hydroxy group: one (or none) in primary alcohols, two in secondary alcohols and three in tertiary alcohols.
Primary alcohols such as ethanol are oxidized to aldehydes and then to carboxylic acids, and secondary alcohols such as propan-2-ol are oxidized to ketones. Tertiary alcohols are hardly oxidized, so the solution stays orange.
When ethanol is heated with concentrated sulfuric acid at about 130 to 140 °C, a molecule of water is removed from two molecules of ethanol, and diethyl ether forms.
2\mathrm{C_2H_5OH} \longrightarrow \mathrm{C_2H_5OC_2H_5} + \mathrm{H_2O}
At about 160 to 170 °C, a molecule of water is removed from within one molecule of ethanol, and ethene forms.
\mathrm{C_2H_5OH} \longrightarrow \mathrm{CH_2{=}CH_2} + \mathrm{H_2O}
Diethyl ether is a volatile and highly flammable liquid that boils at 34 °C. It was once used as an anaesthetic.