Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol, commonly abbreviated as IPA) is a colorless, flammable, organic compound with a pungent odor. Isopropyl alcohol, an organic polar molecule, is miscible in water, ethanol, and chloroform, demonstrating its ability to dissolve a wide range of substances including ethyl cellulose, polyvinyl butyral, oils, alkaloids, and natural resins. Notably, it is not miscible with salt solutions and can be separated by adding sodium chloride in a process known as salting out. It forms an azeotrope with water, resulting in a boiling point of 80.37 °C and is characterized by its slightly bitter taste. Isopropyl alcohol becomes viscous at lower temperatures, freezing at −89.5 °C, and has significant ultraviolet-visible absorbance at 205 nm. Chemically, it can be oxidized to acetone or undergo various reactions to form compounds such as isopropoxides, e.g. aluminium isopropoxide. As an isopropyl group linked to a hydroxyl group (chemical formula (CH3)2CHOH) it is the simplest example of a secondary alcohol, where the alcohol carbon atom is attached to two other carbon atoms. It is a structural isomer of propan-1-ol and ethyl methyl ether, all of which share the formula C3H8O. It was first synthesized in 1853 by Alexander William Williamson and later produced for cordite preparation. It is produced through hydration of propene or hydrogenation of acetone, with modern processes achieving anhydrous alcohol through azeotropic distillation. Isopropyl alcohol serves in medical settings as a rubbing alcohol and hand sanitizer, and in industrial and household applications as a solvent. It is a common ingredient in products such as antiseptics, disinfectants, and detergents. More than a million tonnes are produced worldwide annually. Isopropyl alcohol poses safety risks due to its flammability and potential for peroxide formation. Its ingestion or absorption leads to toxic effects including central nervous system depression and coma.
Properties Isopropyl alcohol is miscible in water, ethanol, and chloroform, as it is an organic polar molecule. It dissolves ethyl cellulose, polyvinyl butyral, many oils, alkaloids, and natural resins. Unlike ethanol or methanol, isopropyl alcohol is not miscible with salt solutions and can be separated from aqueous solutions by adding a salt such as sodium chloride. The process is colloquially called salting out, and causes concentrated isopropyl alcohol to separate into a distinct layer. Isopropyl alcohol forms an azeotrope with water, which gives a boiling point of 80.37 °C (176.67 °F) and a composition of 87.7% by mass (91% by volume) isopropyl alcohol. It has a slightly bitter taste, and is toxic when ingested. Isopropyl alcohol becomes increasingly viscous with decreasing temperature and freezes at −89.5 °C (−129.1 °F). Mixtures with water have higher freezing points: 99% at −89.5 °C (−129.1 °F), 91% (the azeotrope) at −75.5 °C (−103.9 °F), and 70% at −61.7 °C (−79.1 °F). Isopropyl alcohol has a maximal absorbance at 205 nm in an ultraviolet-visible spectrum.
Reactions Isopropyl alcohol can be oxidized to acetone, which is the corresponding ketone. This can be achieved using oxidizing agents such as chromic acid, or by dehydrogenation of isopropyl alcohol over a heated copper catalyst:
Isopropyl alcohol is often used as both solvent and hydride source in the Meerwein-Ponndorf-Verley reduction and other transfer hydrogenation reactions. Isopropyl alcohol may be converted to 2-bromopropane using phosphorus tribromide, or dehydrated to propene by heating with sulfuric acid.
Like most alcohols, isopropyl alcohol reacts with active metals such as potassium to form alkoxides that are called isopropoxides. With titanium tetrachloride, isopropyl alcohol reacts to give titanium isopropoxide:
This and similar reactions are often conducted in the presence of base. The reaction with aluminium is initiated by a trace of mercury to give aluminium isopropoxide.
History Isopropyl alcohol was first synthesized by the chemist Alexander William Williamson in 1853. He achieved this by heating a mixture of propene and sulfuric acid. Standard Oil produced isopropyl alcohol by hydrating propene. Isopropyl alcohol was oxidized to acetone for the preparation of cordite, a smokeless, low explosive propellant.
Production In 1994, 1.5 million tonnes of isopropyl alcohol were produced in the United States, Europe, and Japan. It is primarily produced by combining water and propene in a hydration reaction or by hydrogenating acetone. There are two routes for the hydration process and both processes require that the isopropyl alcohol be separated from water and other by-products by distillation. Isopropyl alcohol and water form an azeotrope, and simple distillation gives a material that is 87.9% by mass isopropyl alcohol and 12.1% by mass water. Pure (anhydrous) isopropyl alcohol is made by azeotropic distillation of the wet isopropyl alcohol using either diisopropyl ether or cyclohexane as azeotroping agents.
Biological Small amounts of isopropyl alcohol are produced in the body in diabetic ketoacidosis.
Indirect hydration Indirect hydration reacts propene with sulfuric acid to form a mixture of sulfate esters. This process can use low-quality propene, and is predominant in the USA. These processes give primarily isopropyl alcohol rather than 1-propanol, because adding water or sulfuric acid to propene follows Markovnikov's rule. Subsequent hydrolysis of these esters by steam produces isopropyl alcohol, by distillation. Diisopropyl ether is a significant by-product of this process; it is recycled back to the process and hydrolyzed to give the desired product.
Direct hydration
Direct hydration reacts propene and water, either in gas or liquid phase, at high pressures in the presence of solid or supported acidic catalysts. This type of process usually requires higher-purity propylene (> 90%). Direct hydration is more commonly used in Europe.
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